The Transport Layer Security (TLS) Protocol Version 1.3RTFM, 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 is a WIP draft of TLS 1.3 and has not yet seen significant security analysis.RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH
The source for this draft is maintained in GitHub. Suggested changes
should be submitted as pull requests at
https://github.com/tlswg/tls13-spec. Instructions are on that page as
well. Editorial changes can be managed in GitHub, but any substantive
change should be discussed on the TLS mailing list.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 , 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, i.e., in integrity-only
modes.The connection is reliable. Messages include an authentication
tag which protects them against modification.The Record Protocol can operate in an insecure mode 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”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this
document are to be interpreted as described in RFC 2119 .The following terms are used:client: The endpoint initiating the TLS connection.connection: A transport-layer connection between two endpoints.endpoint: Either the client or server of the connection.handshake: An initial negotiation between client and server that establishes the parameters of their transactions.peer: An endpoint. When discussing a particular endpoint, “peer” refers to the endpoint that is remote to the primary subject of discussion.receiver: An endpoint that is receiving records.sender: An endpoint that is transmitting records.session: An association between a client and a server resulting from a handshake.server: The endpoint which did not initiate the TLS connection.draft-06Prohibit RC4 negotiation for backwards compatibility.Freeze & deprecate record layer version field.Update format of signatures with context.Remove explicit IV.draft-05Prohibit SSL negotiation for backwards compatibility.Fix which MS is used for exporters.draft-04Modify key computations to include session hash.Remove ChangeCipherSpecRenumber the new handshake messages to be somewhat more
consistent with existing convention and to remove a duplicate
registration.Remove renegotiation.Remove point format negotiation.draft-03Remove GMT time.Merge in support for ECC from RFC 4492 but without explicit
curves.Remove the unnecessary length field from the AD input to AEAD
ciphers.Rename {Client,Server}KeyExchange to {Client,Server}KeyShareAdd an explicit HelloRetryRequest to reject the client’sdraft-02Increment version number.Reworked handshake to provide 1-RTT mode.Remove custom DHE groups.Removed support for compression.Removed support for static RSA and DH key exchange.Removed support for non-AEAD ciphersThe 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 record protection 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 two cryptographic operations — digital signing, and authenticated
encryption with additional data (AEAD) — are designated digitally-signed,
and aead-ciphered, 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 algorithm
field was introduced in TLS 1.2, and is not in earlier 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 previous versions of TLS, the ServerKeyExchange format meant that attackers
can obtain a signature of a message with a chosen, 32-byte prefix. Because TLS
1.3 servers are likely to also implement prior versions, the contents of the
element always start with 64 bytes of octet 32 in order to clear that
chosen-prefix.Following that padding is a NUL-terminated context string in order to
disambiguate signatures for different purposes. The context string will be
specified whenever a digitally-signed element is used.Finally, the specified contents of the digitally-signed structure follow the
NUL at the end of the context string. (See the example at the end of this
section.)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.All ECDSA computations MUST be performed according to ANSI X9.62
or its successors. Data to be signed/verified is hashed, and the
result run directly through the ECDSA algorithm with no additional
hashing. The default hash function is SHA-1 . However, an
alternative hash function, such as one of the new SHA hash functions
specified in FIPS 180-2 may be used instead if the certificate
containing the EC public key explicitly requires use of another hash
function. (The mechanism for specifying the required hash function
has not been standardized, but this provision anticipates such
standardization and obviates the need to update this document in
response. Future PKIX RFCs may choose, for example, to specify the
hash function to be used with a public key in the parameters field of
subjectPublicKeyInfo.) [[OPEN ISSUE: This needs updating per 4492-bis
https://github.com/tlswg/tls13-spec/issues/59]]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 the following exampleAssume that the context string for the signature was specified as “Example”.
The input for the signature/hash algorithm would be:followed by the encoding of the inner struct (field3 and field4).The length of the 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: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 or later 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, protects the records, and transmits the
result. Received data is decrypted and verified, reassembled, and then
delivered to higher-level clients.Three protocols that use the record protocol are described in this document: the
handshake protocol, the alert 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 record protection algorithm and its
parameters as well as the record protection keys and IVs for the
connection in both the read and the write directions. The security
parameters are set by the TLS Handshake Protocol, which also determines
when new cryptographic keys are installed and used for record
protection.
The initial current state always specifies that records are
not protected.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 ).
The algorithm to be used for record protection. This algorithm must
be of the AEAD type and thus provides integrity and confidentiality
as a single primitive. It is possible to have AEAD algorithms which
do not provide any confidentiality and
defines a special NULL_NULL AEAD
algorithm for use in the initial handshake). This specification
includes the key size of this algorithm and of the nonce for
the AEAD algorithm.
A 48-byte secret shared between the two peers in the connection and
used to generate keys for protecting the handshake.
A 48-byte secret shared between the two peers in the connection
and used to generate keys for protecting application data.
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 four
items: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 encryption algorithm. This will consist
of the scheduled key for that connection.
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 MUST NOT
exceed 2^64-1. Sequence numbers do not wrap. If a TLS
implementation would need to wrap a sequence number, it MUST
terminate the connection. 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 protocol version the current record is compatible with.
