The Transport Layer Security (TLS) Protocol Version 1.3Independenttim@dierks.orgRTFM, Inc.ekr@rtfm.com
General
Internet-DraftThis document specifies Version 1.3 of the Transport Layer Security
(TLS) protocol. The TLS protocol provides communications security
over the Internet. The protocol allows client/server applications to
communicate in a way that is designed to prevent eavesdropping,
tampering, or message forgery.DISCLAIMER:
This document is simply a copy of RFC 5246 translated
into markdown format with no intentional technical or editorial
changes beyond updating the references and minor reformatting introduced
by the translation. It is being submitted
as-is to create a clearer revision history for future versions.
Any errata in TLS 1.2 remain in this version. Thanks to Mark
Nottingham for doing the markdown translation.The primary goal of the TLS protocol is to provide privacy and data integrity
between two communicating applications. The protocol is composed of two layers:
the TLS Record Protocol and the TLS Handshake Protocol. At the lowest level,
layered on top of some reliable transport protocol (e.g., TCP ), is
the TLS Record Protocol. The TLS Record Protocol provides connection security
that has two basic properties:The connection is private. Symmetric cryptography is used for
data encryption (e.g., AES , RC4 , etc.). The keys for
this symmetric encryption are generated uniquely for each
connection and are based on a secret negotiated by another
protocol (such as the TLS Handshake Protocol). The Record
Protocol can also be used without encryption.The connection is reliable. Message transport includes a message
integrity check using a keyed MAC. Secure hash functions (e.g.,
SHA-1, etc.) are used for MAC computations. The Record Protocol
can operate without a MAC, but is generally only used in this mode
while another protocol is using the Record Protocol as a transport
for negotiating security parameters.The TLS Record Protocol is used for encapsulation of various higher- level
protocols. One such encapsulated protocol, the TLS Handshake Protocol, allows
the server and client to authenticate each other and to negotiate an encryption
algorithm and cryptographic keys before the application protocol transmits or
receives its first byte of data. The TLS Handshake Protocol provides connection
security that has three basic properties:The peer’s identity can be authenticated using asymmetric, or
public key, cryptography (e.g., RSA , DSA , etc.). This
authentication can be made optional, but is generally required for
at least one of the peers.The negotiation of a shared secret is secure: the negotiated
secret is unavailable to eavesdroppers, and for any authenticated
connection the secret cannot be obtained, even by an attacker who
can place himself in the middle of the connection.The negotiation is reliable: no attacker can modify the
negotiation communication without being detected by the parties to
the communication.One advantage of TLS is that it is application protocol independent.
Higher-level protocols can layer on top of the TLS protocol transparently. The
TLS standard, however, does not specify how protocols add security with TLS;
the decisions on how to initiate TLS handshaking and how to interpret the
authentication certificates exchanged are left to the judgment of the designers
and implementors of protocols that run on top of TLS.The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
“SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be
interpreted as described in RFC 2119 .This document is a revision of the TLS 1.1 protocol which contains
improved flexibility, particularly for negotiation of cryptographic algorithms.
The major changes are:The MD5/SHA-1 combination in the pseudorandom function (PRF) has
been replaced with cipher-suite-specified PRFs. All cipher suites
in this document use P_SHA256.The MD5/SHA-1 combination in the digitally-signed element has been
replaced with a single hash. Signed elements now include a field
that explicitly specifies the hash algorithm used.Substantial cleanup to the client’s and server’s ability to
specify which hash and signature algorithms they will accept.
Note that this also relaxes some of the constraints on signature
and hash algorithms from previous versions of TLS.Addition of support for authenticated encryption with additional
data modes.TLS Extensions definition and AES Cipher Suites were merged in
from external and .Tighter checking of EncryptedPreMasterSecret version numbers.Tightened up a number of requirements.Verify_data length now depends on the cipher suite (default is
still 12).Cleaned up description of Bleichenbacher/Klima attack defenses.Alerts MUST now be sent in many cases.After a certificate_request, if no certificates are available,
clients now MUST send an empty certificate list.TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement
cipher suite.Added HMAC-SHA256 cipher suites.Removed IDEA and DES cipher suites. They are now deprecated and
will be documented in a separate document.Support for the SSLv2 backward-compatible hello is now a MAY, not
a SHOULD, with sending it a SHOULD NOT. Support will probably
become a SHOULD NOT in the future.Added limited “fall-through” to the presentation language to allow
multiple case arms to have the same encoding.Added an Implementation Pitfalls sectionsThe usual clarifications and editorial work.The goals of the TLS protocol, in order of priority, are as follows:Cryptographic security: TLS should be used to establish a secure connection
between two parties.Interoperability: Independent programmers should be able to develop
applications utilizing TLS that can successfully exchange cryptographic
parameters without knowledge of one another’s code.Extensibility: TLS seeks to provide a framework into which new public key
and bulk encryption methods can be incorporated as necessary. This will also
accomplish two sub-goals: preventing the need to create a new protocol (and
risking the introduction of possible new weaknesses) and avoiding the need to
implement an entire new security library.Relative efficiency: Cryptographic operations tend to be highly CPU
intensive, particularly public key operations. For this reason, the TLS
protocol has incorporated an optional session caching scheme to reduce the
number of connections that need to be established from scratch. Additionally,
care has been taken to reduce network activity.This document and the TLS protocol itself are based on the SSL 3.0 Protocol
Specification as published by Netscape. The differences between this protocol
and SSL 3.0 are not dramatic, but they are significant enough that the various
versions of TLS and SSL 3.0 do not interoperate (although each protocol
incorporates a mechanism by which an implementation can back down to prior
versions). This document is intended primarily for readers who will be
implementing the protocol and for those doing cryptographic analysis of it. The
specification has been written with this in mind, and it is intended to reflect
the needs of those two groups. For that reason, many of the algorithm-dependent
data structures and rules are included in the body of the text (as opposed to
in an appendix), providing easier access to them.This document is not intended to supply any details of service definition or of
interface definition, although it does cover select areas of policy as they are
required for the maintenance of solid security.This document deals with the formatting of data in an external representation.
The following very basic and somewhat casually defined presentation syntax will
be used. The syntax draws from several sources in its structure. Although it
resembles the programming language “C” in its syntax and XDR in
both its syntax and intent, it would be risky to draw too many parallels. The
purpose of this presentation language is to document TLS only; it has no
general application beyond that particular goal.The representation of all data items is explicitly specified. The basic data
block size is one byte (i.e., 8 bits). Multiple byte data items are
concatenations of bytes, from left to right, from top to bottom. From the byte
stream, a multi-byte item (a numeric in the example) is formed (using C
notation) by:This byte ordering for multi-byte values is the commonplace network byte order
or big-endian format.Comments begin with “/*” and end with “*/”.Optional components are denoted by enclosing them in “[[ ]]” double
brackets.Single-byte entities containing uninterpreted data are of type
opaque.A vector (single-dimensioned array) is a stream of homogeneous data elements.
The size of the vector may be specified at documentation time or left
unspecified until runtime. In either case, the length declares the number of
bytes, not the number of elements, in the vector. The syntax for specifying a
new type, T’, that is a fixed- length vector of type T isHere, T’ occupies n bytes in the data stream, where n is a multiple of the size
of T. The length of the vector is not included in the encoded stream.In the following example, Datum is defined to be three consecutive bytes that
the protocol does not interpret, while Data is three consecutive Datum,
consuming a total of nine bytes.Variable-length vectors are defined by specifying a subrange of legal lengths,
inclusively, using the notation <floor..ceiling>. When these are encoded, the
actual length precedes the vector’s contents in the byte stream. The length
will be in the form of a number consuming as many bytes as required to hold the
vector’s specified maximum (ceiling) length. A variable-length vector with an
actual length field of zero is referred to as an empty vector.In the following example, mandatory is a vector that must contain between 300
and 400 bytes of type opaque. It can never be empty. The actual length field
consumes two bytes, a uint16, which is sufficient to represent the value 400
(see ). On the other hand, longer can represent up to 800 bytes of
data, or 400 uint16 elements, and it may be empty. Its encoding will include a
two-byte actual length field prepended to the vector. The length of an encoded
vector must be an even multiple of the length of a single element (for example,
a 17-byte vector of uint16 would be illegal).The basic numeric data type is an unsigned byte (uint8). All larger numeric
data types are formed from fixed-length series of bytes concatenated as
described in and are also unsigned. The following numeric
types are predefined.All values, here and elsewhere in the specification, are stored in network byte
(big-endian) order; the uint32 represented by the hex bytes 01 02 03 04 is
equivalent to the decimal value 16909060.Note that in some cases (e.g., DH parameters) it is necessary to represent
integers as opaque vectors. In such cases, they are represented as unsigned
integers (i.e., leading zero octets are not required even if the most
significant bit is set).An additional sparse data type is available called enum. A field of type enum
can only assume the values declared in the definition. Each definition is a
different type. Only enumerateds of the same type may be assigned or compared.
Every element of an enumerated must be assigned a value, as demonstrated in the
following example. Since the elements of the enumerated are not ordered, they
can be assigned any unique value, in any order.An enumerated occupies as much space in the byte stream as would its maximal
defined ordinal value. The following definition would cause one byte to be used
to carry fields of type Color.One may optionally specify a value without its associated tag to force the
width definition without defining a superfluous element.In the following example, Taste will consume two bytes in the data stream but
can only assume the values 1, 2, or 4.The names of the elements of an enumeration are scoped within the defined type.
In the first example, a fully qualified reference to the second element of the
enumeration would be Color.blue. Such qualification is not required if the
target of the assignment is well specified.For enumerateds that are never converted to external representation, the
numerical information may be omitted.Structure types may be constructed from primitive types for convenience. Each
specification declares a new, unique type. The syntax for definition is much
like that of C.The fields within a structure may be qualified using the type’s name, with a
syntax much like that available for enumerateds. For example, T.f2 refers to
the second field of the previous declaration. Structure definitions may be
embedded.Defined structures may have variants based on some knowledge that is available
within the environment. The selector must be an enumerated type that defines
the possible variants the structure defines. There must be a case arm for every
element of the enumeration declared in the select. Case arms have limited
fall-through: if two case arms follow in immediate succession with no fields in
between, then they both contain the same fields. Thus, in the example below,
“orange” and “banana” both contain V2. Note that this is a new piece of syntax
in TLS 1.2.The body of the variant structure may be given a label for reference. The
mechanism by which the variant is selected at runtime is not prescribed by the
presentation language.For example:The five cryptographic operations — digital signing, stream cipher encryption,
block cipher encryption, authenticated encryption with additional data (AEAD)
encryption, and public key encryption — are designated digitally-signed,
stream-ciphered, block-ciphered, aead- ciphered, and public-key-encrypted,
respectively. A field’s cryptographic processing is specified by prepending an
appropriate key word designation before the field’s type specification.
Cryptographic keys are implied by the current session state (see
).A digitally-signed element is encoded as a struct DigitallySigned:The algorithm field specifies the algorithm used (see
for the definition of this field). Note that the introduction of the algorithm
field is a change from previous versions. The signature is a digital signature
using those algorithms over the contents of the element. The contents
themselves do not appear on the wire but are simply calculated. The length of
the signature is specified by the signing algorithm and key.In RSA signing, the opaque vector contains the signature generated using the
RSASSA-PKCS1-v1_5 signature scheme defined in . As discussed in
, the DigestInfo MUST be DER-encoded . For hash
algorithms without parameters (which includes SHA-1), the
DigestInfo.AlgorithmIdentifier.parameters field MUST be NULL, but
implementations MUST accept both without parameters and with NULL parameters.
Note that earlier versions of TLS used a different RSA signature scheme that
did not include a DigestInfo encoding.In DSA, the 20 bytes of the SHA-1 hash are run directly through the Digital
Signing Algorithm with no additional hashing. This produces two values, r and
s. The DSA signature is an opaque vector, as above, the contents of which are
the DER encoding of:Note: In current terminology, DSA refers to the Digital Signature Algorithm and
DSS refers to the NIST standard. In the original SSL and TLS specs, “DSS” was
used universally. This document uses “DSA” to refer to the algorithm, “DSS” to
refer to the standard, and it uses “DSS” in the code point definitions for
historical continuity.In stream cipher encryption, the plaintext is exclusive-ORed with an identical
amount of output generated from a cryptographically secure keyed pseudorandom
number generator.In block cipher encryption, every block of plaintext encrypts to a block of
ciphertext. All block cipher encryption is done in CBC (Cipher Block Chaining)
mode, and all items that are block-ciphered will be an exact multiple of the
cipher block length.In AEAD encryption, the plaintext is simultaneously encrypted and integrity
protected. The input may be of any length, and aead- ciphered output is
generally larger than the input in order to accommodate the integrity check
value.In public key encryption, a public key algorithm is used to encrypt data in
such a way that it can be decrypted only with the matching private key. A
public-key-encrypted element is encoded as an opaque vector <0..2^16-1>, where
the length is specified by the encryption algorithm and key.RSA encryption is done using the RSAES-PKCS1-v1_5 encryption scheme defined in
.In the following exampleThe contents of the inner struct (field3 and field4) are used as input for the
signature/hash algorithm, and then the entire structure is encrypted with a
stream cipher. The length of this structure, in bytes, would be equal to two
bytes for field1 and field2, plus two bytes for the signature and hash
algorithm, plus two bytes for the length of the signature, plus the length of
the output of the signing algorithm. The length of the signature is known
because the algorithm and key used for the signing are known prior to encoding
or decoding this structure.Typed constants can be defined for purposes of specification by declaring a
symbol of the desired type and assigning values to it.Under-specified types (opaque, variable-length vectors, and structures that
contain opaque) cannot be assigned values. No fields of a multi-element
structure or vector may be elided.For example:The TLS record layer uses a keyed Message Authentication Code (MAC) to protect
message integrity. The cipher suites defined in this document use a
construction known as HMAC, described in , which is based on a hash
function. Other cipher suites MAY define their own MAC constructions, if needed.In addition, a construction is required to do expansion of secrets into blocks
of data for the purposes of key generation or validation. This pseudorandom
function (PRF) takes as input a secret, a seed, and an identifying label and
produces an output of arbitrary length.In this section, we define one PRF, based on HMAC. This PRF with the SHA-256
hash function is used for all cipher suites defined in this document and in TLS
documents published prior to this document when TLS 1.2 is negotiated. New
cipher suites MUST explicitly specify a PRF and, in general, SHOULD use the TLS
PRF with SHA-256 or a stronger standard hash function.First, we define a data expansion function, P_hash(secret, data), that uses a
single hash function to expand a secret and seed into an arbitrary quantity of
output:where + indicates concatenation.A() is defined as:P_hash can be iterated as many times as necessary to produce the required
quantity of data. For example, if P_SHA256 is being used to create 80 bytes of
data, it will have to be iterated three times (through A(3)), creating 96 bytes
of output data; the last 16 bytes of the final iteration will then be
discarded, leaving 80 bytes of output data.TLS’s PRF is created by applying P_hash to the secret as:The label is an ASCII string. It should be included in the exact
form it is given without a length byte or trailing null character.
