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 allows client/server applications to
communicate over the Internet in a way that is designed to prevent eavesdropping,
tampering, and 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 peers. The TLS 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 connection is reliable. Messages include an authentication
tag which protects them against modification.Note: The TLS Record Protocol can operate in an insecure mode but is generally
only used in this mode while another protocol is using the TLS 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 , ECDSA , 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-09Change to RSA-PSS signatures for handshake messages.Remove support for DSA.Update key schedule per suggestions by Hugo, Hoeteck, and Bjoern Tackmann.Add support for per-record padding.Switch to encrypted record ContentType.Change HKDF labeling to include protocol version and value lengths.Shift the final decision to abort a handshake due to incompatible
certificates to the client rather than having servers abort early.Deprecate SHA-1 with signatures.Add MTI algorithms.draft-08Remove support for weak and lesser used named curves.Remove support for MD5 and SHA-224 hashes with signatures.Update lists of available AEAD cipher suites and error alerts.Reduce maximum permitted record expansion for AEAD from 2048 to 256 octets.Require digital signatures even when a previous configuration is used.Merge EarlyDataIndication and KnownConfiguration.Change code point for server_configuration to avoid collision with
server_hello_done.Relax certificate_list ordering requirement to match current practice.draft-07Integration of semi-ephemeral DH proposal.Add initial 0-RTT support.Remove resumption and replace with PSK + tickets.Move ClientKeyShare into an extension.Move to HKDF.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 ChangeCipherSpec.Renumber 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}KeyShare.Add an explicit HelloRetryRequest to reject the client’s.draft-02Increment version number.Rework handshake to provide 1-RTT mode.Remove custom DHE groups.Remove support for compression.Remove support for static RSA and DH key exchange.Remove support for non-AEAD ciphers.The goals of the TLS protocol, in order of priority, are as follows:Cryptographic security: TLS should be used to establish a secure connection
between two parties.Interoperability: Independent programmers should be able to develop
applications utilizing TLS that can successfully exchange cryptographic
parameters without knowledge of one another’s code.Extensibility: TLS seeks to provide a framework into which new public key
and 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 have evolved from the SSL 3.0 Protocol
Specification as published by Netscape. The differences between this version
and previous versions 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:Typed constants can be defined for purposes of specification by declaring a
symbol of the desired type and assigning values to it.Under-specified types (opaque, variable-length vectors, and structures that
contain opaque) cannot be assigned values. No fields of a multi-element
structure or vector may be elided.For example:The following common primitive types are defined and used subsequently: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-PSS signature scheme defined in with
MGF1. The digest used in the mask generation function MUST be the same
as the digest which is being signed (i.e., what appears in
algorithm.signature). Note that previous versions of TLS used
RSASSA-PKCS1-v1_5, not RSASSA-PSS.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 SignatureAndHashAlgorithm parameter in the DigitallySigned
object indicates the digest algorithm which was used in the signature.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.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.The TLS Record Protocol is a layered protocol. At each layer, messages
may include fields for length, description, and content. The TLS 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 TLS Record Protocol are described in this document: the TLS
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 TLS 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 the length of a record or absence of traffic
itself is not protected by encryption unless the sender uses the
supplied padding mechanism – see for more details.[[TODO: I plan to totally rewrite or remove this. IT seems like just cruft.]]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 appropriate 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 only 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 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:[TODO: update this to handle new key hierarchy.]The connection state 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 is set to zero at the beginning of a connection and
incremented by one thereafter. 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.
NOTE: This is a change from previous versions of TLS, where
sequence numbers were reset whenever keys were changed.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).
Alert messages MUST NOT be fragmented across 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, even if those fragments contain padding. Zero-length
fragments of Application data MAY be sent as they are potentially
useful as a traffic analysis countermeasure.When record protection has not yet been engaged, TLSPlaintext
structures are written directly onto the wire. Once record protection
has started, TLSPlaintext records are protected and sent as
described in the following section.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 outer opaque_type field of a TLSCiphertext record is always set to the
value 23 (application_data) for outward compatibility with
middleboxes used to parsing previous versions of TLS. The
actual content type of the record is found in fragment.type after
decryption.