This value MUST be set to { 3, 1 } for all records.
This field is deprecated and MUST be ignored for all purposes.
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.This document describes TLS Version 1.3, which uses the version { 3, 4 }.
The version value 3.4 is historical, deriving from the use of { 3, 1 }
for TLS 1.0 and { 3, 0 } for SSL 3.0. In order to maximize backwards
compatibility, the record layer version identifies as simply TLS 1.0.
Endpoints supporting other versions negotiate the version to use
by following the procedure and requirements in .Implementations MUST NOT send zero-length fragments of Handshake or Alert
types. Zero-length fragments of Application data MAY
be sent as they are potentially useful as a traffic analysis countermeasure.The record protection functions translate a TLSPlaintext structure into a
TLSCiphertext. The deprotection functions reverse the process. In TLS 1.3
as opposed to previous versions of TLS, all ciphers are modeled as
“Authenticated Encryption with Additional Data” (AEAD) .
AEAD functions provide a unified encryption and authentication
operation which turns plaintext into authenticated ciphertext and
back again.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.
The type field is identical to TLSPlaintext.type.
The record_version field is identical to TLSPlaintext.record_version and is always { 3, 1 }.
The length (in bytes) of the following TLSCiphertext.fragment.
The length MUST NOT exceed 2^14 + 2048.
The AEAD encrypted form of TLSPlaintext.fragment.The length of the per-record nonce (iv_length) is set to max(8 bytes,
N_MIN) for the AEAD algorithm (see Section 4). An AEAD
algorithm where N_MAX is less than 8 bytes MUST not be used with TLS.
The per-record nonce for the AEAD construction is formed as follows:The 64-bit record sequence number is padded to the left with zeroes
to iv_length.The padded sequence number is XORed with the static client_write_iv
or server_write_iv, depending on the role.The resulting quantity (of length iv_length) is used as the per-record
nonce. Note: This is a different construction from that in TLS 1.2, which
specified a partially explicit nonce.The plaintext is the TLSPlaintext.fragment.The additional authenticated data, which we denote as additional_data, is
defined as follows:where “+” denotes concatenation.Note: In versions of TLS prior to 1.3, the additional_data included a
length field. This presents a problem for cipher constructions with
data-dependent padding (such as CBC). TLS 1.3 removes the length
field and relies on the AEAD cipher to provide integrity for the
length of the data.The AEAD output consists of the ciphertext output by the AEAD encryption
operation. The length will generally be larger than TLSPlaintext.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
TLSPlaintext.length values. Each AEAD cipher MUST NOT produce an expansion of
greater than 1024 bytes. Symbolically,[[OPEN ISSUE: Reduce these values?
https://github.com/tlswg/tls13-spec/issues/55]]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.As a special case, we define the NULL_NULL AEAD cipher which is simply
the identity operation and thus provides no security. This cipher
MUST ONLY be used with the initial TLS_NULL_WITH_NULL_NULL cipher suite.[[OPEN ISSUE: This needs to be revised. See https://github.com/tlswg/tls13-spec/issues/5]]
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 encryption key and a server write
encryption key. Each of these is generated from the byte sequence in
that order. Unused values are empty.When keys are generated, the current master secret (MS) is used
as an entropy source. For handshake records, this means the
hs_master_secret. For application data records, this means the
regular master_secret.To generate the key material, computewhere MS is the relevant master secret. The PRF is computed enough
times to generate the necessary amount of data for the key_block,
which is then partitioned as follows: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.
Specifies the authentication and key establishment algorithms,
the pseudorandom function (PRF) used to generate keying
material, and the record protection algorithm (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.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
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 which cannot be
deprotected. Because AEAD algorithms combine decryption and
verification, this message is used for all deprotection failures.
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. This message is always fatal.
A TLSCiphertext record was received that had a length more than
2^14+2048 bytes, or a record decrypted to a TLSPlaintext record
with more than 2^14 bytes. 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 previous versions of TLS. TLS 1.3 does not
include compression and TLS 1.3 implementations MUST NOT send this
alert when in TLS 1.3 mode. This message is always fatal.
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 message is always fatal.
This alert was used in SSL 3.0 but not any version of TLS. It MUST
NOT be sent by compliant implementations.
This message is always fatal.
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. This message is always fatal.
The protocol version the peer 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 HelloRequest or by the server
in response to a ClientHello after initial handshaking. Versions
of TLS prior to TLS 1.3 supported renegotiation of a previously
established connection; TLS 1.3 removes this feature. This
message is always fatal.
sent by clients that receive an extended ServerHello containing
an extension that they did not put in the corresponding ClientHello.