For example, the label “slithy toves” would be processed by hashing
the following bytes:The TLS Record Protocol is a layered protocol. At each layer, messages may
include fields for length, description, and content. The Record Protocol takes
messages to be transmitted, fragments the data into manageable blocks,
optionally compresses the data, applies a MAC, encrypts, and transmits the
result. Received data is decrypted, verified, decompressed, reassembled, and
then delivered to higher-level clients.Four protocols that use the record protocol are described in this document: the
handshake protocol, the alert protocol, the change cipher spec protocol, and
the application data protocol. In order to allow extension of the TLS protocol,
additional record content types can be supported by the record protocol. New
record content type values are assigned by IANA in the TLS Content Type
Registry as described in .Implementations MUST NOT send record types not defined in this document unless
negotiated by some extension. If a TLS implementation receives an unexpected
record type, it MUST send an unexpected_message alert.Any protocol designed for use over TLS must be carefully designed to deal with
all possible attacks against it. As a practical matter, this means that the
protocol designer must be aware of what security properties TLS does and does
not provide and cannot safely rely on the latter.Note in particular that type and length of a record are not protected by
encryption. If this information is itself sensitive, application designers may
wish to take steps (padding, cover traffic) to minimize information leakage.A TLS connection state is the operating environment of the TLS Record Protocol.
It specifies a compression algorithm, an encryption algorithm, and a MAC
algorithm. In addition, the parameters for these algorithms are known: the MAC
key and the bulk encryption keys for the connection in both the read and the
write directions. Logically, there are always four connection states
outstanding: the current read and write states, and the pending read and write
states. All records are processed under the current read and write states. The
security parameters for the pending states can be set by the TLS Handshake
Protocol, and the ChangeCipherSpec can selectively make either of the pending
states current, in which case the appropriate current state is disposed of and
replaced with the pending state; the pending state is then reinitialized to an
empty state. It is illegal to make a state that has not been initialized with
security parameters a current state. The initial current state always specifies
that no encryption, compression, or MAC will be used.The security parameters for a TLS Connection read and write state are set by
providing the following values:
Whether this entity is considered the “client” or the “server” in
this connection.
An algorithm used to generate keys from the master secret (see
and ).
An algorithm to be used for bulk encryption. This specification
includes the key size of this algorithm, whether it is a block,
stream, or AEAD cipher, the block size of the cipher (if
appropriate), and the lengths of explicit and implicit
initialization vectors (or nonces).
An algorithm to be used for message authentication. This
specification includes the size of the value returned by the MAC
algorithm.
An algorithm to be used for data compression. This specification
must include all information the algorithm requires to do
compression.
A 48-byte secret shared between the two peers in the connection.
A 32-byte value provided by the client.
A 32-byte value provided by the server.These parameters are defined in the presentation language as:The record layer will use the security parameters to generate the following six
items (some of which are not required by all ciphers, and are thus empty):The client write parameters are used by the server when receiving and
processing records and vice versa. The algorithm used for generating these
items from the security parameters is described in Once the security parameters have been set and the keys have been generated,
the connection states can be instantiated by making them the current states.
These current states MUST be updated for each record processed. Each connection
state includes the following elements:
The current state of the compression algorithm.
The current state of the encryption algorithm. This will consist
of the scheduled key for that connection. For stream ciphers,
this will also contain whatever state information is necessary to
allow the stream to continue to encrypt or decrypt data.
The MAC key for this connection, as generated above.
Each connection state contains a sequence number, which is
maintained separately for read and write states. The sequence
number MUST be set to zero whenever a connection state is made the
active state. Sequence numbers are of type uint64 and may not
exceed 2^64-1. Sequence numbers do not wrap. If a TLS
implementation would need to wrap a sequence number, it must
renegotiate instead. A sequence number is incremented after each
record: specifically, the first record transmitted under a
particular connection state MUST use sequence number 0.The TLS record layer receives uninterpreted data from higher layers in
non-empty blocks of arbitrary size.The record layer fragments information blocks into TLSPlaintext records
carrying data in chunks of 2^14 bytes or less. Client message boundaries are
not preserved in the record layer (i.e., multiple client messages of the same
ContentType MAY be coalesced into a single TLSPlaintext record, or a single
message MAY be fragmented across several records).
The higher-level protocol used to process the enclosed fragment.
The version of the protocol being employed. This document
describes TLS Version 1.2, which uses the version { 3, 3 }. The
version value 3.3 is historical, deriving from the use of {3, 1}
for TLS 1.0. (See .) Note that a client that
supports multiple versions of TLS may not know what version will
be employed before it receives the ServerHello. See
for discussion about what record layer
version number should be employed for ClientHello.
The length (in bytes) of the following TLSPlaintext.fragment. The
length MUST NOT exceed 2^14.
The application data. This data is transparent and treated as an
independent block to be dealt with by the higher-level protocol
specified by the type field.Implementations MUST NOT send zero-length fragments of Handshake, Alert, or
ChangeCipherSpec content types. Zero-length fragments of Application data MAY
be sent as they are potentially useful as a traffic analysis countermeasure.Note: Data of different TLS record layer content types MAY be interleaved.
Application data is generally of lower precedence for transmission than other
content types. However, records MUST be delivered to the network in the same
order as they are protected by the record layer. Recipients MUST receive and
process interleaved application layer traffic during handshakes subsequent to
the first one on a connection.All records are compressed using the compression algorithm defined in the
current session state. There is always an active compression algorithm;
however, initially it is defined as CompressionMethod.null. The compression
algorithm translates a TLSPlaintext structure into a TLSCompressed structure.
Compression functions are initialized with default state information whenever a
connection state is made active. describes compression algorithms
for TLS.Compression must be lossless and may not increase the content length by more
than 1024 bytes. If the decompression function encounters a
TLSCompressed.fragment that would decompress to a length in excess of 2^14
bytes, it MUST report a fatal decompression failure error.
The length (in bytes) of the following TLSCompressed.fragment.
The length MUST NOT exceed 2^14 + 1024.
The compressed form of TLSPlaintext.fragment.Note: A CompressionMethod.null operation is an identity operation; no fields
are altered.Implementation note: Decompression functions are responsible for ensuring that
messages cannot cause internal buffer overflows.The encryption and MAC functions translate a TLSCompressed structure into a
TLSCiphertext. The decryption functions reverse the process. The MAC of the
record also includes a sequence number so that missing, extra, or repeated
messages are detectable.
The type field is identical to TLSCompressed.type.
The version field is identical to TLSCompressed.version.
The length (in bytes) of the following TLSCiphertext.fragment.
The length MUST NOT exceed 2^14 + 2048.
The encrypted form of TLSCompressed.fragment, with the MAC.Stream ciphers (including BulkCipherAlgorithm.null; see
) convert TLSCompressed.fragment structures to and
from stream TLSCiphertext.fragment structures.The MAC is generated as:where “+” denotes concatenation.
The sequence number for this record.
The MAC algorithm specified by SecurityParameters.mac_algorithm.Note that the MAC is computed before encryption. The stream cipher encrypts the
entire block, including the MAC. For stream ciphers that do not use a
synchronization vector (such as RC4), the stream cipher state from the end of
one record is simply used on the subsequent packet. If the cipher suite is
TLS_NULL_WITH_NULL_NULL, encryption consists of the identity operation (i.e.,
the data is not encrypted, and the MAC size is zero, implying that no MAC is
used). For both null and stream ciphers, TLSCiphertext.length is
TLSCompressed.length plus SecurityParameters.mac_length.For block ciphers (such as 3DES or AES), the encryption and MAC functions
convert TLSCompressed.fragment structures to and from block
TLSCiphertext.fragment structures.The MAC is generated as described in .
The Initialization Vector (IV) SHOULD be chosen at random, and
MUST be unpredictable. Note that in versions of TLS prior to 1.1,
there was no IV field, and the last ciphertext block of the
previous record (the “CBC residue”) was used as the IV. This was
changed to prevent the attacks described in . For block
ciphers, the IV length is of length
SecurityParameters.record_iv_length, which is equal to the
SecurityParameters.block_size.
Padding that is added to force the length of the plaintext to be
an integral multiple of the block cipher’s block length. The
padding MAY be any length up to 255 bytes, as long as it results
in the TLSCiphertext.length being an integral multiple of the
block length. Lengths longer than necessary might be desirable to
frustrate attacks on a protocol that are based on analysis of the
lengths of exchanged messages. Each uint8 in the padding data
vector MUST be filled with the padding length value. The receiver
MUST check this padding and MUST use the bad_record_mac alert to
indicate padding errors.
The padding length MUST be such that the total size of the
GenericBlockCipher structure is a multiple of the cipher’s block
length. Legal values range from zero to 255, inclusive. This
length specifies the length of the padding field exclusive of the
padding_length field itself.The encrypted data length (TLSCiphertext.length) is one more than the sum of
SecurityParameters.block_length, TLSCompressed.length,
SecurityParameters.mac_length, and padding_length.Example: If the block length is 8 bytes, the content length
(TLSCompressed.length) is 61 bytes, and the MAC length is 20 bytes, then the
length before padding is 82 bytes (this does not include the IV. Thus, the
padding length modulo 8 must be equal to 6 in order to make the total length an
even multiple of 8 bytes (the block length). The padding length can be 6, 14,
22, and so on, through 254. If the padding length were the minimum necessary,
6, the padding would be 6 bytes, each containing the value 6. Thus, the last 8
octets of the GenericBlockCipher before block encryption would be xx 06 06 06
06 06 06 06, where xx is the last octet of the MAC.Note: With block ciphers in CBC mode (Cipher Block Chaining), it is critical
that the entire plaintext of the record be known before any ciphertext is
transmitted. Otherwise, it is possible for the attacker to mount the attack
described in .Implementation note: Canvel et al. have demonstrated a timing
attack on CBC padding based on the time required to compute the MAC. In order
to defend against this attack, implementations MUST ensure that record
processing time is essentially the same whether or not the padding is correct.
In general, the best way to do this is to compute the MAC even if the padding
is incorrect, and only then reject the packet. For instance, if the pad appears
to be incorrect, the implementation might assume a zero-length pad and then
compute the MAC. This leaves a small timing channel, since MAC performance
depends to some extent on the size of the data fragment, but it is not believed
to be large enough to be exploitable, due to the large block size of existing
MACs and the small size of the timing signal.For AEAD ciphers (such as or ), the AEAD function
converts TLSCompressed.fragment structures to and from AEAD
TLSCiphertext.fragment structures.AEAD ciphers take as input a single key, a nonce, a plaintext, and “additional
data” to be included in the authentication check, as described in Section 2.1
of . The key is either the client_write_key or the server_write_key.
No MAC key is used.Each AEAD cipher suite MUST specify how the nonce supplied to the AEAD
operation is constructed, and what is the length of the
GenericAEADCipher.nonce_explicit part. In many cases, it is appropriate to use
the partially implicit nonce technique described in Section 3.2.1 of
; with record_iv_length being the length of the explicit part. In
this case, the implicit part SHOULD be derived from key_block as
client_write_iv and server_write_iv (as described in ), and
the explicit part is included in GenericAEAEDCipher.nonce_explicit.The plaintext is the TLSCompressed.fragment.The additional authenticated data, which we denote as additional_data, is
defined as follows:where “+” denotes concatenation.The aead_output consists of the ciphertext output by the AEAD encryption
operation. The length will generally be larger than TLSCompressed.length, but
by an amount that varies with the AEAD cipher. Since the ciphers might
incorporate padding, the amount of overhead could vary with different
TLSCompressed.length values. Each AEAD cipher MUST NOT produce an expansion of
greater than 1024 bytes. Symbolically,In order to decrypt and verify, the cipher takes as input the key, nonce, the
“additional_data”, and the AEADEncrypted value. The output is either the
plaintext or an error indicating that the decryption failed. There is no
separate integrity check. That is:If the decryption fails, a fatal bad_record_mac alert MUST be generated.The Record Protocol requires an algorithm to generate keys required by the
current connection state (see ) from the security
parameters provided by the handshake protocol.The master secret is expanded into a sequence of secure bytes, which is then
split to a client write MAC key, a server write MAC key, a client write
encryption key, and a server write encryption key. Each of these is generated
from the byte sequence in that order. Unused values are empty. Some AEAD
ciphers may additionally require a client write IV and a server write IV (see
).When keys and MAC keys are generated, the master secret is used as an entropy
source.To generate the key material, computeuntil enough output has been generated. Then, the key_block is partitioned as
follows:Currently, the client_write_IV and server_write_IV are only generated for
implicit nonce techniques as described in Section 3.2.1 of .Implementation note: The currently defined cipher suite which requires the most
material is AES_256_CBC_SHA256. It requires 2 x 32 byte keys and 2 x 32 byte
MAC keys, for a total 128 bytes of key material.TLS has three subprotocols that are used to allow peers to agree upon security
parameters for the record layer, to authenticate themselves, to instantiate
negotiated security parameters, and to report error conditions to each other.The Handshake Protocol is responsible for negotiating a session, which consists
of the following items:
An arbitrary byte sequence chosen by the server to identify an
active or resumable session state.
X509v3 certificate of the peer. This element of the state
may be null.
The algorithm used to compress data prior to encryption.
Specifies the pseudorandom function (PRF) used to generate keying
material, the bulk data encryption algorithm (such as null, AES,
etc.) and the MAC algorithm (such as HMAC-SHA1). It also defines
cryptographic attributes such as the mac_length. (See
for formal definition.)
48-byte secret shared between the client and server.