The record_version field is identical to TLSPlaintext.record_version and is always { 3, 1 }.
Note that the handshake protocol including the ClientHello and ServerHello messages authenticates
the protocol version, so this value is redundant.
The length (in bytes) of the following TLSCiphertext.fragment. The length
MUST NOT exceed 2^14 + 256. An endpoint that receives a record that exceeds
this length MUST generate a fatal “record_overflow” alert.
The cleartext of TLSPlaintext.fragment.
The actual content type of the record.
An arbitrary-length run of zero-valued bytes may
appear in the cleartext after the type field. This provides an
opportunity for senders to pad any TLS record by a chosen amount as
long as the total stays within record size limits. See
for more details.
The AEAD encrypted form of TLSPlaintext.fragment + TLSPlaintext.type + zeros,
where “+” denotes concatenation.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 concatenation of TLSPlaintext.fragment and TLSPlaintext.type.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 of the plaintext is greater than
TLSPlaintext.length due to the inclusion of TLSPlaintext.type and
however much padding is supplied by the sender. The length of
aead_output will generally be larger than the plaintext, but by an
amount that varies with the AEAD cipher. Since the ciphers might
incorporate padding, the amount of overhead could vary with different
lengths of plaintext. Symbolically,In order to decrypt and verify, the cipher takes as input the key, nonce, the
“additional_data”, and the AEADEncrypted value. The output is either the
plaintext or an error indicating that the decryption failed. There is no
separate integrity check. That is:If the decryption fails, a fatal “bad_record_mac” alert MUST be generated.An AEAD cipher MUST NOT produce an expansion of greater than 255
bytes. An endpoint that receives a record from its peer with
TLSCipherText.length larger than 2^14 + 256 octets MUST generate a
fatal “record_overflow” alert. This limit is derived from the maximum
TLSPlaintext length of 2^14 octets + 1 octet for ContentType + the
maximum AEAD expansion of 255 octets.All encrypted TLS records can be padded to inflate the size of the
TLSCipherText. This allows the sender to hide the size of the
traffic from an observer.When generating a TLSCiphertext record, implementations MAY choose to
pad. An unpadded record is just a record with a padding length of
zero. Padding is a string of zero-valued bytes appended
to the ContentType field before encryption. Implementations MUST set
the padding octets to all zeros before encrypting.Application Data records may contain a zero-length fragment.content if
the sender desires. This permits generation of plausibly-sized cover
traffic in contexts where the presence or absence of activity may be
sensitive. Implementations MUST NOT send Handshake or Alert records
that have a zero-length fragment.content.The padding sent is automatically verified by the record protection
mechanism: Upon successful decryption of a TLSCiphertext.fragment,
the receiving implementation scans the field from the end toward the
beginning until it finds a non-zero octet. This non-zero octet is the
content type of the message.Implementations MUST limit their scanning to the cleartext returned
from the AEAD decryption. If a receiving implementation does not find
a non-zero octet in the cleartext, it should treat the record as
having an unexpected ContentType, sending an “unexpected_message”
alert.The presence of padding does not change the overall record size
limitations – the full fragment plaintext may not exceed 2^14 octets.Versions of TLS prior to 1.3 had limited support for padding. This
padding scheme was selected because it allows padding of any encrypted
TLS record by an arbitrary size (from zero up to TLS record size
limits) without introducing new content types. The design also
enforces all-zero padding octets, which allows for quick detection of
padding errors.Selecting a padding policy that suggests when and how much to pad is a
complex topic, and is beyond the scope of this specification. If the
application layer protocol atop TLS permits padding, it may be
preferable to pad application_data TLS records within the application
layer. Padding for encrypted handshake and alert TLS records must
still be handled at the TLS layer, though. Later documents may define
padding selection algorithms, or define a padding policy request
mechanism through TLS extensions or some other means.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 TLS Handshake Protocol is responsible for negotiating a session, which consists
of the following items:
X509v3 certificate of the peer. This element of the state
may be null.