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 a protocol version,
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 AES-GCM with
a 255-bit ECDHE key exchange with a host whose certificate
chain you have verified, you can expect that to be reasonably secure.These goals are achieved by the handshake protocol, which can be
summarized as follows: The client sends a ClientHello message which
contains a random nonce (ClientHello.random), its preferences for
Protocol Version, Cipher Suite, and a variety of extensions. In
the same flight, it sends a ClientKeyShare message which contains its
share of the parameters for key agreement for some set of expected
server parameters (DHE/ECDHE groups, etc.).If the client has provided a ClientKeyShare with an appropriate set of
keying material, the server responds to the ClientHello with a ServerHello
message. The ServerHello contains the server’s nonce
(ServerHello.random), the server’s choice of the Protocol Version,
Session ID and Cipher Suite, and the server’s response to the
extensions the client offered.The server can then generate its own keying material share and send a
ServerKeyShare message which contains its share of the parameters for
the key agreement. The server can now compute the shared secret (the
premaster secret). At this point, the server starts encrypting all
remaining handshake traffic with the negotiated cipher suite using a key
derived from the premaster secret (via the “handshake master secret”).
The remainder of the server’s
handshake messages will be encrypted using that key.Following these messages, the server will send an EncryptedExtensions
message which contains a response to any client’s extensions which are
not necessary to establish the Cipher Suite. The server will then send
its certificate in a Certificate message if it is to be authenticated.
The server may optionally request a certificate from the client by
sending a CertificateRequest message at this point.
Finally, if the server is authenticated, it will send a CertificateVerify
message which provides a signature over the entire handshake up to
this point. This serves both to authenticate the server and to establish
the integrity of the negotiation. Finally, the server sends a Finished
message which includes an integrity check over the handshake keyed
by the shared secret and demonstrates that the server and client have
agreed upon the same keys.
[[TODO: If the server is not requesting client authentication,
it MAY start sending application data following the Finished, though
the server has no way of knowing who will be receiving the data. Add this.]]Once the client receives the ServerKeyShare, it can also compute the
premaster secret and decrypt the server’s remaining handshake messages.
The client generates its own sending keys based on the premaster secret
and will encrypt the remainder of its handshake messages using those keys
and the newly established cipher suite. If the server has sent a
CertificateRequest message, the client MUST send the Certificate
message, though it may contain zero certificates. If the client has
sent a certificate, a digitally-signed CertificateVerify message is
sent to explicitly verify possession of the private key in the
certificate. Finally, the client sends the Finished message.At this point, the handshake is complete, and the
client and server may exchange application layer data, which is
protected using a new set of keys derived from both the premaster
secret and the handshake transcript (See
for the security rationale for this.)Application data MUST NOT be sent prior to the Finished message.
[[TODO: can we make this clearer and more clearly match the text above
about server-side False Start.]]
Client Server* Indicates optional or situation-dependent messages that are not always sent.{} Indicates messages protected using keys derived from the handshake master
secret.[] Indicates messages protected using keys derived from the master secret.If the client has not provided an appropriate ClientKeyShare (e.g. it
includes only DHE or ECDHE groups unacceptable or unsupported by the
server), the server corrects the mismatch with a HelloRetryRequest and
the client will need to restart the handshake with an appropriate
ClientKeyShare, as shown in Figure 2:Figure 2. Message flow for a full handshake with mismatched parameters[[OPEN ISSUE: Should we restart the handshake hash?
https://github.com/tlswg/tls13-spec/issues/104.]]
[[OPEN ISSUE: We need to make sure that this flow doesn’t introduce
downgrade issues. Potential options include continuing the handshake
hashes (as long as clients don’t change their opinion of the server’s
capabilities with aborted handshakes) and requiring the client to send
the same ClientHello (as is currently done) and then checking you get
the same negotiated parameters.]]If no common cryptographic parameters can be negotiated, the server
will send a fatal alert.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 proceed directly to sending Finished messages, which
are protected using handshake keys as described above, computed using
resumption premaster secret created in the first handshake as the
premaster secret. Once the
re-establishment is complete, the client and server MAY begin to
exchange application layer data, which is protected using the
application secrets (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.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 AEAD algorithm is initialized to NULL_NULL.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 will also send a
ClientHello when the server has responded to its ClientHello with a
ServerHello that selects cryptographic parameters that don’t match the
client’s ClientKeyShare. In that case, the client MUST send the same
ClientHello (without modification) along with the new ClientKeyShare.
If a server receives a ClientHello at any other time, it MUST send
a fatal “no_renegotiation” alert.Structure of this message:The ClientHello message includes a random structure, which is used later in
the protocol.
32 bytes generated by a secure random number generator.Note: Versions of TLS prior to TLS 1.3 used the top 32 bits of
the Random value to encode the time since the UNIX epoch.Note:
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 confidentiality or
integrity 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 record protection algorithm (including
secret key length) 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.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 3.4. (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 .
Versions of TLS before 1.3 supported compression and the list of
compression methods was supplied in this field. For any TLS 1.3
ClientHello, this field MUST contain only the “null” compression
method with the code point of 0. If a TLS 1.3 ClientHello is
received with any other value in this field, the server MUST
generate a fatal “illegal_parameter” alert. Note that TLS 1.3
servers may receive TLS 1.2 or prior ClientHellos which contain
other compression methods and MUST follow the procedures for
the appropriate prior version of TLS.