A flag indicating whether the session can be used to initiate new
connections.These items are then used to create security parameters for use by the record
layer when protecting application data. Many connections can be instantiated
using the same session through the resumption feature of the TLS Handshake
Protocol.The change cipher spec protocol exists to signal transitions in ciphering
strategies. The protocol consists of a single message, which is encrypted and
compressed under the current (not the pending) connection state. The message
consists of a single byte of value 1.The ChangeCipherSpec message is sent by both the client and the server to
notify the receiving party that subsequent records will be protected under the
newly negotiated CipherSpec and keys. Reception of this message causes the
receiver to instruct the record layer to immediately copy the read pending
state into the read current state. Immediately after sending this message, the
sender MUST instruct the record layer to make the write pending state the write
active state. (See .) The ChangeCipherSpec message is sent
during the handshake after the security parameters have been agreed upon, but
before the verifying Finished message is sent.Note: If a rehandshake occurs while data is flowing on a connection, the
communicating parties may continue to send data using the old CipherSpec.
However, once the ChangeCipherSpec has been sent, the new CipherSpec MUST be
used. The first side to send the ChangeCipherSpec does not know that the other
side has finished computing the new keying material (e.g., if it has to perform
a time-consuming public key operation). Thus, a small window of time, during
which the recipient must buffer the data, MAY exist. In practice, with modern
machines this interval is likely to be fairly short.One of the content types supported by the TLS record layer is the alert type.
Alert messages convey the severity of the message (warning or fatal) and a
description of the alert. Alert messages with a level of fatal result in the
immediate termination of the connection. In this case, other connections
corresponding to the session may continue, but the session identifier MUST be
invalidated, preventing the failed session from being used to establish new
connections. Like other messages, alert messages are encrypted and compressed,
as specified by the current connection state.The client and the server must share knowledge that the connection is ending in
order to avoid a truncation attack. Either party may initiate the exchange of
closing messages.
This message notifies the recipient that the sender will not send
any more messages on this connection. Note that as of TLS 1.1,
failure to properly close a connection no longer requires that a
session not be resumed. This is a change from TLS 1.0 to conform
with widespread implementation practice.Either party may initiate a close by sending a close_notify alert. Any data
received after a closure alert is ignored.Unless some other fatal alert has been transmitted, each party is required to
send a close_notify alert before closing the write side of the connection. The
other party MUST respond with a close_notify alert of its own and close down
the connection immediately, discarding any pending writes. It is not required
for the initiator of the close to wait for the responding close_notify alert
before closing the read side of the connection.If the application protocol using TLS provides that any data may be carried
over the underlying transport after the TLS connection is closed, the TLS
implementation must receive the responding close_notify alert before indicating
to the application layer that the TLS connection has ended. If the application
protocol will not transfer any additional data, but will only close the
underlying transport connection, then the implementation MAY choose to close
the transport without waiting for the responding close_notify. No part of this
standard should be taken to dictate the manner in which a usage profile for TLS
manages its data transport, including when connections are opened or closed.Note: It is assumed that closing a connection reliably delivers pending data
before destroying the transport.Error handling in the TLS Handshake protocol is very simple. When an error is
detected, the detecting party sends a message to the other party. Upon
transmission or receipt of a fatal alert message, both parties immediately
close the connection. Servers and clients MUST forget any session-identifiers,
keys, and secrets associated with a failed connection. Thus, any connection
terminated with a fatal alert MUST NOT be resumed.Whenever an implementation encounters a condition which is defined as a fatal
alert, it MUST send the appropriate alert prior to closing the connection. For
all errors where an alert level is not explicitly specified, the sending party
MAY determine at its discretion whether to treat this as a fatal error or not.
If the implementation chooses to send an alert but intends to close the
connection immediately afterwards, it MUST send that alert at the fatal alert
level.If an alert with a level of warning is sent and received, generally the
connection can continue normally. If the receiving party decides not to proceed
with the connection (e.g., after having received a no_renegotiation alert that
it is not willing to accept), it SHOULD send a fatal alert to terminate the
connection. Given this, the sending party cannot, in general, know how the
receiving party will behave. Therefore, warning alerts are not very useful when
the sending party wants to continue the connection, and thus are sometimes
omitted. For example, if a peer decides to accept an expired certificate
(perhaps after confirming this with the user) and wants to continue the
connection, it would not generally send a certificate_expired alert.The following error alerts are defined:
An inappropriate message was received. This alert is always fatal
and should never be observed in communication between proper
implementations.
This alert is returned if a record is received with an incorrect
MAC. This alert also MUST be returned if an alert is sent because
a TLSCiphertext decrypted in an invalid way: either it wasn’t an
even multiple of the block length, or its padding values, when
checked, weren’t correct. This message is always fatal and should
never be observed in communication between proper implementations
(except when messages were corrupted in the network).
This alert was used in some earlier versions of TLS, and may have
permitted certain attacks against the CBC mode . It MUST
NOT be sent by compliant implementations.
A TLSCiphertext record was received that had a length more than
2^14+2048 bytes, or a record decrypted to a TLSCompressed record
with more than 2^14+1024 bytes. This message is always fatal and
should never be observed in communication between proper
implementations (except when messages were corrupted in the
network).
The decompression function received improper input (e.g., data
that would expand to excessive length). This message is always
fatal and should never be observed in communication between proper
implementations.
Reception of a handshake_failure alert message indicates that the
sender was unable to negotiate an acceptable set of security
parameters given the options available. This is a fatal error.
This alert was used in SSLv3 but not any version of TLS. It MUST
NOT be sent by compliant implementations.
A certificate was corrupt, contained signatures that did not
verify correctly, etc.
A certificate was of an unsupported type.
A certificate was revoked by its signer.
A certificate has expired or is not currently valid.
Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.
A field in the handshake was out of range or inconsistent with
other fields. This message is always fatal.
A valid certificate chain or partial chain was received, but the
certificate was not accepted because the CA certificate could not
be located or couldn’t be matched with a known, trusted CA. This
message is always fatal.
A valid certificate was received, but when access control was
applied, the sender decided not to proceed with negotiation. This
message is always fatal.
A message could not be decoded because some field was out of the
specified range or the length of the message was incorrect. This
message is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).
A handshake cryptographic operation failed, including being unable
to correctly verify a signature or validate a Finished message.
This message is always fatal.
This alert was used in some earlier versions of TLS. It MUST NOT
be sent by compliant implementations.
The protocol version the client has attempted to negotiate is
recognized but not supported. (For example, old protocol versions
might be avoided for security reasons.) This message is always
fatal.
Returned instead of handshake_failure when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client. This message is always
fatal.
An internal error unrelated to the peer or the correctness of the
protocol (such as a memory allocation failure) makes it impossible
to continue. This message is always fatal.
This handshake is being canceled for some reason unrelated to a
protocol failure. If the user cancels an operation after the
handshake is complete, just closing the connection by sending a
close_notify is more appropriate. This alert should be followed
by a close_notify. This message is generally a warning.
Sent by the client in response to a hello request or by the server
in response to a client hello after initial handshaking. Either
of these would normally lead to renegotiation; when that is not
appropriate, the recipient should respond with this alert. At
that point, the original requester can decide whether to proceed
with the connection. One case where this would be appropriate is
where a server has spawned a process to satisfy a request; the
process might receive security parameters (key length,
authentication, etc.) at startup, and it might be difficult to
communicate changes to these parameters after that point. This
message is always a warning.
sent by clients that receive an extended server hello containing
an extension that they did not put in the corresponding client
hello. This message is always fatal.New Alert values are assigned by IANA as described in .The cryptographic parameters of the session state are produced by the TLS
Handshake Protocol, which operates on top of the TLS record layer. When a TLS
client and server first start communicating, they agree on a protocol version,
select cryptographic algorithms, optionally authenticate each other, and use
public-key encryption techniques to generate shared secrets.The TLS Handshake Protocol involves the following steps:Exchange hello messages to agree on algorithms, exchange random
values, and check for session resumption.Exchange the necessary cryptographic parameters to allow the
client and server to agree on a premaster secret.Exchange certificates and cryptographic information to allow the
client and server to authenticate themselves.Generate a master secret from the premaster secret and exchanged
random values.Provide security parameters to the record layer.Allow the client and server to verify that their peer has
calculated the same security parameters and that the handshake
occurred without tampering by an attacker.Note that higher layers should not be overly reliant on whether TLS always
negotiates the strongest possible connection between two peers. There are a
number of ways in which a man-in-the-middle attacker can attempt to make two
entities drop down to the least secure method they support. The protocol has
been designed to minimize this risk, but there are still attacks available: for
example, an attacker could block access to the port a secure service runs on,
or attempt to get the peers to negotiate an unauthenticated connection. The
fundamental rule is that higher levels must be cognizant of what their security
requirements are and never transmit information over a channel less secure than
what they require. The TLS protocol is secure in that any cipher suite offers
its promised level of security: if you negotiate 3DES with a 1024-bit RSA key
exchange with a host whose certificate you have verified, you can expect to be
that secure.These goals are achieved by the handshake protocol, which can be summarized as
follows: The client sends a ClientHello message to which the server must
respond with a ServerHello message, or else a fatal error will occur and the
connection will fail. The ClientHello and ServerHello are used to establish
security enhancement capabilities between client and server. The ClientHello
and ServerHello establish the following attributes: Protocol Version, Session
ID, Cipher Suite, and Compression Method. Additionally, two random values are
generated and exchanged: ClientHello.random and ServerHello.random.The actual key exchange uses up to four messages: the server Certificate, the
ServerKeyExchange, the client Certificate, and the ClientKeyExchange. New key
exchange methods can be created by specifying a format for these messages and
by defining the use of the messages to allow the client and server to agree
upon a shared secret. This secret MUST be quite long; currently defined key
exchange methods exchange secrets that range from 46 bytes upwards.Following the hello messages, the server will send its certificate in a
Certificate message if it is to be authenticated. Additionally, a
ServerKeyExchange message may be sent, if it is required (e.g., if the server
has no certificate, or if its certificate is for signing only). If the server
is authenticated, it may request a certificate from the client, if that is
appropriate to the cipher suite selected. Next, the server will send the
ServerHelloDone message, indicating that the hello-message phase of the
handshake is complete. The server will then wait for a client response. If the
server has sent a CertificateRequest message, the client MUST send the
Certificate message. The ClientKeyExchange message is now sent, and the content
of that message will depend on the public key algorithm selected between the
ClientHello and the ServerHello. If the client has sent a certificate with
signing ability, a digitally-signed CertificateVerify message is sent to
explicitly verify possession of the private key in the certificate.At this point, a ChangeCipherSpec message is sent by the client, and the client
copies the pending Cipher Spec into the current Cipher Spec. The client then
immediately sends the Finished message under the new algorithms, keys, and
secrets. In response, the server will send its own ChangeCipherSpec message,
transfer the pending to the current Cipher Spec, and send its Finished message
under the new Cipher Spec. At this point, the handshake is complete, and the
client and server may begin to exchange application layer data. (See flow chart
below.) Application data MUST NOT be sent prior to the completion of the first
handshake (before a cipher suite other than TLS_NULL_WITH_NULL_NULL is
established).* Indicates optional or situation-dependent messages that are not always sent.Note: To help avoid pipeline stalls, ChangeCipherSpec is an independent TLS
protocol content type, and is not actually a TLS handshake message.When the client and server decide to resume a previous session or duplicate an
existing session (instead of negotiating new security parameters), the message
flow is as follows:The client sends a ClientHello using the Session ID of the session to be
resumed. The server then checks its session cache for a match. If a match is
found, and the server is willing to re-establish the connection under the
specified session state, it will send a ServerHello with the same Session ID
value. At this point, both client and server MUST send ChangeCipherSpec
messages and proceed directly to Finished messages. Once the re-establishment
is complete, the client and server MAY begin to exchange application layer
data. (See flow chart below.) If a Session ID match is not found, the server
generates a new session ID, and the TLS client and server perform a full
handshake.The contents and significance of each message will be presented in detail in
the following sections.The TLS Handshake Protocol is one of the defined higher-level clients of the
TLS Record Protocol. This protocol is used to negotiate the secure attributes
of a session. Handshake messages are supplied to the TLS record layer, where
they are encapsulated within one or more TLSPlaintext structures, which are
processed and transmitted as specified by the current active session state.The handshake protocol messages are presented below in the order they MUST be
sent; sending handshake messages in an unexpected order results in a fatal
error. Unneeded handshake messages can be omitted, however. Note one exception
to the ordering: the Certificate message is used twice in the handshake (from
server to client, then from client to server), but described only in its first
position. The one message that is not bound by these ordering rules is the
HelloRequest message, which can be sent at any time, but which SHOULD be
ignored by the client if it arrives in the middle of a handshake.New handshake message types are assigned by IANA as described in
.The hello phase messages are used to exchange security enhancement capabilities
between the client and server. When a new session begins, the record layer’s
connection state encryption, hash, and compression algorithms are initialized
to null. The current connection state is used for renegotiation messages.When this message will be sent:The HelloRequest message MAY be sent by the server at any time.Meaning of this message:HelloRequest is a simple notification that the client should begin the
negotiation process anew. In response, the client should send a ClientHello
message when convenient. This message is not intended to establish which side
is the client or server but merely to initiate a new negotiation. Servers
SHOULD NOT send a HelloRequest immediately upon the client’s initial
connection. It is the client’s job to send a ClientHello at that time.This message will be ignored by the client if the client is currently
negotiating a session. This message MAY be ignored by the client if it does not
wish to renegotiate a session, or the client may, if it wishes, respond with a
no_renegotiation alert. Since handshake messages are intended to have
transmission precedence over application data, it is expected that the
negotiation will begin before no more than a few records are received from the
client. If the server sends a HelloRequest but does not receive a ClientHello
in response, it may close the connection with a fatal alert.After sending a HelloRequest, servers SHOULD NOT repeat the request until the
subsequent handshake negotiation is complete.Structure of this message:This message MUST NOT be included in the message hashes that are maintained
throughout the handshake and used in the Finished messages and the certificate
verify message.When this message will be sent:When a client first connects to a server, it is required to send the
ClientHello as its first message. The client can also send a ClientHello in
response to a HelloRequest or on its own initiative in order to renegotiate the
security parameters in an existing connection.Structure of this message:The ClientHello message includes a random structure, which is used later in
the protocol.