Specifies the authentication and key establishment algorithms,
the hash for use with HKDF to generate keying
material, and the record protection algorithm (See
for formal definition.)
a secret shared between the client and server that can be used
as a PSK in future 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 using a PSK established in an initial handshake.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. Failure to properly close a connection does
not prohibit a session from being resumed.
This message notifies the recipient that the sender will not send
any more messages on this connection. Any data received after a
closure MUST be ignored.
This message notifies the recipient that the sender is canceling the
handshake for some reason unrelated to a protocol failure. If a 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 alert is generally a warning.Either party MAY initiate a close by sending a “close_notify” alert. Any data
received after a closure alert is ignored. If a transport-level close is
received prior to a “close_notify”, the receiver cannot know that all the
data that was sent has been received.Each party MUST send a “close_notify” alert before closing the write side
of the connection, unless some other fatal alert has been transmitted. The
other party MUST respond with a “close_notify” alert of its own and close down
the connection immediately, discarding any pending writes. The initiator of the
close need not 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 its peer. 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 “user_canceled” alert that
it is not willing to accept), it SHOULD send a fatal alert to terminate the
connection. Given this, the sending peer 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 party 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 alert is used for all deprotection failures.
This alert is always fatal and should never be observed in
communication between proper implementations (except when messages
were corrupted in the network).
A TLSCiphertext record was received that had a length more than
2^14 + 256 bytes, or a record decrypted to a TLSPlaintext record
with more than 2^14 bytes. This alert is always fatal and
should never be observed in communication between proper
implementations (except when messages were corrupted in the
network).
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 alert 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 alert 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
alert is always fatal.
A valid certificate or PSK was received, but when access control was
applied, the sender decided not to proceed with negotiation. This
alert 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
alert 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 alert 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 alert 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 alert 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 alert is always fatal.
Sent by a server in response to an invalid connection retry attempt
from a client. (see [RFC7507]) This alert is always fatal.
Sent by endpoints that receive a hello message not containing an
extension that is mandatory to send for the offered TLS version.
This message is always fatal.
[[TODO: IANA Considerations.]]
Sent by endpoints receiving any hello message containing an extension
known to be prohibited for inclusion in the given hello message, including
any extensions in a ServerHello not first offered in the corresponding
ClientHello. This alert is always fatal.
Sent by servers when unable to obtain a certificate from a URL
provided by the client via the “client_certificate_url” extension
[RFC6066].
Sent by servers when no server exists identified by the name
provided by the client via the “server_name” extension
[RFC6066].
Sent by clients when an invalid or unacceptable OCSP response is
provided by the server via the “status_request” extension
[RFC6066]. This alert is always fatal.
Sent by servers when a retrieved object does not have the correct hash
provided by the client via the “client_certificate_url” extension
[RFC6066]. This alert is always fatal.
Sent by servers when a PSK cipher suite is selected but no
acceptable PSK identity is provided by the client. Sending this alert
is OPTIONAL; servers MAY instead choose to send a “decrypt_error”
alert to merely indicate an invalid PSK identity.