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
or HelloRetryRequest message.When this message will be sent:This message is always sent by the client. It MUST immediately follow the
ClientHello message. In backward compatibility mode (see Section XXX)
it will be included in the EarlyData extension ()
in the ClientHello.Meaning of this message:This message contains the client’s cryptographic parameters
for zero or more key establishment methods.Structure of this message:
The named group for the key share offer. This identifies the
specific key exchange method that the ClientKeyShareOffer describes.
Finite Field Diffie-Hellman parameters are described in
; Elliptic Curve Diffie-Hellman parameters are
described in .
Key exchange information. The contents of this field are
determined by the value of NamedGroup entry and its corresponding
definition.
A list of ClientKeyShareOffer values.Clients may offer an arbitrary number of ClientKeyShareOffer
values, each representing a single set of key agreement parameters;
for instance a client might offer shares for several elliptic curves
or multiple integer DH groups. The shares for each ClientKeyShareOffer
MUST by generated independently. Clients MUST NOT offer multiple
ClientKeyShareOffers for the same parameters. It is explicitly
permitted to send an empty ClientKeyShare message, as this is used
to elicit the server’s parameters if the client has no useful
information.
[TODO: Recommendation about what the client offers. Presumably which integer
DH groups and which curves.]
[TODO: Work out how this interacts with PSK and SRP.]Diffie-Hellman parameters for both clients and servers are encoded in
the opaque key_exchange field of the ClientKeyShareOffer or
ServerKeyShare structures. The opaque value contains the
Diffie-Hellman public value (dh_Y = g^X mod p),
encoded as a big-endian integer.ECDHE parameters for both clients and servers are encoded in the
opaque key_exchange field of the ClientKeyShareOffer or
ServerKeyShare structures. The opaque value conveys the Elliptic
Curve Diffie-Hellman public value (ecdh_Y) represented as a byte
string ECPoint.point.
This is the byte string representation of an elliptic curve
point following the conversion routine in Section 4.3.6 of ANSI
X9.62 .Although X9.62 supports multiple point formats, any given curve
MUST specify only a single point format. All curves currently
specified in this document MUST only be used with the uncompressed
point format.Note: Versions of TLS prior to 1.3 permitted point negotiation;
TLS 1.3 removes this feature in favor of a single point format
for each curve.[[OPEN ISSUE: We will need to adjust the compressed/uncompressed point issue
if we have new curves that don’t need point compression. This depends
on the CFRG’s recommendations. The expectation is that future curves will
come with defined point formats and that existing curves conform to
X9.62.]]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 and the client’s
ClientKeyShare message was acceptable. If the client proposed groups are not
acceptable by the server, it will respond with an “insufficient_security” fatal alert.Structure of this message:The presence of extensions can be detected by determining whether there are
bytes following the cipher_suite field at the end of the ServerHello.
This field will contain the lower of that suggested by the client
in the ClientHello and the highest supported by the server. For
this version of the specification, the version is 3.4. (See
for details about backward compatibility.)
This structure is generated by the server and MUST be
generated independently of 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.
A list of extensions. Note that only extensions offered by the
client can appear in the server’s list. In TLS 1.3 as opposed to
previous versions of TLS, the server’s extensions are split between
the ServerHello and the EncryptedExtensions
message. The ServerHello
MUST only include extensions which are required to establish
the cryptographic context.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 but
the client’s ClientKeyShare message did not contain an acceptable
offer. If it cannot find such a match, it will respond with a
“handshake_failure” alert.Structure of this message:[[OPEN ISSUE: Merge in DTLS Cookies?]]
The group which the client MUST use for its new ClientHello.The “server_version”, “cipher_suite” and “extensions” fields have the
same meanings as their corresponding values in the ServerHello. The
server SHOULD send only the extensions necessary for the client to
generate a correct ClientHello/ClientKeyShare pair.Upon receipt of a HelloRetryRequest, the client MUST send a new
ClientHello/ClientKeyShare pair to the server. The ClientKeyShare MUST
contain both the groups in the original ClientKeyShare as well as a
ClientKeyShareOffer consistent with the “selected_group” field.
I.e., it MUST be a superset of the previous ClientKeyShareOffer.Upon re-sending the ClientHello/ClientKeyShare and receiving the
server’s ServerHello/ServerKeyShare, the client MUST verify that
the selected CipherSuite and NamedGroup match that supplied in
the HelloRetryRequest.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 .
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
ClientHello, and does not include them in ServerHello. 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 share), 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 (DHE_RSA, ECDHE_RSA), behave as if client had sent the value
{sha1,rsa}.If the negotiated key exchange algorithm is DHE_DSS, behave
as if the client had sent the value {sha1,dsa}.If the negotiated key exchange algorithm is ECDHE_ECDSA,
behave as if the client had sent value {sha1,ecdsa}.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 ServerHello, and the server ignores the extension in ClientHello (if present).When sent by the client, the “supported_groups” extension indicates
the named groups which the client supports, ordered from most
preferred to least preferred.Note: In versions of TLS prior to TLS 1.3, this extension was named
“elliptic curves” and only contained elliptic curve groups. See
and .The “extension_data” field of this extension SHALL contain a
“NamedGroupList” value:
Indicates support of the corresponding named curve
The named curves defined here are those specified in SEC 2 [13].