The current time and date in standard UNIX 32-bit format
(seconds since the midnight starting Jan 1, 1970, UTC, ignoring
leap seconds) according to the sender’s internal clock. Clocks
are not required to be set correctly by the basic TLS protocol;
higher-level or application protocols may define additional
requirements. Note that, for historical reasons, the data
element is named using GMT, the predecessor of the current
worldwide time base, UTC.
28 bytes generated by a secure random number generator.The ClientHello message includes a variable-length session identifier. If not
empty, the value identifies a session between the same client and server whose
security parameters the client wishes to reuse. The session identifier MAY be
from an earlier connection, this connection, or from another currently active
connection. The second option is useful if the client only wishes to update the
random structures and derived values of a connection, and the third option
makes it possible to establish several independent secure connections without
repeating the full handshake protocol. These independent connections may occur
sequentially or simultaneously; a SessionID becomes valid when the handshake
negotiating it completes with the exchange of Finished messages and persists
until it is removed due to aging or because a fatal error was encountered on a
connection associated with the session. The actual contents of the SessionID
are defined by the server.Warning: Because the SessionID is transmitted without encryption or immediate
MAC protection, servers MUST NOT place confidential information in session
identifiers or let the contents of fake session identifiers cause any breach of
security. (Note that the content of the handshake as a whole, including the
SessionID, is protected by the Finished messages exchanged at the end of the
handshake.)The cipher suite list, passed from the client to the server in the ClientHello
message, contains the combinations of cryptographic algorithms supported by the
client in order of the client’s preference (favorite choice first). Each cipher
suite defines a key exchange algorithm, a bulk encryption algorithm (including
secret key length), a MAC algorithm, and a PRF. The server will select a cipher
suite or, if no acceptable choices are presented, return a handshake failure
alert and close the connection. If the list contains cipher suites the server
does not recognize, support, or wish to use, the server MUST ignore those
cipher suites, and process the remaining ones as usual.The ClientHello includes a list of compression algorithms supported by the
client, ordered according to the client’s preference.TLS allows extensions to follow the compression_methods field in an extensions
block. The presence of extensions can be detected by determining whether there
are bytes following the compression_methods at the end of the ClientHello. Note
that this method of detecting optional data differs from the normal TLS method
of having a variable-length field, but it is used for compatibility with TLS
before extensions were defined.
The version of the TLS protocol by which the client wishes to
communicate during this session. This SHOULD be the latest
(highest valued) version supported by the client. For this
version of the specification, the version will be 3.3 (see
for details about backward compatibility).
A client-generated random structure.
The ID of a session the client wishes to use for this connection.
This field is empty if no session_id is available, or if the
client wishes to generate new security parameters.
This is a list of the cryptographic options supported by the
client, with the client’s first preference first. If the
session_id field is not empty (implying a session resumption
request), this vector MUST include at least the cipher_suite from
that session. Values are defined in .
This is a list of the compression methods supported by the client,
sorted by client preference. If the session_id field is not empty
(implying a session resumption request), it MUST include the
compression_method from that session. This vector MUST contain,
and all implementations MUST support, CompressionMethod.null.
Thus, a client and server will always be able to agree on a
compression method.
Clients MAY request extended functionality from servers by sending
data in the extensions field. The actual “Extension” format is
defined in .In the event that a client requests additional functionality using extensions,
and this functionality is not supplied by the server, the client MAY abort the
handshake. A server MUST accept ClientHello messages both with and without the
extensions field, and (as for all other messages) it MUST check that the amount
of data in the message precisely matches one of these formats; if not, then it
MUST send a fatal “decode_error” alert.After sending the ClientHello message, the client waits for a ServerHello
message. Any handshake message returned by the server, except for a
HelloRequest, is treated as a fatal error.When this message will be sent:The server will send this message in response to a ClientHello message when
it was able to find an acceptable set of algorithms. If it cannot find such a
match, it will respond with a handshake failure alert.Structure of this message:The presence of extensions can be detected by determining whether there are
bytes following the compression_method field at the end of the ServerHello.
This field will contain the lower of that suggested by the client
in the client hello and the highest supported by the server. For
this version of the specification, the version is 3.3. (See
for details about backward compatibility.)
This structure is generated by the server and MUST be
independently generated from the ClientHello.random.
This is the identity of the session corresponding to this
connection. If the ClientHello.session_id was non-empty, the
server will look in its session cache for a match. If a match is
found and the server is willing to establish the new connection
using the specified session state, the server will respond with
the same value as was supplied by the client. This indicates a
resumed session and dictates that the parties must proceed
directly to the Finished messages. Otherwise, this field will
contain a different value identifying the new session. The server
may return an empty session_id to indicate that the session will
not be cached and therefore cannot be resumed. If a session is
resumed, it must be resumed using the same cipher suite it was
originally negotiated with. Note that there is no requirement
that the server resume any session even if it had formerly
provided a session_id. Clients MUST be prepared to do a full
negotiation — including negotiating new cipher suites — during
any handshake.
The single cipher suite selected by the server from the list in
ClientHello.cipher_suites. For resumed sessions, this field is
the value from the state of the session being resumed.
The single compression algorithm selected by the server from the
list in ClientHello.compression_methods. For resumed sessions,
this field is the value from the resumed session state.
A list of extensions. Note that only extensions offered by the
client can appear in the server’s list.The extension format is:Here:“extension_type” identifies the particular extension type.“extension_data” contains information specific to the particular
extension type.The initial set of extensions is defined in a companion document .
The list of extension types is maintained by IANA as described in
.An extension type MUST NOT appear in the ServerHello unless the same extension
type appeared in the corresponding ClientHello. If a client receives an
extension type in ServerHello that it did not request in the associated
ClientHello, it MUST abort the handshake with an unsupported_extension fatal
alert.Nonetheless, “server-oriented” extensions may be provided in the future within
this framework. Such an extension (say, of type x) would require the client to
first send an extension of type x in a ClientHello with empty extension_data to
indicate that it supports the extension type. In this case, the client is
offering the capability to understand the extension type, and the server is
taking the client up on its offer.When multiple extensions of different types are present in the ClientHello or
ServerHello messages, the extensions MAY appear in any order. There MUST NOT be
more than one extension of the same type.Finally, note that extensions can be sent both when starting a new session and
when requesting session resumption. Indeed, a client that requests session
resumption does not in general know whether the server will accept this
request, and therefore it SHOULD send the same extensions as it would send if
it were not attempting resumption.In general, the specification of each extension type needs to describe the
effect of the extension both during full handshake and session resumption. Most
current TLS extensions are relevant only when a session is initiated: when an
older session is resumed, the server does not process these extensions in
Client Hello, and does not include them in Server Hello. However, some
extensions may specify different behavior during session resumption.There are subtle (and not so subtle) interactions that may occur in this
protocol between new features and existing features which may result in a
significant reduction in overall security. The following considerations should
be taken into account when designing new extensions:Some cases where a server does not agree to an extension are error
conditions, and some are simply refusals to support particular features. In
general, error alerts should be used for the former, and a field in the
server extension response for the latter.Extensions should, as far as possible, be designed to prevent any attack that
forces use (or non-use) of a particular feature by manipulation of handshake
messages. This principle should be followed regardless of whether the feature
is believed to cause a security problem.
Often the fact that the extension fields are included in the inputs to the
Finished message hashes will be sufficient, but extreme care is needed when
the extension changes the meaning of messages sent in the handshake phase.
Designers and implementors should be aware of the fact that until the
handshake has been authenticated, active attackers can modify messages and
insert, remove, or replace extensions.It would be technically possible to use extensions to change major aspects
of the design of TLS; for example the design of cipher suite negotiation.
This is not recommended; it would be more appropriate to define a new version
of TLS — particularly since the TLS handshake algorithms have specific
protection against version rollback attacks based on the version number, and
the possibility of version rollback should be a significant consideration in
any major design change.The client uses the “signature_algorithms” extension to indicate to the server
which signature/hash algorithm pairs may be used in digital signatures. The
“extension_data” field of this extension contains a
“supported_signature_algorithms” value.Each SignatureAndHashAlgorithm value lists a single hash/signature pair that
the client is willing to verify. The values are indicated in descending order
of preference.Note: Because not all signature algorithms and hash algorithms may be accepted
by an implementation (e.g., DSA with SHA-1, but not SHA-256), algorithms here
are listed in pairs.
This field indicates the hash algorithm which may be used. The
values indicate support for unhashed data, MD5 , SHA-1,
SHA-224, SHA-256, SHA-384, and SHA-512 , respectively. The
“none” value is provided for future extensibility, in case of a
signature algorithm which does not require hashing before signing.
This field indicates the signature algorithm that may be used.
The values indicate anonymous signatures, RSASSA-PKCS1-v1_5
and DSA , and ECDSA , respectively. The
“anonymous” value is meaningless in this context but used in
. It MUST NOT appear in this extension.The semantics of this extension are somewhat complicated because the cipher
suite indicates permissible signature algorithms but not hash algorithms.
and describe the
appropriate rules.If the client supports only the default hash and signature algorithms (listed
in this section), it MAY omit the signature_algorithms extension. If the client
does not support the default algorithms, or supports other hash and signature
algorithms (and it is willing to use them for verifying messages sent by the
server, i.e., server certificates and server key exchange), it MUST send the
signature_algorithms extension, listing the algorithms it is willing to accept.If the client does not send the signature_algorithms extension, the server MUST
do the following:If the negotiated key exchange algorithm is one of (RSA, DHE_RSA, DH_RSA,
RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had sent the value
{sha1,rsa}.If the negotiated key exchange algorithm is one of (DHE_DSS, DH_DSS), behave
as if the client had sent the value {sha1,dsa}.If the negotiated key exchange algorithm is one of (ECDH_ECDSA, ECDHE_ECDSA),
behave as if the client had sent value {sha1,ecdsa}.Note: this is a change from TLS 1.1 where there are no explicit rules, but as a
practical matter one can assume that the peer supports MD5 and SHA-1.Note: this extension is not meaningful for TLS versions prior to 1.2. Clients
MUST NOT offer it if they are offering prior versions. However, even if clients
do offer it, the rules specified in require servers to ignore
extensions they do not understand.Servers MUST NOT send this extension. TLS servers MUST support receiving this
extension.When performing session resumption, this extension is not included in Server
Hello, and the server ignores the extension in Client Hello (if present).When this message will be sent:The server MUST send a Certificate message whenever the agreed- upon key
exchange method uses certificates for authentication (this includes all key
exchange methods defined in this document except DH_anon). This message will
always immediately follow the ServerHello message.Meaning of this message:This message conveys the server’s certificate chain to the client.The certificate MUST be appropriate for the negotiated cipher suite’s key
exchange algorithm and any negotiated extensions.Structure of this message:
This is a sequence (chain) of certificates. The sender’s
certificate MUST come first in the list. Each following
certificate MUST directly certify the one preceding it. Because
certificate validation requires that root keys be distributed
independently, the self-signed certificate that specifies the root
certificate authority MAY be omitted from the chain, under the
assumption that the remote end must already possess it in order to
validate it in any case.The same message type and structure will be used for the client’s response to a
certificate request message. Note that a client MAY send no certificates if it
does not have an appropriate certificate to send in response to the server’s
authentication request.Note: PKCS #7 is not used as the format for the certificate vector
because PKCS #6 extended certificates are not used. Also, PKCS #7
defines a SET rather than a SEQUENCE, making the task of parsing the list more
difficult.The following rules apply to the certificates sent by the server:The certificate type MUST be X.509v3, unless explicitly negotiated
otherwise (e.g., ).The end entity certificate’s public key (and associated
restrictions) MUST be compatible with the selected key exchange
algorithm.The “server_name” and “trusted_ca_keys” extensions are used to
guide certificate selection.If the client provided a “signature_algorithms” extension, then all
certificates provided by the server MUST be signed by a hash/signature
algorithm pair that appears in that extension. Note that this implies that a
certificate containing a key for one signature algorithm MAY be signed using a
different signature algorithm (for instance, an RSA key signed with a DSA key).
This is a departure from TLS 1.1, which required that the algorithms be the
same. Note that this also implies that the DH_DSS, DH_RSA, ECDH_ECDSA, and
ECDH_RSA key exchange algorithms do not restrict the algorithm used to sign the
certificate. Fixed DH certificates MAY be signed with any hash/signature
algorithm pair appearing in the extension. The names DH_DSS, DH_RSA,
ECDH_ECDSA, and ECDH_RSA are historical.If the server has multiple certificates, it chooses one of them based on the
above-mentioned criteria (in addition to other criteria, such as transport
layer endpoint, local configuration and preferences, etc.). If the server has a
single certificate, it SHOULD attempt to validate that it meets these criteria.Note that there are certificates that use algorithms and/or algorithm
combinations that cannot be currently used with TLS. For example, a certificate
with RSASSA-PSS signature key (id-RSASSA-PSS OID in SubjectPublicKeyInfo)
cannot be used because TLS defines no corresponding signature algorithm.As cipher suites that specify new key exchange methods are specified for the
TLS protocol, they will imply the certificate format and the required encoded
keying information.When this message will be sent:This message will be sent immediately after the server Certificate message (or
the ServerHello message, if this is an anonymous negotiation).The ServerKeyExchange message is sent by the server only when the server
Certificate message (if sent) does not contain enough data to allow the client
to exchange a premaster secret. This is true for the following key exchange
methods:It is not legal to send the ServerKeyExchange message for the following key
exchange methods:Other key exchange algorithms, such as those defined in , MUST
specify whether the ServerKeyExchange message is sent or not; and if the
message is sent, its contents.Meaning of this message:This message conveys cryptographic information to allow the client to
communicate the premaster secret: a Diffie-Hellman public key with which the
client can complete a key exchange (with the result being the premaster secret)
or a public key for some other algorithm.Structure of this message:
The server’s key exchange parameters.