[[TODO: This doesn’t really make sense with the current PSK
negotiation scheme where the client provides multiple PSKs in
flight 1. https://github.com/tlswg/tls13-spec/issues/230]]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 establish shared secret keying
material.TLS supports three basic key exchange modes:Diffie-Hellman (of both the finite field and elliptic curve
varieties).A pre-shared symmetric key (PSK)A combination of a symmetric key and Diffie-HellmanWhich mode is used depends on the negotiated cipher suite. Conceptually,
the handshake establishes two secrets which are used to derive all the
keys.Ephemeral Secret (ES): A secret which is derived from fresh (EC)DHE
shares for this connection. Keying material derived from ES is
intended to be forward secure (with the exception of pre-shared
key only modes).Static Secret (SS): A secret which may be derived from static or
semi-static keying material, such as a pre-shared key or the
server’s semi-static (EC)DH share.In some cases, as with the DH handshake shown in , these
secrets are the same, but having both allows for a uniform key
derivation scheme for all cipher modes.The basic TLS Handshake for DH is shown in :The first message sent by the client is the ClientHello which contains
a random nonce (ClientHello.random), its offered protocol version,
cipher suite, and extensions, and one or more Diffie-Hellman key
shares in the ClientKeyShare extension .The server processes the ClientHello and determines the appropriate
cryptographic parameters for the connection. It then responds with
the following messages:
indicates the negotiated connection parameters. []
the server’s ephemeral Diffie-Hellman Share which must be in the
same group as one of the shares offered by the client. This
message will be omitted if DH is not in use (i.e., a pure PSK
cipher suite is selected). The ClientKeyShare and ServerKeyShare
are used together to derive the Static Secret and Ephemeral
Secret (in this mode they are the same). []
supplies a configuration for 0-RTT handshakes (see ).
[]
responses to any extensions which are not required in order to
determine the cryptographic parameters. []
the server certificate. This message will be omitted if the
server is not authenticating via a certificates. []
if certificate-based client authentication is desired, the
desired parameters for that certificate. This message will
be omitted if client authentication is not desired.
[[OPEN ISSUE: See https://github.com/tlswg/tls13-spec/issues/184]].
[]
a signature over the entire handshake using the public key
in the Certificate message. This message will be omitted if the
server is not authenticating via a certificate. []
a MAC over the entire handshake computed using the Static Secret.
This message provides key confirmation and
In some modes (see ) it also authenticates the handshake using the
the Static Secret. []Upon receiving the server’s messages, the client responds with his final
flight of messages:
the client’s certificate. This message will be omitted if the
client is not authenticating via a certificates. []
a signature over the entire handshake using the private key corresponding
to the public key in the Certificate message. This message will be omitted if the
client is not authenticating via a certificate. []
a MAC over the entire handshake computed using the Static Secret
and providing key confirmation. []At this point, the handshake is complete, and the client and server may exchange
application layer data. Application data MUST NOT be sent prior to sending the
Finished message. If client authentication is requested, the server MUST NOT
send application data before it receives the client’s Finished.[[TODO: Move this elsewhere?
Note that higher layers should not be overly reliant on whether TLS always
negotiates the strongest possible connection between two endpoints. 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
(i.e., perform a downgrade attack). The TLS 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”
against algorithmic attacks, at least in the year 2015.]]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:[[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 “handshake_failure” or “insufficient_security” fatal alert
(see ).TLS also allows several optimized variants of the basic handshake, as
described below.TLS 1.3 supports a “0-RTT” mode in which the client can send
application data as well as its Certificate and CertificateVerify (if
client authentication is requested) on its first flight, thus reducing
handshake latency. In order to enable this functionality, the server
provides a ServerConfiguration message containing a long-term (EC)DH
share. On future connections to the same server, the client can use
that share to encrypt the first-flight data.Note: because sequence numbers continue to increment between the
initial (early) application data and the application data sent
after the handshake has completed, an attacker cannot remove
early application data messages.IMPORTANT NOTE: The security properties for 0-RTT data (regardless of
the cipher suite) are weaker than those for other kinds of TLS data.
Specifically.This data is not forward secure, because it is encrypted solely
with the server’s semi-static (EC)DH share.There are no guarantees of non-replay between connections.
Unless the server takes special measures outside those provided by TLS (See
), the server has no guarantee that the same
0-RTT data was not transmitted on multiple 0-RTT connections.