Note that many of these curves are also recommended in ANSI
X9.62 and FIPS 186-2 . Values 0xFE00 through 0xFEFF are
reserved for private use. Values 0xFF01 and 0xFF02 were used in
previous versions of TLS but MUST NOT be offered by TLS 1.3
implementations.
[[OPEN ISSUE: Triage curve list.]]
Indicates support of the corresponding finite field
group, defined in Items in named_curve_list are ordered according to the client’s
preferences (favorite choice first).As an example, a client that only supports secp192r1 (aka NIST P-192;
value 19 = 0x0013) and secp224r1 (aka NIST P-224; value 21 = 0x0015)
and prefers to use secp192r1 would include a TLS extension consisting
of the following octets. Note that the first two octets indicate the
extension type (Supported Group Extension):The client MUST supply a “named_groups” extension containing at
least one group for each key exchange algorithm (currently
DHE and ECDHE) for which it offers a cipher suite.
If the client does not supply a “named_groups” extension with a
compatible group, the server MUST NOT negotiate a cipher suite of the
relevant type. For instance, if a client supplies only ECDHE groups,
the server MUST NOT negotiate finite field Diffie-Hellman. If no
acceptable group can be selected across all cipher suites, then the
server MUST generate a fatal “handshake_failure” alert.NOTE: A server participating in an ECDHE-ECDSA key exchange may use
different curves for (i) the ECDSA key in its certificate, and (ii)
the ephemeral ECDH key in the ServerKeyExchange message. The server
must consider the supported groups in both cases.[[TODO: IANA Considerations.]]TLS versions before 1.3 have a strict message ordering and do not
permit additional messages to follow the ClientHello. The EarlyData
extension allows TLS messages which would otherwise be sent as
separate records to be instead inserted in the ClientHello. The
extension simply contains the TLS records which would otherwise have
been included in the client’s first flight.Extra messages for the client’s first flight MAY either be transmitted
standalone or sent as EarlyData. However, when a client does not know
whether TLS 1.3 can be negotiated – e.g., because the server may
support a prior version of TLS or because of network intermediaries –
it SHOULD use the EarlyData extension. If the EarlyData extension
is used, then clients MUST NOT send any messages other than the
ClientHello in their initial flight.Any data included in EarlyData is not integrated into the handshake
hashes directly. E.g., if the ClientKeyShare is included in
EarlyData, then the handshake hashes consist of ClientHello +
ServerHello, etc. However, because the ClientKeyShare is in a
ClientHello extension, it is still hashed transitively. This procedure
guarantees that the Finished message covers these messages even if
they are ultimately ignored by the server (e.g., because it is sent to
a TLS 1.2 server). TLS 1.3 servers MUST understand messages sent in
EarlyData, and aside from hashing them differently, MUST treat them as
if they had been sent immediately after the ClientHello.Servers MUST NOT send the EarlyData extension. Negotiating TLS 1.3
serves as acknowledgment that it was processed as described above.[[OPEN ISSUE: This is a fairly general mechanism which is possibly
overkill in the 1-RTT case, where it would potentially be more
attractive to just have a “ClientKeyShare” extension. However,
for the 0-RTT case we will want to send the Certificate, CertificateVerify,
and application data, so a more general extension seems appropriate
at least until we have determined we don’t need it for 0-RTT.]]When this message will be sent:This message will be sent immediately after the ServerHello message if
the client has provided a ClientKeyShare message which is compatible
with the selected cipher suite and group parameters.Meaning of this message:This message conveys cryptographic information to allow the client to
compute 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 named group for the key share offer. This identifies the
selected key exchange method from the ClientKeyShare message
(), identifying which value from the
ClientKeyShareOffer the server has accepted as is responding to.
Key exchange information. The contents of this field are
determined by the value of NamedGroup entry and its corresponding
definition.When this message will be sent:If this message is sent, it MUST be sent immediately after the server’s
ServerKeyShare.Meaning of this message:The EncryptedExtensions message simply contains any extensions
which should be protected, i.e., any which are not needed to
establish the cryptographic context. The same extension types
MUST NOT appear in both the ServerHello and EncryptedExtensions.
If the same extension appears in both locations, the client
MUST rely only on the value in the EncryptedExtensions block.