For non-anonymous key exchanges, a signature over the server’s
key exchange parameters.If the client has offered the “signature_algorithms” extension, the signature
algorithm and hash algorithm MUST be a pair listed in that extension. Note that
there is a possibility for inconsistencies here. For instance, the client might
offer DHE_DSS key exchange but omit any DSA pairs from its
“signature_algorithms” extension. In order to negotiate correctly, the server
MUST check any candidate cipher suites against the “signature_algorithms”
extension before selecting them. This is somewhat inelegant but is a compromise
designed to minimize changes to the original cipher suite design.In addition, the hash and signature algorithms MUST be compatible with the key
in the server’s end-entity certificate. RSA keys MAY be used with any permitted
hash algorithm, subject to restrictions in the certificate, if any.Because DSA signatures do not contain any secure indication of hash algorithm,
there is a risk of hash substitution if multiple hashes may be used with any
key. Currently, DSA may only be used with SHA-1. Future revisions of
DSS are expected to allow the use of other digest algorithms with
DSA, as well as guidance as to which digest algorithms should be used with each
key size. In addition, future revisions of may specify mechanisms
for certificates to indicate which digest algorithms are to be used with DSA.As additional cipher suites are defined for TLS that include new key exchange
algorithms, the server key exchange message will be sent if and only if the
certificate type associated with the key exchange algorithm does not provide
enough information for the client to exchange a premaster secret.When this message will be sent:A non-anonymous server can optionally request a certificate from the client,
if appropriate for the selected cipher suite. This message, if sent, will
immediately follow the ServerKeyExchange message (if it is sent; otherwise,
this message follows the server’s Certificate message).Structure of this message:
A list of the types of certificate types that the client may
offer.
A list of the hash/signature algorithm pairs that the server is
able to verify, listed in descending order of preference.
A list of the distinguished names of acceptable
certificate_authorities, represented in DER-encoded format. These
distinguished names may specify a desired distinguished name for a
root CA or for a subordinate CA; thus, this message can be used to
describe known roots as well as a desired authorization space. If
the certificate_authorities list is empty, then the client MAY
send any certificate of the appropriate ClientCertificateType,
unless there is some external arrangement to the contrary.The interaction of the certificate_types and
supported_signature_algorithms fields is somewhat complicated.
certificate_types has been present in TLS since SSLv3, but was
somewhat underspecified. Much of its functionality is superseded by
supported_signature_algorithms. The following rules apply:Any certificates provided by the client MUST be signed using a
hash/signature algorithm pair found in
supported_signature_algorithms.The end-entity certificate provided by the client MUST contain a
key that is compatible with certificate_types. If the key is a
signature key, it MUST be usable with some hash/signature
algorithm pair in supported_signature_algorithms.For historical reasons, the names of some client certificate types
include the algorithm used to sign the certificate. For example,
in earlier versions of TLS, rsa_fixed_dh meant a certificate
signed with RSA and containing a static DH key. In TLS 1.2, this
functionality has been obsoleted by the
supported_signature_algorithms, and the certificate type no longer
restricts the algorithm used to sign the certificate. For
example, if the server sends dss_fixed_dh certificate type and
{{sha1, dsa}, {sha1, rsa}} signature types, the client MAY reply
with a certificate containing a static DH key, signed with RSA-
SHA1.New ClientCertificateType values are assigned by IANA as described in
.Note: Values listed as RESERVED may not be used. They were used in SSLv3.Note: It is a fatal handshake_failure alert for an anonymous server to request
client authentication.When this message will be sent:The ServerHelloDone message is sent by the server to indicate the end of the
ServerHello and associated messages. After sending this message, the server
will wait for a client response.Meaning of this message:This message means that the server is done sending messages to support the
key exchange, and the client can proceed with its phase of the key exchange.Upon receipt of the ServerHelloDone message, the client SHOULD verify that
the server provided a valid certificate, if required, and check that the server
hello parameters are acceptable.Structure of this message:When this message will be sent:This is the first message the client can send after receiving a
ServerHelloDone message. This message is only sent if the server requests a
certificate. If no suitable certificate is available, the client MUST send a
certificate message containing no certificates. That is, the certificate_list
structure has a length of zero. If the client does not send any certificates,
the server MAY at its discretion either continue the handshake without client
authentication, or respond with a fatal handshake_failure alert. Also, if some
aspect of the certificate chain was unacceptable (e.g., it was not signed by a
known, trusted CA), the server MAY at its discretion either continue the
handshake (considering the client unauthenticated) or send a fatal alert.Client certificates are sent using the Certificate structure defined in
.Meaning of this message:This message conveys the client’s certificate chain to the server; the server
will use it when verifying the CertificateVerify message (when the client
authentication is based on signing) or calculating the premaster secret (for
non-ephemeral Diffie- Hellman). The certificate MUST be appropriate for the
negotiated cipher suite’s key exchange algorithm, and any negotiated extensions.In particular:The certificate type MUST be X.509v3, unless explicitly negotiated
otherwise (e.g., ).The end-entity certificate’s public key (and associated
restrictions) has to be compatible with the certificate types
listed in CertificateRequest: If the certificate_authorities list in the certificate request
message was non-empty, one of the certificates in the certificate
chain SHOULD be issued by one of the listed CAs.The certificates MUST be signed using an acceptable hash/
signature algorithm pair, as described in . Note
that this relaxes the constraints on certificate-signing
algorithms found in prior versions of TLS.Note that, as with the server certificate, there are certificates that use
algorithms/algorithm combinations that cannot be currently used with TLS.When this message will be sent:This message is always sent by the client. It MUST immediately follow the
client certificate message, if it is sent. Otherwise, it MUST be the first
message sent by the client after it receives the ServerHelloDone message.Meaning of this message:With this message, the premaster secret is set, either by direct transmission
of the RSA-encrypted secret or by the transmission of Diffie-Hellman parameters
that will allow each side to agree upon the same premaster secret.When the client is using an ephemeral Diffie-Hellman exponent, then this
message contains the client’s Diffie-Hellman public value. If the client is
sending a certificate containing a static DH exponent (i.e., it is doing
fixed_dh client authentication), then this message MUST be sent but MUST be
empty.Structure of this message:The choice of messages depends on which key exchange method has been
selected. See for the KeyExchangeAlgorithm
definition.Meaning of this message:If RSA is being used for key agreement and authentication, the client
generates a 48-byte premaster secret, encrypts it using the public key from the
server’s certificate, and sends the result in an encrypted premaster secret
message. This structure is a variant of the ClientKeyExchange message and is
not a message in itself.Structure of this message:Note: The version number in the PreMasterSecret is the version offered by the
client in the ClientHello.client_version, not the version negotiated for the
connection. This feature is designed to prevent rollback attacks.
Unfortunately, some old implementations use the negotiated version instead, and
therefore checking the version number may lead to failure to interoperate with
such incorrect client implementations.Client implementations MUST always send the correct version number in
PreMasterSecret. If ClientHello.client_version is TLS 1.1 or higher, server
implementations MUST check the version number as described in the note below.
If the version number is TLS 1.0 or earlier, server implementations SHOULD
check the version number, but MAY have a configuration option to disable the
check. Note that if the check fails, the PreMasterSecret SHOULD be randomized
as described below.Note: Attacks discovered by Bleichenbacher and Klima et al.
can be used to attack a TLS server that reveals whether a particular message,
when decrypted, is properly PKCS#1 formatted, contains a valid PreMasterSecret
structure, or has the correct version number.As described by Klima , these vulnerabilities can be avoided by
treating incorrectly formatted message blocks and/or mismatched version numbers
in a manner indistinguishable from correctly formatted RSA blocks. In other
words:Generate a string R of 46 random bytesDecrypt the message to recover the plaintext MIf the PKCS#1 padding is not correct, or the length of message
M is not exactly 48 bytes:
else If ClientHello.client_version <= TLS 1.0, and version
number check is explicitly disabled:
else: Note that explicitly constructing the pre_master_secret with the
ClientHello.client_version produces an invalid master_secret if the client has
sent the wrong version in the original pre_master_secret.An alternative approach is to treat a version number mismatch as a PKCS-1
formatting error and randomize the premaster secret completely:Generate a string R of 48 random bytesDecrypt the message to recover the plaintext MIf the PKCS#1 padding is not correct, or the length of message
M is not exactly 48 bytes:
else If ClientHello.client_version <= TLS 1.0, and version
number check is explicitly disabled:
else If M[0..1] != ClientHello.client_version:
else: Although no practical attacks against this construction are known, Klima et al.
describe some theoretical attacks, and therefore the first
construction described is RECOMMENDED.In any case, a TLS server MUST NOT generate an alert if processing an
RSA-encrypted premaster secret message fails, or the version number is not as
expected. Instead, it MUST continue the handshake with a randomly generated
premaster secret. It may be useful to log the real cause of failure for
troubleshooting purposes; however, care must be taken to avoid leaking the
information to an attacker (through, e.g., timing, log files, or other
channels.)The RSAES-OAEP encryption scheme defined in is more secure against
the Bleichenbacher attack. However, for maximal compatibility with earlier
versions of TLS, this specification uses the RSAES-PKCS1-v1_5 scheme. No
variants of the Bleichenbacher attack are known to exist provided that the
above recommendations are followed.Implementation note: Public-key-encrypted data is represented as an opaque
vector <0..2^16-1> (see ). Thus, the RSA-encrypted
PreMasterSecret in a ClientKeyExchange is preceded by two length bytes. These
bytes are redundant in the case of RSA because the EncryptedPreMasterSecret is
the only data in the ClientKeyExchange and its length can therefore be
unambiguously determined. The SSLv3 specification was not clear about the
encoding of public-key- encrypted data, and therefore many SSLv3
implementations do not include the length bytes — they encode the
RSA-encrypted data directly in the ClientKeyExchange message.This specification requires correct encoding of the EncryptedPreMasterSecret
complete with length bytes. The resulting PDU is incompatible with many SSLv3
implementations. Implementors upgrading from SSLv3 MUST modify their
implementations to generate and accept the correct encoding. Implementors who
wish to be compatible with both SSLv3 and TLS should make their
implementation’s behavior dependent on the protocol version.Implementation note: It is now known that remote timing-based attacks on TLS
are possible, at least when the client and server are on the same LAN.
Accordingly, implementations that use static RSA keys MUST use RSA blinding or
some other anti-timing technique, as described in .Meaning of this message:This structure conveys the client’s Diffie-Hellman public value (Yc) if it
was not already included in the client’s certificate. The encoding used for Yc
is determined by the enumerated PublicValueEncoding. This structure is a
variant of the client key exchange message, and not a message in itself.Structure of this message:
The client’s Diffie-Hellman public value (Yc).When this message will be sent:This message is used to provide explicit verification of a client
certificate. This message is only sent following a client certificate that has
signing capability (i.e., all certificates except those containing fixed
Diffie-Hellman parameters). When sent, it MUST immediately follow the client
key exchange message.Structure of this message:Here handshake_messages refers to all handshake messages sent or received,
starting at client hello and up to, but not including, this message, including
the type and length fields of the handshake messages. This is the concatenation
of all the Handshake structures (as defined in )
exchanged thus far. Note that this requires both sides to either buffer the
messages or compute running hashes for all potential hash algorithms up to the
time of the CertificateVerify computation. Servers can minimize this
computation cost by offering a restricted set of digest algorithms in the
CertificateRequest message.The hash and signature algorithms used in the signature MUST be one of those
present in the supported_signature_algorithms field of the CertificateRequest
message. In addition, the hash and signature algorithms MUST be compatible with
the key in the client’s end-entity certificate. RSA keys MAY be used with any
permitted hash algorithm, subject to restrictions in the certificate, if any.Because DSA signatures do not contain any secure indication of hash
algorithm, there is a risk of hash substitution if multiple hashes may be used
with any key. Currently, DSA may only be used with SHA-1. Future
revisions of DSS are expected to allow the use of other digest
algorithms with DSA, as well as guidance as to which digest algorithms should
be used with each key size. In addition, future revisions of may
specify mechanisms for certificates to indicate which digest algorithms are to
be used with DSA.When this message will be sent:A Finished message is always sent immediately after a change cipher spec
message to verify that the key exchange and authentication processes were
successful. It is essential that a change cipher spec message be received
between the other handshake messages and the Finished message.Meaning of this message:The Finished message is the first one protected with the just negotiated
algorithms, keys, and secrets. Recipients of Finished messages MUST verify that
the contents are correct. Once a side has sent its Finished message and
received and validated the Finished message from its peer, it may begin to send
and receive application data over the connection.Structure of this message:Hash denotes a Hash of the handshake messages. For the PRF defined in
, the Hash MUST be the Hash used as the basis for the PRF. Any cipher
suite which defines a different PRF MUST also define the Hash to use in the
Finished computation.In previous versions of TLS, the verify_data was always 12 octets long. In
the current version of TLS, it depends on the cipher suite. Any cipher suite
which does not explicitly specify verify_data_length has a verify_data_length
equal to 12. This includes all existing cipher suites. Note that this
representation has the same encoding as with previous versions. Future cipher
suites MAY specify other lengths but such length MUST be at least 12 bytes.
All of the data from all messages in this handshake (not
including any HelloRequest messages) up to, but not including,
this message. This is only data visible at the handshake layer
and does not include record layer headers. This is the
concatenation of all the Handshake structures as defined in
, exchanged thus far.It is a fatal error if a Finished message is not preceded by a ChangeCipherSpec
message at the appropriate point in the handshake.The value handshake_messages includes all handshake messages starting at
ClientHello up to, but not including, this Finished message. This may be
different from handshake_messages in because it would
include the CertificateVerify message (if sent). Also, the handshake_messages
for the Finished message sent by the client will be different from that for the
Finished message sent by the server, because the one that is sent second will
include the prior one.Note: ChangeCipherSpec messages, alerts, and any other record types are not
handshake messages and are not included in the hash computations. Also,
HelloRequest messages are omitted from handshake hashes.In order to begin connection protection, the TLS Record Protocol requires
specification of a suite of algorithms, a master secret, and the client and
server random values. The authentication, encryption, and MAC algorithms are
determined by the cipher_suite selected by the server and revealed in the
ServerHello message. The compression algorithm is negotiated in the hello
messages, and the random values are exchanged in the hello messages. All that
remains is to calculate the master secret.For all key exchange methods, the same algorithm is used to convert the
pre_master_secret into the master_secret. The pre_master_secret should be
deleted from memory once the master_secret has been computed.The master secret is always exactly 48 bytes in length. The length of the
premaster secret will vary depending on key exchange method.When RSA is used for server authentication and key exchange, a 48- byte
pre_master_secret is generated by the client, encrypted under the server’s
public key, and sent to the server. The server uses its private key to decrypt
the pre_master_secret. Both parties then convert the pre_master_secret into the
master_secret, as specified above.A conventional Diffie-Hellman computation is performed. The negotiated key (Z)
is used as the pre_master_secret, and is converted into the master_secret, as
specified above. Leading bytes of Z that contain all zero bits are stripped
before it is used as the pre_master_secret.Note: Diffie-Hellman parameters are specified by the server and may be either
ephemeral or contained within the server’s certificate.In the absence of an application profile standard specifying otherwise, a
TLS-compliant application MUST implement the cipher suite
TLS_RSA_WITH_AES_128_CBC_SHA (see for the definition).Application data messages are carried by the record layer and are fragmented,
compressed, and encrypted based on the current connection state. The messages
are treated as transparent data to the record layer.Security issues are discussed throughout this memo, especially in Appendices D,
E, and F.This document uses several registries that were originally created in
. IANA has updated these to reference this document. The registries
and their allocation policies (unchanged from ) are listed below.TLS ClientCertificateType Identifiers Registry: Future values in
the range 0-63 (decimal) inclusive are assigned via Standards
Action . Values in the range 64-223 (decimal) inclusive
are assigned via Specification Required . Values from
224-255 (decimal) inclusive are reserved for Private Use
.TLS Cipher Suite Registry: Future values with the first byte in
the range 0-191 (decimal) inclusive are assigned via Standards
Action . Values with the first byte in the range 192-254
(decimal) are assigned via Specification Required .