This is especially relevant if the data is authenticated either
with TLS client authentication or inside the application layer
protocol. However, 0-RTT data cannot be duplicated within a connection (i.e., the server
will not process the same data twice for the same connection) and also
cannot be sent as if it were ordinary TLS data.If the server key is compromised, and client authentication is used, then
the attacker can impersonate the client to the server (as it knows the
traffic key).Finally, TLS provides a pre-shared key (PSK) mode which allows a
client and server who share an existing secret (e.g., a key
established out of band) to establish a connection authenticated by
that key. PSKs can also be established in a previous session and
then reused (“session resumption”). Once a handshake has completed, the server can
send the client a PSK identity which corresponds to a key derived from
the initial handshake (See ). The client
can then use that PSK identity in future handshakes to negotiate use
of the PSK; if the server accepts it, then the security context of the
original connection is tied to the new connection. In TLS 1.2 and
below, this functionality was provided by “session resumption” and
“session tickets” . Both mechanisms are obsoleted in TLS
1.3.PSK cipher suites can either use PSK in combination with
an (EC)DHE exchange in order to provide forward secrecy in combination
with shared keys, or can use PSKs alone, at the cost of losing forward
secrecy. shows a pair of handshakes in which the first establishes
a PSK and the second uses it:As the server is authenticating via a PSK, it does not
send a Certificate or a CertificateVerify. PSK-based resumption cannot
be used to provide a new ServerConfiguration. Note that the client supplies a ClientKeyShare to the server as well, which
allows the server to decline resumption and fall back to 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 or TLSCiphertext structures, which are
processed and transmitted as specified by the current active session state.The TLS Handshake Protocol messages are presented below in the order they
MUST be sent; sending handshake messages in an unexpected order
results in an “unexpected_message” 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) except including a new ClientKeyShare.
[[OPEN ISSUE: New random values? See:
https://github.com/tlswg/tls13-spec/issues/185]]
If a server receives a ClientHello at any other time, it MUST send
a fatal “unexpected_message” alert and close the connection.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.
See for additional information.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.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 hash to be used with HKDF. 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.
Versions of TLS prior to TLS 1.3 supported a session resumption
feature which has been merged with Pre-Shared Keys in this version
(see ).
This field MUST be ignored by a server negotiating TLS 1.3 and
should be set as a zero length vector (i.e., a single zero byte
length field) by clients which do not have a cached session_id
set by a pre-TLS 1.3 server.
This is a list of the cryptographic options supported by the
client, with the client’s first preference first.
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: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 extension was acceptable. If the client proposed groups are not
acceptable by the server, it will respond with a “handshake_failure” 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.
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.
[[TODO: interaction with PSK.]]
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:Servers send this message in response to a ClientHello
message when it was able to find an acceptable set of algorithms and
groups that are mutually supported, but
the client’s ClientKeyShare 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 pair.Upon receipt of a HelloRetryRequest, the client MUST first verify
that the “selected_group” field does not identify a group which
was not in the original ClientHello. If it was present, then
the client MUST abort the handshake with a fatal “handshake_failure”
alert. Clients SHOULD also abort with “handshake_failure” in response to any second
HelloRetryRequest which was sent in the same connection (i.e.,
where the ClientHello was itself in response to a HelloRetryRequest).Otherwise, the client MUST send a ClientHello with a new
ClientKeyShare extension to the server. The ClientKeyShare MUST append
a new ClientKeyShareOffer which is consistent with the
“selected_group” field to the groups in the original ClientKeyShare.Upon re-sending the ClientHello 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 or HelloRetryRequest
unless the same extension type appeared in the corresponding ClientHello.
If a client receives an extension type in ServerHello or HelloRetryRequest
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 or 0-RTT mode. 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.
[[TODO: update this and the previous paragraph to cover PSK-based 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.All clients MUST send a valid “signature_algorithms” extension containing
at least one supported SignatureAndHashAlgorithm when offering any
certificate authenticated cipher suites.
Servers MUST NOT negotiate use of a certificate authenticated cipher suite
unless the client supplies a supported SignatureAndHashAlgorithm.
If the extension is not provided and no alternative cipher suite is available,
the server MUST close the connection with a fatal “missing_extension” alert.