[[OPEN ISSUE: Should we just produce a canonical list of what
goes where and have it be an error to have it in the wrong
place? That seems simpler. Perhaps have a whitelist of which
extensions can be unencrypted and everything else MUST be
encrypted.]]Structure of this message:
A list of extensions.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 either the EncryptedExtensions message if one is
sent or the ServerKeyShare 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).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: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 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 SSL 3.0, 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 MUST NOT be used. They were used in SSL 3.0.Note: It is a fatal “handshake_failure” alert for an anonymous server to request
client authentication.When this message will be sent:This message is used to provide explicit proof that the server
possesses the private key corresponding to its certificate
and also provides integrity for the handshake up
to this point. This message is only sent when the server is
authenticated via a certificate. When sent, it MUST be the
last server handshake message prior to the Finished.Structure of this message:Here handshake_messages_hash is a digest of all handshake messages
sent or received, starting at ClientHello and up to, but not
including, this message, including the type and length fields of the
handshake messages. This is a digest of the concatenation of all the
Handshake structures (as defined in ) exchanged
thus far. For the PRF defined in Section 5, the digest 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 this
computation. Note that this is the same running hash that is used in
the Finished message .The context string for the signature is “TLS 1.3, server CertificateVerify”. A
hash of the handshake messages is signed rather than the messages themselves
because the digitally-signed format requires padding and context bytes at the
beginning of the input. Thus, by signing a digest of the messages, an
implementation need only maintain one running hash per hash type for
CertificateVerify, Finished and other messages.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.
[[TODO: Update this to deal with DSS-3 and DSS-4.
https://github.com/tlswg/tls13-spec/issues/59]]When this message will be sent:The Server’s Finished message is the final message sent by the server
and indicates that the key exchange and authentication processes were successful.Meaning of this message: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. This data will be
protected under keys derived from the hs_master_secret (see
).Structure of this message:
PRF(hs_master_secret, finished_label, Hash(handshake_messages))
[0..verify_data_length-1];
For Finished messages sent by the client, the string
“client finished”. For Finished messages sent by the server,
the string “server finished”.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.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 or
. 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: 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.When this message will be sent:This message is the first handshake message the client can send
after receiving the server’s Finished. 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 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’s
Certificate message. The contents of the message are computed as described
in , except that the context string is
“TLS 1.3, client CertificateVerify”.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.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, key
agreement, and record protection algorithms are determined by the
cipher_suite selected by the server and revealed in the ServerHello
message. The random values are exchanged in the hello messages. All
that remains is to calculate the master secret.The pre_master_secret is used to generate a series of master secret values,
as shown in the following diagram and described below.First, as soon as the ClientKeyShare and ServerKeyShare messages
have been exchanged, the client and server each use the
unauthenticated key shares to generate a master secret which is used
for the protection of the remaining handshake records. Specifically,
they generate:During resumption, the premaster secret is initialized with the
“resumption premaster secret”, rather than using the values from the
ClientKeyShare/ServerKeyShare exchange.This master secret value is used to generate the record protection
keys used for the handshake, as described in .Once the hs_master_secret has been computed, the premaster secret SHOULD
be deleted from memory.Once the last non-Finished message has been sent, the client and
server then compute the master secret which will be used for the
remainder of the session. It is also used with TLS Exporters .If the server does not request client authentication, the master
secret can be computed at the time that the server sends its Finished,
thus allowing the server to send traffic on its first flight (See
[TODO] for security considerations on this practice.) If the server
requests client authentication, this secret can be computed after the
client’s Certificate and CertificateVerify have been sent, or, if the
client refuses client authentication, after the client’s empty
Certificate message has been sent.For full handshakes, each side also derives a new secret which will
be used as the premaster_secret for future resumptions of the
newly established session. This is computed as:The session_hash value is a running hash of the handshake as
defined in . Thus, the hs_master_secret
is generated using a different session_hash from the other
two secrets.All master secrets are always exactly 48 bytes in length. The length
of the premaster secret will vary depending on key exchange method.When a handshake takes place, we definewhere “handshake_messages” refers to all handshake messages sent or
received, starting at ClientHello up to the present time, with the
exception of the Finished message, including the type and length
fields of the handshake messages. This is the concatenation of all the
exchanged Handshake structures.For concreteness, at the point where the handshake master secret
is derived, the session hash includes the ClientHello, ClientKeyShare,
ServerHello, and ServerKeyShare, and HelloRetryRequest (if any)
(though see [https://github.com/tlswg/tls13-spec/issues/104]).
At the point where the master secret is derived, it includes every
handshake message, with the exception of the Finished messages.