Values with the first byte 255 (decimal) are reserved for Private
Use .This document defines several new HMAC-SHA256-based cipher suites,
whose values (in ) have been allocated from the TLS
Cipher Suite registry.TLS ContentType Registry: Future values are allocated via
Standards Action .TLS Alert Registry: Future values are allocated via Standards
Action .TLS HandshakeType Registry: Future values are allocated via
Standards Action .This document also uses a registry originally created in . IANA has
updated it to reference this document. The registry and its allocation policy
(unchanged from ) is listed below:TLS ExtensionType Registry: Future values are allocated via IETF
Consensus . IANA has updated this registry to include
the signature_algorithms extension and its corresponding value
(see ).In addition, this document defines two new registries to be maintained by IANA:TLS SignatureAlgorithm Registry: The registry has been initially
populated with the values described in . Future
values in the range 0-63 (decimal) inclusive are assigned via
Standards Action . Values in the range 64-223 (decimal)
inclusive are assigned via Specification Required .
Values from 224-255 (decimal) inclusive are reserved for Private
Use .TLS HashAlgorithm Registry: The registry has been initially
populated with the values described in . Future
values in the range 0-63 (decimal) inclusive are assigned via
Standards Action . Values in the range 64-223 (decimal)
inclusive are assigned via Specification Required .
Values from 224-255 (decimal) inclusive are reserved for Private
Use .This document also uses the TLS Compression Method Identifiers Registry,
defined in . IANA has allocated value 0 for the “null” compression
method.HMAC: Keyed-Hashing for Message AuthenticationIBM, T.J. Watson Research CenterP.O.Box 704Yorktown HeightsNY10598UShugo@watson.ibm.comUniversity of California at San Diego, Dept of Computer Science and Engineering9500 Gilman DriveMail Code 0114La JollaCA92093USmihir@cs.ucsd.eduIBM T.J. Watson Research CenterP.O.Box 704Yorktown HeightsNY10598UScanetti@watson.ibm.comThis document describes HMAC, a mechanism for message authentication using cryptographic hash functions. HMAC can be used with any iterative cryptographic hash function, e.g., MD5, SHA-1, in combination with a secret shared key. The cryptographic strength of HMAC depends on the properties of the underlying hash function.Key words for use in RFCs to Indicate Requirement LevelsHarvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864sob@harvard.edu
General
keyword
In many standards track documents several words are used to signify
the requirements in the specification. These words are often
capitalized. This document defines these words as they should be
interpreted in IETF documents. Authors who follow these guidelines
should incorporate this phrase near the beginning of their document:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
Note that the force of these words is modified by the requirement
level of the document in which they are used.
Guidelines for Writing an IANA Considerations Section in RFCsIBM Corporation3039 Cornwallis Ave.PO Box 12195 - BRQA/502Research Triangle ParkNC 27709-2195919-254-7798narten@raleigh.ibm.comMaxwarePirsenteretN-7005 TrondheimNorway+47 73 54 57 97Harald@Alvestrand.no
General
Internet Assigned Numbers AuthorityIANA
Many protocols make use of identifiers consisting of constants and
other well-known values. Even after a protocol has been defined and
deployment has begun, new values may need to be assigned (e.g., for a
new option type in DHCP, or a new encryption or authentication
algorithm for IPSec). To insure that such quantities have consistent
values and interpretations in different implementations, their
assignment must be administered by a central authority. For IETF
protocols, that role is provided by the Internet Assigned Numbers
Authority (IANA).
In order for the IANA to manage a given name space prudently, it
needs guidelines describing the conditions under which new values can
be assigned. If the IANA is expected to play a role in the management
of a name space, the IANA must be given clear and concise
instructions describing that role. This document discusses issues
that should be considered in formulating a policy for assigning
values to a name space and provides guidelines to document authors on
the specific text that must be included in documents that place
demands on the IANA.
The MD5 Message-Digest AlgorithmMassachusetts Institute of Technology, (MIT) Laboratory for Computer Science545 Technology SquareNE43-324CambridgeMA02139-1986US+1 617 253 5880rivest@theory.lcs.mit.eduPublic-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1This memo represents a republication of PKCS #1 v2.1 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document is taken directly from the PKCS #1 v2.1 document, with certain corrections made during the publication process. This memo provides information for the Internet community.Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) ProfileThis memo profiles the X.509 v3 certificate and X.509 v2 Certificate Revocation List (CRL) for use in the Internet. [STANDARDS-TRACK]Specification for the Advanced Encryption Standard (AES)National Institute of Standards and TechnologyRecommendation for the Triple Data Encryption Algorithm (TDEA) Block CipherNational Institute of Standards and TechnologyDigital Signature StandardNational Institute of Standards and Technology, U.S. Department of CommerceApplied Cryptography: Protocols, Algorithms, and Source Code in C, 2nd ed.Secure Hash StandardNational Institute of Standards and Technology, U.S. Department of CommerceInformation technology - Abstract Syntax Notation One (ASN.1): Specification of basic notationITU-TInformation technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)ITU-TTransmission Control ProtocolUniversity of Southern California (USC)/Information Sciences Institute4676 Admiralty WayMarina del ReyCA90291USDefending Against Sequence Number AttacksAT&T Research600 Mountain AvenueMurray HillNJ07974US+1 908 582 5886smb@research.att.comIP spoofing attacks based on sequence number spoofing have become a serious threat on the Internet (CERT Advisory CA-95:01). While ubiquitous crypgraphic authentication is the right answer, we propose a simple modification to TCP implementations that should be a very substantial block to the current wave of attacks.The TLS Protocol Version 1.0Certicomtdierks@certicom.comCerticomcallen@certicom.comThis document specifies Version 1.0 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications privacy over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery.Methods for Avoiding the "Small-Subgroup" Attacks on the Diffie-Hellman Key Agreement Method for S/MIMEEntrust Technologies750 Heron RoadOttawaOntarioK1V 1A7CArobert.zuccherato@entrust.comIn some circumstances the use of the Diffie-Hellman key agreement scheme in a prime order subgroup of a large prime p is vulnerable to certain attacks known as "small-subgroup" attacks. Methods exist, however, to prevent these attacks. This document will describe the situations relevant to implementations of S/MIME version 3 in which protection is necessary and the methods that can be used to prevent these attacks.Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)This document proposes several new ciphersuites. At present, the symmetric ciphers supported by Transport Layer Security (TLS) are RC2, RC4, International Data Encryption Algorithm (IDEA), Data Encryption Standard (DES), and triple DES. The protocol would be enhanced by the addition of Advanced Encryption Standard (AES) ciphersuites. [STANDARDS-TRACK]More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)This document defines new Modular Exponential (MODP) Groups for the Internet Key Exchange (IKE) protocol. It documents the well known and used 1536 bit group 5, and also defines new 2048, 3072, 4096, 6144, and 8192 bit Diffie-Hellman groups numbered starting at 14. The selection of the primes for theses groups follows the criteria established by Richard Schroeppel. [STANDARDS-TRACK]Transport Layer Security Protocol Compression MethodsThe Transport Layer Security (TLS) protocol (RFC 2246) includes features to negotiate selection of a lossless data compression method as part of the TLS Handshake Protocol and to then apply the algorithm associated with the selected method as part of the TLS Record Protocol. TLS defines one standard compression method which specifies that data exchanged via the record protocol will not be compressed. This document describes an additional compression method associated with a lossless data compression algorithm for use with TLS, and it describes a method for the specification of additional TLS compression methods. [STANDARDS-TRACK]Determining Strengths For Public Keys Used For Exchanging Symmetric KeysImplementors of systems that use public key cryptography to exchange symmetric keys need to make the public keys resistant to some predetermined level of attack. That level of attack resistance is the strength of the system, and the symmetric keys that are exchanged must be at least as strong as the system strength requirements. The three quantities, system strength, symmetric key strength, and public key strength, must be consistently matched for any network protocol usage. While it is fairly easy to express the system strength requirements in terms of a symmetric key length and to choose a cipher that has a key length equal to or exceeding that requirement, it is harder to choose a public key that has a cryptographic strength meeting a symmetric key strength requirement. This document explains how to determine the length of an asymmetric key as a function of a symmetric key strength requirement. Some rules of thumb for estimating equivalent resistance to large-scale attacks on various algorithms are given. The document also addresses how changing the sizes of the underlying large integers (moduli, group sizes, exponents, and so on) changes the time to use the algorithms for key exchange. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Randomness Requirements for SecuritySecurity systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space.</t><t> Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)This document specifies three sets of new ciphersuites for the Transport Layer Security (TLS) protocol to support authentication based on pre-shared keys (PSKs). These pre-shared keys are symmetric keys, shared in advance among the communicating parties. The first set of ciphersuites uses only symmetric key operations for authentication. The second set uses a Diffie-Hellman exchange authenticated with a pre-shared key, and the third set combines public key authentication of the server with pre-shared key authentication of the client. [STANDARDS-TRACK]IP Authentication HeaderThis document describes an updated version of the IP Authentication Header (AH), which is designed to provide authentication services in IPv4 and IPv6. This document obsoletes RFC 2402 (November 1998). [STANDARDS-TRACK]IP Encapsulating Security Payload (ESP)This document describes an updated version of the Encapsulating Security Payload (ESP) protocol, which is designed to provide a mix of security services in IPv4 and IPv6. ESP is used to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality. This document obsoletes RFC 2406 (November 1998). [STANDARDS-TRACK]Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2)The IPsec series of protocols makes use of various cryptographic algorithms in order to provide security services. The Internet Key Exchange (IKE (RFC 2409) and IKEv2) provide a mechanism to negotiate which algorithms should be used in any given association. However, to ensure interoperability between disparate implementations, it is necessary to specify a set of mandatory-to-implement algorithms to ensure that there is at least one algorithm that all implementations will have available. This document defines the current set of algorithms that are mandatory to implement as part of IKEv2, as well as algorithms that should be implemented because they may be promoted to mandatory at some future time. [STANDARDS-TRACK]The Transport Layer Security (TLS) Protocol Version 1.1This document specifies Version 1.1 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]Transport Layer Security (TLS) ExtensionsThis document describes extensions that may be used to add functionality to Transport Layer Security (TLS). It provides both generic extension mechanisms for the TLS handshake client and server hellos, and specific extensions using these generic mechanisms.</t><t> The extensions may be used by TLS clients and servers. The extensions are backwards compatible: communication is possible between TLS clients that support the extensions and TLS servers that do not support the extensions, and vice versa. [STANDARDS-TRACK]Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)This document describes new key exchange algorithms based on Elliptic Curve Cryptography (ECC) for the Transport Layer Security (TLS) protocol. In particular, it specifies the use of Elliptic Curve Diffie-Hellman (ECDH) key agreement in a TLS handshake and the use of Elliptic Curve Digital Signature Algorithm (ECDSA) as a new authentication mechanism. This memo provides information for the Internet community.XDR: External Data Representation StandardThis document describes the External Data Representation Standard (XDR) protocol as it is currently deployed and accepted. This document obsoletes RFC 1832. [STANDARDS-TRACK]Using OpenPGP Keys for Transport Layer Security (TLS) AuthenticationThis memo proposes extensions to the Transport Layer Security (TLS) protocol to support the OpenPGP key format. The extensions discussed here include a certificate type negotiation mechanism, and the required modifications to the TLS Handshake Protocol. This memo defines an Experimental Protocol for the Internet community.An Interface and Algorithms for Authenticated EncryptionThis document defines algorithms for Authenticated Encryption with Associated Data (AEAD), and defines a uniform interface and a registry for such algorithms. The interface and registry can be used as an application-independent set of cryptoalgorithm suites. This approach provides advantages in efficiency and security, and promotes the reuse of crypto implementations. [STANDARDS-TRACK]Chosen Ciphertext Attacks against Protocols Based on RSA Encryption Standard PKCSSecurity of CBC Ciphersuites in SSL/TLS: Problems and CountermeasuresPassword Interception in a SSL/TLS ChannelNIST Special Publication 800-38C: The CCM Mode for Authentication and ConfidentialityData Encryption Standard (DES)Digital Signature StandardNational Institute of Standards and Technology, U.S.Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)American National Standards InstituteThe Order of Encryption and Authentication for Protecting Communications (Or: How Secure is SSL?)Bleichenbacher's RSA signature forgery based on implementation errorRecommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMACAttacking RSA-based Sessions in SSL/TLSPKCS #6: RSA Extended Certificate Syntax Standard, version 1.5RSA LaboratoriesPKCS #7: RSA Cryptographic Message Syntax Standard, version 1.5RSA LaboratoriesA Method for Obtaining Digital Signatures and Public-Key CryptosystemsThe SSL ProtocolNetscape Communications Corp.The SSL 3.0 ProtocolNetscape Communications Corp.Netscape Communications Corp.Netscape Communications Corp.Remote timing attacks are practicalTransport Layer Security (TLS) Extensions: Extension DefinitionsInformation Technology - Open Systems Interconnection - The Directory: ModelsThis section describes protocol types and constants.The following values define the cipher suite codes used in the ClientHello and
ServerHello messages.A cipher suite defines a cipher specification supported in TLS Version 1.2.TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a TLS
connection during the first handshake on that channel, but MUST NOT be
negotiated, as it provides no more protection than an unsecured connection.The following CipherSuite definitions require that the server provide an RSA
certificate that can be used for key exchange. The server may request any
signature-capable certificate in the certificate request message.The following cipher suite definitions are used for server- authenticated (and
optionally client-authenticated) Diffie-Hellman. DH denotes cipher suites in
which the server’s certificate contains the Diffie-Hellman parameters signed by
the certificate authority (CA). DHE denotes ephemeral Diffie-Hellman, where the
Diffie-Hellman parameters are signed by a signature-capable certificate, which
has been signed by the CA. The signing algorithm used by the server is
specified after the DHE component of the CipherSuite name. The server can
request any signature-capable certificate from the client for client
authentication, or it may request a Diffie-Hellman certificate. Any
Diffie-Hellman certificate provided by the client must use the parameters
(group and generator) described by the server.The following cipher suites are used for completely anonymous Diffie-Hellman
communications in which neither party is authenticated. Note that this mode is
vulnerable to man-in-the- middle attacks. Using this mode therefore is of
limited use: These cipher suites MUST NOT be used by TLS 1.2 implementations
unless the application layer has specifically requested to allow anonymous key
exchange. (Anonymous key exchange may sometimes be acceptable, for example, to
support opportunistic encryption when no set-up for authentication is in place,
or when TLS is used as part of more complex security protocols that have other
means to ensure authentication.)Note that using non-anonymous key exchange without actually verifying the key
exchange is essentially equivalent to anonymous key exchange, and the same
precautions apply. While non-anonymous key exchange will generally involve a
higher computational and communicational cost than anonymous key exchange, it
may be in the interest of interoperability not to disable non-anonymous key
exchange when the application layer is allowing anonymous key exchange.New cipher suite values have been assigned by IANA as described in
.Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are
reserved to avoid collision with Fortezza-based cipher suites in
SSL 3.These security parameters are determined by the TLS Handshake Protocol and
provided as parameters to the TLS record layer in order to initialize a
connection state. SecurityParameters includes:RFC 4492 adds Elliptic Curve cipher suites to TLS. This document
changes some of the structures used in that document. This section details the
required changes for implementors of both RFC 4492 and TLS 1.2. Implementors of
TLS 1.2 who are not implementing RFC 4492 do not need to read this section.This document adds a “signature_algorithm” field to the digitally- signed
element in order to identify the signature and digest algorithms used to create
a signature. This change applies to digital signatures formed using ECDSA as
well, thus allowing ECDSA signatures to be used with digest algorithms other
than SHA-1, provided such use is compatible with the certificate and any
restrictions imposed by future revisions of .As described in and , the
restrictions on the signature algorithms used to sign certificates are no
longer tied to the cipher suite (when used by the server) or the
ClientCertificateType (when used by the client). Thus, the restrictions on the
algorithm used to sign certificates specified in Sections 2 and 3 of RFC 4492
are also relaxed. As in this document, the restrictions on the keys in the
end-entity certificate remain.