(see )Servers MUST NOT send this extension. TLS servers MUST support receiving
this extension. Clients receiving this extension MUST respond with an
“unsupported_extension” alert and close the connection.The “extension_data” field of this extension contains a
“supported_signature_algorithms” value:[[TODO: IANA considerations for new SignatureAlgorithm 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., ECDSA with SHA-256, but not SHA-384), algorithms here
are listed in pairs.
This field indicates the hash algorithms which may be used. The
values indicate support for unhashed data, SHA-1, 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. Previous versions of TLS
supported MD5 and SHA-1. These algorithms are now deprecated and
MUST NOT be offered by TLS 1.3 implementations. SHA-1 SHOULD NOT be
offered, however clients willing to negotiate use of TLS 1.2 MAY
offer support for SHA-1 for backwards compatibility with old
servers.
This field indicates the signature algorithm that may be used.
The values indicate RSASSA-PKCS1-v1_5 ,
DSA , ECDSA , and RSASSA-PSS
respectively. Because all RSA signatures used in
signed TLS handshake messages (see ),
as opposed to those in certificates, are RSASSA-PSS, the “rsa”
value refers solely to signatures which appear in certificates.
The use of DSA and anonymous is deprecated. Previous versions
of TLS supported DSA. DSA is deprecated as of TLS 1.3 and
SHOULD NOT be offered or negotiated by any implementation.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.Clients offering support for SHA-1 for TLS 1.2 servers MUST do so by listing
those hash/signature pairs as the lowest priority (listed after all other
pairs in the supported_signature_algorithms vector). TLS 1.3 servers MUST NOT
offer a SHA-1 signed certificate unless no valid certificate chain can be
produced without it (see ).Note: TLS 1.3 servers MAY receive TLS 1.2 ClientHellos which do not contain
this extension. If those servers are willing to negotiate TLS 1.2, they MUST
behave in accordance with the requirements of .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 .All clients MUST send a valid “supported_groups” extension containing
at least one group for each ephemeral key exchange algorithm (currently
DHE and ECDHE) for which it offers a cipher suite.
Servers MUST NOT negotiate use of a DHE or ECDHE cipher suites
unless the client supplies a supported NamedGroup.
If the extension is not provided and no alternative cipher suite is available,
the server MUST close the connection with a fatal “missing_extension” alert.
(see )
If the extension is provided, but no compatible group is offered, 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.Servers MUST NOT send this extension. TLS servers MUST support receiving
this extension. Clients receiving this extension MUST respond with an
“unsupported_extension” alert and close the connection.The “extension_data” field of this extension contains a
“NamedGroupList” value:
Indicates support of the corresponding named curve.
Note that some curves are also recommended in ANSI
X9.62 and FIPS 186-4 .
Values 0xFE00 through 0xFEFF are reserved for private use.
Indicates support of the corresponding finite field
group, defined in .
Values 0x01FC through 0x01FF are reserved for private use.Items in named_curve_list are ordered according to the client’s
preferences (most preferred choice first).As an example, a client that only supports secp256r1 (aka NIST P-256;
value 23 = 0x0017) and secp384r1 (aka NIST P-384; value 24 = 0x0018)
and prefers to use secp256r1 would include a TLS extension consisting
of the following octets. Note that the first two octets indicate the
extension type (Supported Group Extension):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 ServerKeyShare message. The server
must consider the supported groups in both cases.[[TODO: IANA Considerations.]]The “client_key_share” extension contains the client’s cryptographic
parameters for zero or more non-PSK key establishment methods (currently
DHE or ECDHE).All clients MUST send a valid “client_key_share” extension when offering
any DHE or ECDHE cipher suites.
Servers MUST NOT negotiate use of a DHE or ECDHE cipher suites
unless the client supplies a (possibly empty) “client_key_share” extension.
If the extension is not provided and no alternative cipher suite is available,
the server MUST close the connection with a fatal “missing_extension” alert.