Note that if client authentication is not used, then the session
hash is complete at the point when the server has sent its first
flight. Otherwise, it is only complete when the client has sent its
first flight, as it covers the client’s Certificate and CertificateVerify.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.All ECDH calculations (including parameter and key generation as well
as the shared secret calculation) are performed according to [6]
using the ECKAS-DH1 scheme with the identity map as key derivation
function (KDF), so that the premaster secret is the x-coordinate of
the ECDH shared secret elliptic curve point represented as an octet
string. Note that this octet string (Z in IEEE 1363 terminology) as
output by FE2OSP, the Field Element to Octet String Conversion
Primitive, has constant length for any given field; leading zeros
found in this octet string MUST NOT be truncated.(Note that this use of the identity KDF is a technicality. The
complete picture is that ECDH is employed with a non-trivial KDF
because TLS does not directly use the premaster secret for anything
other than for computing the master secret.)In the absence of an application profile standard specifying otherwise, a
TLS-compliant application MUST implement the cipher suite TODO:Needs to be selected. (See for the definition.)Application data messages are carried by the record layer and are fragmented
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 C,
D, and E.[[TODO: Update https://github.com/tlswg/tls13-spec/issues/62]]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 .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 ).This document also uses two registries originally created in . IANA
[should update/has updated] it to reference this document. The registries
and their allocation policies are listed below.TLS NamedCurve registry: Future values are allocated via IETF Consensus
.TLS ECPointFormat Registry: Future values are allocated via IETF Consensus
.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 .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. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK]AES Galois Counter Mode (GCM) Cipher Suites for TLSThis memo describes the use of the Advanced Encryption Standard (AES) in Galois/Counter Mode (GCM) as a Transport Layer Security (TLS) authenticated encryption operation. GCM provides both confidentiality and data origin authentication, can be efficiently implemented in hardware for speeds of 10 gigabits per second and above, and is also well-suited to software implementations. This memo defines TLS cipher suites that use AES-GCM with RSA, DSA, and Diffie-Hellman-based key exchange mechanisms. [STANDARDS-TRACK]TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)RFC 4492 describes elliptic curve cipher suites for Transport Layer Security (TLS). However, all those cipher suites use HMAC-SHA-1 as their Message Authentication Code (MAC) algorithm. This document describes sixteen new cipher suites for TLS that specify stronger MAC algorithms. Eight use Hashed Message Authentication Code (HMAC) with SHA-256 or SHA-384, and eight use AES in Galois Counter Mode (GCM). This memo provides information for the Internet community.Specification for the Advanced Encryption Standard (AES)National Institute of Standards and TechnologyDigital Signature StandardNational Institute of Standards and Technology, U.S. Department of CommerceSecure 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-TPublic Key Cryptography For The Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)ANSINew Directions in CryptographyTransmission 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.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]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.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]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]Keying Material Exporters for Transport Layer Security (TLS)A number of protocols wish to leverage Transport Layer Security (TLS) to perform key establishment but then use some of the keying material for their own purposes. This document describes a general mechanism for allowing that. [STANDARDS-TRACK]Transport Layer Security (TLS) Extensions: Extension DefinitionsThis document provides specifications for existing TLS extensions. It is a companion document for RFC 5246, "The Transport Layer Security (TLS) Protocol Version 1.2". The extensions specified are server_name, max_fragment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and status_request. [STANDARDS-TRACK]Prohibiting Secure Sockets Layer (SSL) Version 2.0This document requires that when Transport Layer Security (TLS) clients and servers establish connections, they never negotiate the use of Secure Sockets Layer (SSL) version 2.0. This document updates the backward compatibility sections found in the Transport Layer Security (TLS). [STANDARDS-TRACK]Prohibiting RC4 Cipher SuitesThis document requires that Transport Layer Security (TLS) clients and servers never negotiate the use of RC4 cipher suites when they establish connections. This applies to all TLS versions. This document updates RFCs 5246, 4346, and 2246.Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for TLSTraditional finite-field-based Diffie-Hellman (DH) key exchange during the TLS handshake suffers from a number of security, interoperability, and efficiency shortcomings. These shortcomings arise from lack of clarity about which DH group parameters TLS servers should offer and clients should accept. This document offers a solution to these shortcomings for compatible peers by using a section of the TLS "EC Named Curve Registry" to establish common finite-field DH parameters with known structure and a mechanism for peers to negotiate support for these groups. This draft updates TLS versions 1.0 [RFC2246], 1.1 [RFC4346], and 1.2 [RFC5246], as well as the TLS ECC extensions [RFC4492].Transport Layer Security (TLS) Session Hash and Extended Master Secret ExtensionThe Transport Layer Security (TLS) master secret is not cryptographically bound to important session parameters such as the server certificate. Consequently, it is possible for an active attacker to set up two sessions, one with a client and another with a server, such that the master secrets on the two sessions are the same. Thereafter, any mechanism that relies on the master secret for authentication, including session resumption, becomes vulnerable to a man-in-the-middle attack, where the attacker can simply forward messages back and forth between the client and server. This specification defines a TLS extension that contextually binds the master secret to a log of the full handshake that computes it, thus preventing such attacks.Deprecating Secure Sockets Layer Version 3.0Secure Sockets Layer version 3.0 (SSLv3) [RFC6101] is not sufficiently secure. This document requires that SSLv3 not be used. The replacement versions, in particular Transport Layer Security (TLS) 1.2 [RFC5246], are considerably more secure and capable protocols. This document updates the backward compatibility section of RFC 5246 and its predecessors to prohibit fallback to SSLv3.Security of CBC Ciphersuites in SSL/TLS: Problems and CountermeasuresDigital 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 InstituteBleichenbacher's RSA signature forgery based on implementation errorRecommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMACPKCS #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 practicalInformation 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.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 cipher suite definitions, defined in , are
used for server-authenticated (and optionally client-authenticated)
Diffie-Hellman. 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.The following cipher suite definitions, defined in , are
used for server-authenticated (and optionally client-authenticated)
Elliptic Curve Diffie-Hellman. ECDHE 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.The following ciphers, defined in ,
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 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.)[[TODO: Add all the defined AEAD ciphers. This currently only lists
GCM. https://github.com/tlswg/tls13-spec/issues/53]]
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.The PRFs SHALL be as follows:o For cipher suites ending with _SHA256, the PRF is the TLS PRF
with SHA-256 as the hash function.o For cipher suites ending with _SHA384, the PRF is the TLS PRF
with SHA-384 as the hash function.New cipher suite values are 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 3.0.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.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 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 keys or signatures weaker than 2048-bit RSA or 224-bit ECDSA
are not appropriate for secure 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? (see )Have you ensured that all support for SSL, RC4, and EXPORT ciphers
is completely removed from all possible configurations that support
TLS 1.3 or later, and that attempts to use these obsolete capabilities
fail correctly? (see )Do you handle TLS extensions in ClientHello correctly, including
omitting the extensions field completely?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:What countermeasures do you use to prevent timing attacks against
RSA signing operations .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
)?Do you use a strong and, most importantly, properly seeded random number
generator (see ) Diffie-Hellman private values, the
DSA “k” parameter, and other security-critical values?The TLS protocol provides a built-in mechanism for version negotiation between
endpoints potentially supporting different versions of TLS.TLS 1.x and SSL 3.0 use compatible ClientHello messages. Servers can also 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.Prior versions of TLS used the record layer version number for various
purposes. (TLSPlaintext.record_version & TLSCiphertext.record_version)
As of TLS 1.3, this field is deprecated and its value MUST be ignored by all
implementations. Version negotiation is performed using only the handshake versions.