AES is a widely used symmetric encryption algorithm. AES is
a block cipher with a 128-, 192-, or 256-bit keys and a 16-byte
block size. TLS currently only supports the 128- and 256-bit key
sizes.
An application protocol is a protocol that normally layers
directly on top of the transport layer (e.g., TCP/IP). Examples
include HTTP, TELNET, FTP, and SMTP.
See public key cryptography.
A symmetric encryption algorithm that simultaneously provides
confidentiality and message integrity.
Authentication is the ability of one entity to determine the
identity of another entity.
A block cipher is an algorithm that operates on plaintext in
groups of bits, called blocks. 64 bits was, and 128 bits is, a
common block size.
A symmetric encryption algorithm used to encrypt large quantities
of data.
CBC is a mode in which every plaintext block encrypted with a
block cipher is first exclusive-ORed with the previous ciphertext
block (or, in the case of the first block, with the initialization
vector). For decryption, every block is first decrypted, then
exclusive-ORed with the previous ciphertext block (or IV).
As part of the X.509 protocol (a.k.a. ISO Authentication
framework), certificates are assigned by a trusted Certificate
Authority and provide a strong binding between a party’s identity
or some other attributes and its public key.
The application entity that initiates a TLS connection to a
server. This may or may not imply that the client initiated the
underlying transport connection. The primary operational
difference between the server and client is that the server is
generally authenticated, while the client is only optionally
authenticated.
The key used to encrypt data written by the client.
The secret data used to authenticate data written by the client.
A connection is a transport (in the OSI layering model definition)
that provides a suitable type of service. For TLS, such
connections are peer-to-peer relationships. The connections are
transient. Every connection is associated with one session.
DES still is a very widely used symmetric encryption
algorithm although it is considered as rather weak now. DES is a
block cipher with a 56-bit key and an 8-byte block size. Note
that in TLS, for key generation purposes, DES is treated as having
an 8-byte key length (64 bits), but it still only provides 56 bits
of protection. (The low bit of each key byte is presumed to be
set to produce odd parity in that key byte.) DES can also be
operated in a mode where three independent keys and three
encryptions are used for each block of data; this uses 168 bits of
key (24 bytes in the TLS key generation method) and provides the
equivalent of 112 bits of security.
A standard for digital signing, including the Digital Signing
Algorithm, approved by the National Institute of Standards and
Technology, defined in NIST FIPS PUB 186-2, “Digital Signature
Standard”, published January 2000 by the U.S. Department of
Commerce . A significant update has been drafted and
was published in March 2006.
Digital signatures utilize public key cryptography and one-way
hash functions to produce a signature of the data that can be
authenticated, and is difficult to forge or repudiate.
An initial negotiation between client and server that
establishes the parameters of their transactions.
When a block cipher is used in CBC mode, the initialization vector
is exclusive-ORed with the first plaintext block prior to
encryption.
A Message Authentication Code is a one-way hash computed from a
message and some secret data. It is difficult to forge without
knowing the secret data. Its purpose is to detect if the message
has been altered.
Secure secret data used for generating encryption keys, MAC
secrets, and IVs.
MD5 is a hashing function that converts an arbitrarily long
data stream into a hash of fixed size (16 bytes). Due to
significant progress in cryptanalysis, at the time of publication
of this document, MD5 no longer can be considered a ‘secure’
hashing function.
A class of cryptographic techniques employing two-key ciphers.
Messages encrypted with the public key can only be decrypted with
the associated private key. Conversely, messages signed with the
private key can be verified with the public key.
A one-way transformation that converts an arbitrary amount of data
into a fixed-length hash. It is computationally hard to reverse
the transformation or to find collisions. MD5 and SHA are
examples of one-way hash functions.
A stream cipher invented by Ron Rivest. A compatible cipher is
described in .
A very widely used public key algorithm that can be used for
either encryption or digital signing.
The server is the application entity that responds to requests for
connections from clients. See also “client”.
A TLS session is an association between a client and a server.
Sessions are created by the handshake protocol. Sessions define a
set of cryptographic security parameters that can be shared among
multiple connections. Sessions are used to avoid the expensive
negotiation of new security parameters for each connection.
A session identifier is a value generated by a server that
identifies a particular session.
The key used to encrypt data written by the server.
The secret data used to authenticate data written by the server.
The Secure Hash Algorithm is defined in FIPS PUB 180-2. It
produces a 20-byte output. Note that all references to SHA
(without a numerical suffix) actually use the modified SHA-1
algorithm.
The 256-bit Secure Hash Algorithm is defined in FIPS PUB 180-2.
It produces a 32-byte output.
Netscape’s Secure Socket Layer protocol . TLS is based on
SSL Version 3.0.
An encryption algorithm that converts a key into a
cryptographically strong keystream, which is then exclusive-ORed
with the plaintext.
See bulk cipher.
This protocol; also, the Transport Layer Security working group of
the Internet Engineering Task Force (IETF). See “Working Group
Information” at the end of this document (see page 99).
Indicates whether this is a stream cipher or a block cipher
running in CBC mode.
The number of bytes from the key_block that are used for
generating the write keys.
The amount of data needed to be generated for the initialization
vector. Zero for stream ciphers; equal to the block size for
block ciphers (this is equal to
SecurityParameters.record_iv_length).
The amount of data a block cipher enciphers in one chunk; a block
cipher running in CBC mode can only encrypt an even multiple of
its block size.The TLS protocol cannot prevent many common security mistakes. This section
provides several recommendations to assist implementors.TLS requires a cryptographically secure pseudorandom number generator (PRNG).
Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash
operations, most notably SHA-1, are acceptable, but cannot provide more
security than the size of the random number generator state.To estimate the amount of seed material being produced, add the number of bits
of unpredictable information in each seed byte. For example, keystroke timing
values taken from a PC compatible’s 18.2 Hz timer provide 1 or 2 secure bits
each, even though the total size of the counter value is 16 bits or more.
Seeding a 128-bit PRNG would thus require approximately 100 such timer values. provides guidance on the generation of random values.Implementations are responsible for verifying the integrity of certificates and
should generally support certificate revocation messages. Certificates should
always be verified to ensure proper signing by a trusted Certificate Authority
(CA). The selection and addition of trusted CAs should be done very carefully.
Users should be able to view information about the certificate and root CA.TLS supports a range of key sizes and security levels, including some that
provide no or minimal security. A proper implementation will probably not
support many cipher suites. For instance, anonymous Diffie-Hellman is strongly
discouraged because it cannot prevent man- in-the-middle attacks. Applications
should also enforce minimum and maximum key sizes. For example, certificate
chains containing 512- bit RSA keys or signatures are not appropriate for
high-security applications.Implementation experience has shown that certain parts of earlier TLS
specifications are not easy to understand, and have been a source of
interoperability and security problems. Many of these areas have been clarified
in this document, but this appendix contains a short list of the most important
things that require special attention from implementors.TLS protocol issues:Do you correctly handle handshake messages that are fragmented to
multiple TLS records (see )? Including corner cases
like a ClientHello that is split to several small fragments? Do
you fragment handshake messages that exceed the maximum fragment
size? In particular, the certificate and certificate request
handshake messages can be large enough to require fragmentation.Do you ignore the TLS record layer version number in all TLS
records before ServerHello (see )?Do you handle TLS extensions in ClientHello correctly, including
omitting the extensions field completely?Do you support renegotiation, both client and server initiated?
While renegotiation is an optional feature, supporting it is
highly recommended.When the server has requested a client certificate, but no
suitable certificate is available, do you correctly send an empty
Certificate message, instead of omitting the whole message (see
)?Cryptographic details:In the RSA-encrypted Premaster Secret, do you correctly send and
verify the version number? When an error is encountered, do you
continue the handshake to avoid the Bleichenbacher attack (see
)?What countermeasures do you use to prevent timing attacks against
RSA decryption and signing operations (see
)?When verifying RSA signatures, do you accept both NULL and missing parameters
(see )? Do you verify that the RSA padding
doesn’t have additional data after the hash value? When using Diffie-Hellman key exchange, do you correctly strip
leading zero bytes from the negotiated key (see )?Does your TLS client check that the Diffie-Hellman parameters sent
by the server are acceptable (see
)?How do you generate unpredictable IVs for CBC mode ciphers (see
)?Do you accept long CBC mode padding (up to 255 bytes; see
?How do you address CBC mode timing attacks ()?Do you use a strong and, most importantly, properly seeded random number
generator (see ) for generating the
premaster secret (for RSA key exchange), Diffie-Hellman private values, the
DSA “k” parameter, and other security-critical values?Since there are various versions of TLS (1.0, 1.1, 1.2, and any future
versions) and SSL (2.0 and 3.0), means are needed to negotiate the specific
protocol version to use. The TLS protocol provides a built-in mechanism for
version negotiation so as not to bother other protocol components with the
complexities of version selection.TLS versions 1.0, 1.1, and 1.2, and SSL 3.0 are very similar, and use
compatible ClientHello messages; thus, supporting all of them is relatively
easy. Similarly, servers can easily handle clients trying to use future
versions of TLS as long as the ClientHello format remains compatible, and the
client supports the highest protocol version available in the server.A TLS 1.2 client who wishes to negotiate with such older servers will send a
normal TLS 1.2 ClientHello, containing { 3, 3 } (TLS 1.2) in
ClientHello.client_version. If the server does not support this version, it
will respond with a ServerHello containing an older version number. If the
client agrees to use this version, the negotiation will proceed as appropriate
for the negotiated protocol.If the version chosen by the server is not supported by the client (or not
acceptable), the client MUST send a “protocol_version” alert message and close
the connection.If a TLS server receives a ClientHello containing a version number greater than
the highest version supported by the server, it MUST reply according to the
highest version supported by the server.A TLS server can also receive a ClientHello containing a version number smaller
than the highest supported version. If the server wishes to negotiate with old
clients, it will proceed as appropriate for the highest version supported by
the server that is not greater than ClientHello.client_version. For example, if
the server supports TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the
server will proceed with a TLS 1.0 ServerHello. If server supports (or is
willing to use) only versions greater than client_version, it MUST send a
“protocol_version” alert message and close the connection.Whenever a client already knows the highest protocol version known to a server
(for example, when resuming a session), it SHOULD initiate the connection in
that native protocol.Note: some server implementations are known to implement version negotiation
incorrectly. For example, there are buggy TLS 1.0 servers that simply close the
connection when the client offers a version newer than TLS 1.0. Also, it is
known that some servers will refuse the connection if any TLS extensions are
included in ClientHello. Interoperability with such buggy servers is a complex
topic beyond the scope of this document, and may require multiple connection
attempts by the client.Earlier versions of the TLS specification were not fully clear on what the
record layer version number (TLSPlaintext.version) should contain when sending
ClientHello (i.e., before it is known which version of the protocol will be
employed). Thus, TLS servers compliant with this specification MUST accept any
value {03,XX} as the record layer version number for ClientHello.TLS clients that wish to negotiate with older servers MAY send any value
{03,XX} as the record layer version number. Typical values would be {03,00},
the lowest version number supported by the client, and the value of
ClientHello.client_version. No single value will guarantee interoperability
with all old servers, but this is a complex topic beyond the scope of this
document.TLS 1.2 clients that wish to support SSL 2.0 servers MUST send version 2.0
CLIENT-HELLO messages defined in . The message MUST contain the same
version number as would be used for ordinary ClientHello, and MUST encode the
supported TLS cipher suites in the CIPHER-SPECS-DATA field as described below.Warning: The ability to send version 2.0 CLIENT-HELLO messages will be phased
out with all due haste, since the newer ClientHello format provides better
mechanisms for moving to newer versions and negotiating extensions. TLS 1.2
clients SHOULD NOT support SSL 2.0.However, even TLS servers that do not support SSL 2.0 MAY accept version 2.0
CLIENT-HELLO messages. The message is presented below in sufficient detail for
TLS server implementors; the true definition is still assumed to be .For negotiation purposes, 2.0 CLIENT-HELLO is interpreted the same way as a
ClientHello with a “null” compression method and no extensions. Note that this
message MUST be sent directly on the wire, not wrapped as a TLS record. For the
purposes of calculating Finished and CertificateVerify, the msg_length field is
not considered to be a part of the handshake message.