(see )Servers MUST NOT send this extension. TLS servers MUST support receiving
this extension. Clients receiving this extension MUST respond with an
“unsupported_extension” alert and close the connection.[[OPEN ISSUE: Would it
be better to omit it if it’s empty?.
https://github.com/tlswg/tls13-spec/issues/190]]
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.The “extension_data” field of this extension contains a
“ClientKeyShare” value:
A list of ClientKeyShareOffer values in descending order of
client preference.Clients may offer an arbitrary number of ClientKeyShareOffer
values, each representing a single set of key exchange 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 “client_key_share” extension 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.]]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.]]The “pre_shared_key” extension is used to indicate the identity of the
pre-shared key to be used with a given handshake in association
with a PSK or (EC)DHE-PSK cipher suite (see for background).All clients MUST send a valid “pre_shared_key” extension when offering
any PSK cipher suites.
Servers MUST NOT negotiate use of a PSK cipher suite
unless the client supplies a “pre_shared_key” extension.
If the extension is not provided and no alternative cipher suite is available,
the server MUST close the connection with a fatal “missing_extension” alert.
(see )The “extension_data” field of this extension contains a
“PreSharedKeyExtension” value:
An opaque label for the pre-shared key.If no suitable identity is provided, the server MUST NOT negotiate
a PSK cipher suite and MAY respond with an “unknown_psk_identity”
alert message. Sending this alert is OPTIONAL; servers MAY instead
choose to send a “decrypt_error” alert to merely indicate an
invalid PSK identity or instead negotiate use of a non-PSK cipher
suite, if available.If the server selects a PSK cipher suite, it MUST send a
PreSharedKeyExtension with the identity that it selected.
The client MUST verify that the server has selected one of
the identities that the client supplied. If any other identity
is returned, the client MUST generate a fatal
“unknown_psk_identity” alert and close the connection.In cases where TLS clients have previously interacted with the
server and the server has supplied a ServerConfiguration , the client
can send application data and its Certificate/CertificateVerify
messages (if client authentication is required). If the client
opts to do so, it MUST supply an Early Data Indication
extension.The “extension_data” field of this extension contains an
“EarlyDataIndication” value:
The label for the configuration in question.
The cipher suite which the client is using to encrypt the early data.
The extensions required to define the cryptographic configuration
for the clients early data (see below for details).
An optional context value that can be used for anti-replay
(see below).
The type of early data that is being sent. “client_authentication”
means that only handshake data is being sent. “early_data”
means that only data is being sent. “client_authentication_and_data”
means that both are being sent.The client specifies the cryptographic configuration for the 0-RTT
data using the “configuration”, “cipher_suite”, and “extensions”
values. For configurations received in-band (in a previous TLS connection)
the client MUST:Send the same cryptographic determining parameters (Section
) with the previous
connection. If a 0-RTT handshake is being used with a PSK
that was negotiated via a non-PSK handshake,
then the client MUST use the same symmetric cipher parameters
as were negotiated on that handshake but with a PSK cipher
suite.Indicate the same parameters as the server indicated in that connection.If TLS client authentication is being used, then either
“early_handshake” or “early_handshake_and_data” MUST be indicated in
order to send the client authentication data on the first flight. In
either case, the client Certificate and CertificateVerify (assuming
that the Certificate is non-empty) MUST be sent on the first flight.
A server which receives an initial flight with only “early_data” and
which expects certificate-based client authentication MUST NOT
accept early data.In order to allow servers to readily distinguish between messages sent
in the first flight and in the second flight (in cases where the
server does not accept the EarlyDataIndication extension), the client MUST
send the handshake messages as content type
“early_handshake”. A server which does not accept the extension
proceeds by skipping all records after the ClientHello and until
the next client message of type “handshake”.