(ClientHello.client_version & ServerHello.server_version)
In order to maximize interoperability with older endpoints, implementations
that negotiate the usage of TLS 1.0-1.2 SHOULD set the record layer
version number to the negotiated version for the ServerHello and all
records thereafter.A TLS 1.3 client who wishes to negotiate with such older servers will send a
normal TLS 1.3 ClientHello containing { 3, 4 } (TLS 1.3) 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. A client resuming a session SHOULD initiate the
connection using the version that was previously negotiated.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.Some legacy server implementations are known to not implement the TLS
specification properly and might abort connections upon encountering
TLS extensions or versions which it is not aware of. Interoperability
with buggy servers is a complex topic beyond the scope of this document.
Multiple connection attempts may be required in order to negotiate
a backwards compatible connection, however this practice is vulnerable
to downgrade attacks and is NOT RECOMMENDED.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 the server only supports
versions greater than client_version, it MUST send a “protocol_version”
alert message and close the connection.Note that earlier versions of TLS did not clearly specify the record layer
version number value in all cases (TLSPlaintext.record_version). Servers
will receive various TLS 1.x versions in this field, however its value
MUST always be ignored.If an implementation negotiates usage of TLS 1.2, then negotiation of cipher
suites also supported by TLS 1.3 SHOULD be preferred, if available.The security of RC4 cipher suites is considered insufficient for the reasons
cited in [RFC7465]. Implementations MUST NOT offer or negotiate RC4 cipher suites
for any version of TLS for any reason.Old versions of TLS permitted the usage of very low strength ciphers.
Ciphers with a strength less than 112 bits MUST NOT be offered or
negotiated for any version of TLS for any reason.The security of SSL 2.0 is considered insufficient for the reasons enumerated
in [RFC6176], and MUST NOT be negotiated for any reason.Implementations MUST NOT send an SSL version 2.0 compatible CLIENT-HELLO.
Implementations MUST NOT negotiate TLS 1.3 or later using an SSL version 2.0 compatible
CLIENT-HELLO. Implementations are NOT RECOMMENDED to accept an SSL version 2.0 compatible
CLIENT-HELLO in order to negotiate older versions of TLS.Implementations MUST NOT send or accept any records with a version less than { 3, 0 }.The security of SSL 3.0 is considered insufficient for the reasons enumerated
in [I-D.ietf-tls-sslv3-diediedie], and MUST NOT be negotiated for any reason.Implementations MUST NOT send a ClientHello.client_version or ServerHello.server_version
set to { 3, 0 } or less. Any endpoint receiving a Hello message with
ClientHello.client_version or ServerHello.server_version set to { 3, 0 } MUST respond
with a “protocol_version” alert message and close the connection.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 and record protection 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
share message, and the client’s are sent in the client key share 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.When Diffie-Hellman key exchange is used, the client and server use
the client key exchange and server key exchange messages to send
temporary Diffie-Hellman parameters. The signature in the certificate
verify message (if present) covers the entire handshake up to that
point and thus attests the certificate holder’s desire to use the
the ephemeral DHE keys.Peers SHOULD validate each other’s public key Y (dh_Ys offered by
the server or DH_Yc offered by the client) by ensuring that 1 < Y <
p-1. This simple check ensures that the remote peer is properly
behaved and isn’t forcing the local system into a small subgroup.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 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 record protection keys are secure,
the connection should be secure and effectively independent from previous
connections. Attackers cannot use known keys 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 record protection secrets for each connection.Outgoing data is protected using an AEAD algorithm before transmission. The
authentication data includes the sequence number, message type, message length,
and the message contents. 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 keys.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 doing asymmetric crypto
operations. 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:
https://www.ietf.org/mail-archive/web/tls/current/index.html