The highest bit MUST be 1; the remaining bits contain the length
of the following data in bytes.
This field, in conjunction with the version field, identifies a
version 2 ClientHello message. The value MUST be 1.
Equal to ClientHello.client_version.
This field is the total length of the field cipher_specs. It
cannot be zero and MUST be a multiple of the V2CipherSpec length
(3).
This field MUST have a value of zero for a client that claims to
support TLS 1.2.
The length in bytes of the client’s challenge to the server to
authenticate itself. Historically, permissible values are between
16 and 32 bytes inclusive. When using the SSLv2 backward-
compatible handshake the client SHOULD use a 32-byte challenge.
This is a list of all CipherSpecs the client is willing and able
to use. In addition to the 2.0 cipher specs defined in ,
this includes the TLS cipher suites normally sent in
ClientHello.cipher_suites, with each cipher suite prefixed by a
zero byte. For example, the TLS cipher suite {0x00,0x0A} would be
sent as {0x00,0x00,0x0A}.
This field MUST be empty.
Corresponds to ClientHello.random. If the challenge length is
less than 32, the TLS server will pad the data with leading (note:
not trailing) zero bytes to make it 32 bytes long.Note: Requests to resume a TLS session MUST use a TLS client hello.When TLS clients fall back to Version 2.0 compatibility mode, they MUST use
special PKCS#1 block formatting. This is done so that TLS servers will reject
Version 2.0 sessions with TLS-capable clients.When a client negotiates SSL 2.0 but also supports TLS, it MUST set the
right-hand (least-significant) 8 random bytes of the PKCS padding (not
including the terminal null of the padding) for the RSA encryption of the
ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY to 0x03 (the other padding
bytes are random).When a TLS-capable server negotiates SSL 2.0 it SHOULD, after decrypting the
ENCRYPTED-KEY-DATA field, check that these 8 padding bytes are 0x03. If they
are not, the server SHOULD generate a random value for SECRET-KEY-DATA, and
continue the handshake (which will eventually fail since the keys will not
match). Note that reporting the error situation to the client could make the
server vulnerable to attacks described in .The TLS protocol is designed to establish a secure connection between a client
and a server communicating over an insecure channel. This document makes
several traditional assumptions, including that attackers have substantial
computational resources and cannot obtain secret information from sources
outside the protocol. Attackers are assumed to have the ability to capture,
modify, delete, replay, and otherwise tamper with messages sent over the
communication channel. This appendix outlines how TLS has been designed to
resist a variety of attacks.The handshake protocol is responsible for selecting a cipher spec and
generating a master secret, which together comprise the primary cryptographic
parameters associated with a secure session. The handshake protocol can also
optionally authenticate parties who have certificates signed by a trusted
certificate authority.TLS supports three authentication modes: authentication of both parties, server
authentication with an unauthenticated client, and total anonymity. Whenever
the server is authenticated, the channel is secure against man-in-the-middle
attacks, but completely anonymous sessions are inherently vulnerable to such
attacks. Anonymous servers cannot authenticate clients. If the server is
authenticated, its certificate message must provide a valid certificate chain
leading to an acceptable certificate authority. Similarly, authenticated
clients must supply an acceptable certificate to the server. Each party is
responsible for verifying that the other’s certificate is valid and has not
expired or been revoked.The general goal of the key exchange process is to create a pre_master_secret
known to the communicating parties and not to attackers. The pre_master_secret
will be used to generate the master_secret (see
). The master_secret is required to generate the
Finished messages, encryption keys, and MAC keys (see and
). By sending a correct Finished message, parties thus prove
that they know the correct pre_master_secret.Completely anonymous sessions can be established using Diffie-Hellman for key
exchange. The server’s public parameters are contained in the server key
exchange message, and the client’s are sent in the client key exchange message.
Eavesdroppers who do not know the private values should not be able to find the
Diffie-Hellman result (i.e., the pre_master_secret).Warning: Completely anonymous connections only provide protection against
passive eavesdropping. Unless an independent tamper-proof channel is used to
verify that the Finished messages were not replaced by an attacker, server
authentication is required in environments where active man-in-the-middle
attacks are a concern.With RSA, key exchange and server authentication are combined. The public key
is contained in the server’s certificate. Note that compromise of the server’s
static RSA key results in a loss of confidentiality for all sessions protected
under that static key. TLS users desiring Perfect Forward Secrecy should use
DHE cipher suites. The damage done by exposure of a private key can be limited
by changing one’s private key (and certificate) frequently.After verifying the server’s certificate, the client encrypts a
pre_master_secret with the server’s public key. By successfully decoding the
pre_master_secret and producing a correct Finished message, the server
demonstrates that it knows the private key corresponding to the server
certificate.When RSA is used for key exchange, clients are authenticated using the
certificate verify message (see ). The client signs a
value derived from all preceding handshake messages. These handshake messages
include the server certificate, which binds the signature to the server, and
ServerHello.random, which binds the signature to the current handshake process.When Diffie-Hellman key exchange is used, the server can either supply a
certificate containing fixed Diffie-Hellman parameters or use the server key
exchange message to send a set of temporary Diffie-Hellman parameters signed
with a DSA or RSA certificate. Temporary parameters are hashed with the
hello.random values before signing to ensure that attackers do not replay old
parameters. In either case, the client can verify the certificate or signature
to ensure that the parameters belong to the server.If the client has a certificate containing fixed Diffie-Hellman parameters, its
certificate contains the information required to complete the key exchange.
Note that in this case the client and server will generate the same
Diffie-Hellman result (i.e., pre_master_secret) every time they communicate. To
prevent the pre_master_secret from staying in memory any longer than necessary,
it should be converted into the master_secret as soon as possible. Client
Diffie-Hellman parameters must be compatible with those supplied by the server
for the key exchange to work.If the client has a standard DSA or RSA certificate or is unauthenticated, it
sends a set of temporary parameters to the server in the client key exchange
message, then optionally uses a certificate verify message to authenticate
itself.If the same DH keypair is to be used for multiple handshakes, either because
the client or server has a certificate containing a fixed DH keypair or because
the server is reusing DH keys, care must be taken to prevent small subgroup
attacks. Implementations SHOULD follow the guidelines found in .Small subgroup attacks are most easily avoided by using one of the DHE cipher
suites and generating a fresh DH private key (X) for each handshake. If a
suitable base (such as 2) is chosen, g^X mod p can be computed very quickly;
therefore, the performance cost is minimized. Additionally, using a fresh key
for each handshake provides Perfect Forward Secrecy. Implementations SHOULD
generate a new X for each handshake when using DHE cipher suites.Because TLS allows the server to provide arbitrary DH groups, the client should
verify that the DH group is of suitable size as defined by local policy. The
client SHOULD also verify that the DH public exponent appears to be of adequate
size. provides a useful guide to the strength of various group
sizes. The server MAY choose to assist the client by providing a known group,
such as those defined in or . These can be verified by
simple comparison.Because TLS includes substantial improvements over SSL Version 2.0, attackers
may try to make TLS-capable clients and servers fall back to Version 2.0. This
attack can occur if (and only if) two TLS- capable parties use an SSL 2.0
handshake.Although the solution using non-random PKCS #1 block type 2 message padding is
inelegant, it provides a reasonably secure way for Version 3.0 servers to
detect the attack. This solution is not secure against attackers who can
brute-force the key and substitute a new ENCRYPTED-KEY-DATA message containing
the same key (but with normal padding) before the application-specified wait
threshold has expired. Altering the padding of the least-significant 8 bytes of
the PKCS padding does not impact security for the size of the signed hashes and
RSA key lengths used in the protocol, since this is essentially equivalent to
increasing the input block size by 8 bytes.An attacker might try to influence the handshake exchange to make the parties
select different encryption algorithms than they would normally choose.For this attack, an attacker must actively change one or more handshake
messages. If this occurs, the client and server will compute different values
for the handshake message hashes. As a result, the parties will not accept each
others’ Finished messages. Without the master_secret, the attacker cannot
repair the Finished messages, so the attack will be discovered.When a connection is established by resuming a session, new ClientHello.random
and ServerHello.random values are hashed with the session’s master_secret.
Provided that the master_secret has not been compromised and that the secure
hash operations used to produce the encryption keys and MAC keys are secure,
the connection should be secure and effectively independent from previous
connections. Attackers cannot use known encryption keys or MAC secrets to
compromise the master_secret without breaking the secure hash operations.Sessions cannot be resumed unless both the client and server agree. If either
party suspects that the session may have been compromised, or that certificates
may have expired or been revoked, it should force a full handshake. An upper
limit of 24 hours is suggested for session ID lifetimes, since an attacker who
obtains a master_secret may be able to impersonate the compromised party until
the corresponding session ID is retired. Applications that may be run in
relatively insecure environments should not write session IDs to stable storage.The master_secret is hashed with the ClientHello.random and ServerHello.random
to produce unique data encryption keys and MAC secrets for each connection.Outgoing data is protected with a MAC before transmission. To prevent message
replay or modification attacks, the MAC is computed from the MAC key, the
sequence number, the message length, the message contents, and two fixed
character strings. The message type field is necessary to ensure that messages
intended for one TLS record layer client are not redirected to another. The
sequence number ensures that attempts to delete or reorder messages will be
detected. Since sequence numbers are 64 bits long, they should never overflow.
Messages from one party cannot be inserted into the other’s output, since they
use independent MAC keys. Similarly, the server write and client write keys are
independent, so stream cipher keys are used only once.If an attacker does break an encryption key, all messages encrypted with it can
be read. Similarly, compromise of a MAC key can make message-modification
attacks possible. Because MACs are also encrypted, message-alteration attacks
generally require breaking the encryption algorithm as well as the MAC.Note: MAC keys may be larger than encryption keys, so messages can remain
tamper resistant even if encryption keys are broken. describes a chosen plaintext attack on TLS that depends on knowing
the IV for a record. Previous versions of TLS used the CBC residue
of the previous record as the IV and therefore enabled this attack. This
version uses an explicit IV in order to protect against this attack.TLS secures transmitted application data via the use of symmetric encryption
and authentication functions defined in the negotiated cipher suite. The
objective is to protect both the integrity and confidentiality of the
transmitted data from malicious actions by active attackers in the network. It
turns out that the order in which encryption and authentication functions are
applied to the data plays an important role for achieving this goal .The most robust method, called encrypt-then-authenticate, first applies
encryption to the data and then applies a MAC to the ciphertext. This method
ensures that the integrity and confidentiality goals are obtained with ANY pair
of encryption and MAC functions, provided that the former is secure against
chosen plaintext attacks and that the MAC is secure against chosen-message
attacks. TLS uses another method, called authenticate-then-encrypt, in which
first a MAC is computed on the plaintext and then the concatenation of
plaintext and MAC is encrypted. This method has been proven secure for CERTAIN
combinations of encryption functions and MAC functions, but it is not
guaranteed to be secure in general. In particular, it has been shown that there
exist perfectly secure encryption functions (secure even in the
information-theoretic sense) that combined with any secure MAC function, fail
to provide the confidentiality goal against an active attack. Therefore, new
cipher suites and operation modes adopted into TLS need to be analyzed under
the authenticate-then-encrypt method to verify that they achieve the stated
integrity and confidentiality goals.Currently, the security of the authenticate-then-encrypt method has been proven
for some important cases. One is the case of stream ciphers in which a
computationally unpredictable pad of the length of the message, plus the length
of the MAC tag, is produced using a pseudorandom generator and this pad is
exclusive-ORed with the concatenation of plaintext and MAC tag. The other is
the case of CBC mode using a secure block cipher. In this case, security can be
shown if one applies one CBC encryption pass to the concatenation of plaintext
and MAC and uses a new, independent, and unpredictable IV for each new pair of
plaintext and MAC. In versions of TLS prior to 1.1, CBC mode was used properly
EXCEPT that it used a predictable IV in the form of the last block of the
previous ciphertext. This made TLS open to chosen plaintext attacks. This
version of the protocol is immune to those attacks. For exact details in the
encryption modes proven secure, see .TLS is susceptible to a number of denial-of-service (DoS) attacks. In
particular, an attacker who initiates a large number of TCP connections can
cause a server to consume large amounts of CPU for doing RSA decryption.
However, because TLS is generally used over TCP, it is difficult for the
attacker to hide his point of origin if proper TCP SYN randomization is used
by the TCP stack.Because TLS runs over TCP, it is also susceptible to a number of DoS attacks on
individual connections. In particular, attackers can forge RSTs, thereby
terminating connections, or forge partial TLS records, thereby causing the
connection to stall. These attacks cannot in general be defended against by a
TCP-using protocol. Implementors or users who are concerned with this class of
attack should use IPsec AH or ESP .For TLS to be able to provide a secure connection, both the client and server
systems, keys, and applications must be secure. In addition, the implementation
must be free of security errors.The system is only as strong as the weakest key exchange and authentication
algorithm supported, and only trustworthy cryptographic functions should be
used. Short public keys and anonymous servers should be used with great
caution. Implementations and users must be careful when deciding which
certificates and certificate authorities are acceptable; a dishonest
certificate authority can do tremendous damage.The discussion list for the IETF TLS working group is located at the e-mail
address tls@ietf.org. Information on the group and information on how to
subscribe to the list is at https://www1.ietf.org/mailman/listinfo/tlsArchives of the list can be found at:
http://www.ietf.org/mail-archive/web/tls/current/index.html