[[OPEN ISSUE: This needs replacement when we add encrypted content
types.]]A server which receives an EarlyDataIndication extension
can behave in one of two ways:Ignore the extension and return no response. This indicates
that the server has ignored any early data and an ordinary
1-RTT handshake is required.Return an empty extension, indicating that it intends to
process the early data. It is not possible for the server
to accept only a subset of the early data messages.Prior to accepting the EarlyDataIndication extension, the server
MUST perform the following checks:The configuration_id matches a known server configuration.The client’s cryptographic determining parameters match the
parameters that the server has negotiated based on the
rest of the ClientHello.If any of these checks fail, the server MUST NOT respond
with the extension and must discard all the remaining first
flight data (thus falling back to 1-RTT).[[TODO: How does the client behave if the indication is rejected.]][[OPEN ISSUE: This just specifies the signaling for 0-RTT but
not the the 0-RTT cryptographic transforms, including:What is in the handshake hash (including potentially some
speculative data from the server).What is signed in the client’s CertificateVerify.Whether we really want the Finished to not include the
server’s data at all.What’s here now needs a lot of cleanup before it is clear
and correct.]]In order to allow the server to decrypt 0-RTT data, the client
needs to provide enough information to allow the server to
decrypt the traffic without negotiation. This is accomplished
by having the client indicate the “cryptographic determining
parameters” in its ClientHello, which are necessary to decrypt
the client’s packets. This includes the following
values:The cipher suite identifier.If PSK is being used, the server’s version of the
PreSharedKey extension (indicating the PSK the client is
using).[[TODO: Are there other extensions we need? I’ve gone over the list and I
don’t see any, but…]]
[[TODO: This should be the same list as what you need for !EncryptedExtensions.
Consolidate this list.]]As noted in , TLS does not provide any
inter-connection mechanism for replay protection for data sent by the
client in the first flight. As a special case, implementations where
the server configuration, is delivered out of band (as has been
proposed for DTLS-SRTP ), MAY use a unique server
configuration identifier for each connection, thus preventing
replay. Implementations are responsible for ensuring uniqueness of the
identifier in this case.When this message will be sent:This message will be sent immediately after the ServerHello message if
the client has provided a ClientKeyShare extension 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 a shared secret secret: a Diffie-Hellman public key with which the
client can complete a key exchange (with the result being the shared 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
(), 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. This is the first message that is encrypted under keys
derived from ES.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 PSK).
This message will always immediately follow the EncryptedExtensions
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 SHOULD directly certify one preceding it. Because
certificate validation requires that trust anchors be distributed
independently, a certificate that specifies a
trust anchor MAY be omitted from the chain, provided that
supported peers are known to possess any omitted certificates.Note: Prior to TLS 1.3, “certificate_list” ordering required each certificate
to certify the one immediately preceding it,
however some implementations allowed some flexibility. Servers sometimes send
both a current and deprecated intermediate for transitional purposes, and others
are simply configured incorrectly, but these cases can nonetheless be validated
properly. For maximum compatibility, all implementations SHOULD be prepared to
handle potentially extraneous certificates and arbitrary orderings from any TLS
version, with the exception of the end-entity certificate which MUST be first.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 server’s 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. As servers MAY require the presence of the server_name
extension, clients SHOULD send this extension.All certificates provided by the server MUST be signed by a
hash/signature algorithm pair that appears in the “signature_algorithms”
extension provided by the client, if they are able to provide such
a chain (see ).
If the server cannot produce a certificate chain that is signed only via the
indicated supported pairs, then it SHOULD continue the handshake by sending
the client a certificate chain of its choice that may include algorithms
that are not known to be supported by the client. This fallback chain MAY
use the deprecated SHA-1 hash algorithm.
If the client cannot construct an acceptable chain using the provided
certificates and decides to abort the handshake, then it MUST send an
“unsupported_certificate” alert message and close the connection.Any endpoint receiving any certificate signed using any signature algorithm
using an MD5 hash MUST send a “bad_certificate” alert message and close
the connection.As SHA-1 and SHA-224 are deprecated, support for them is NOT RECOMMENDED.
Endpoints that reject chains due to use of a deprecated hash MUST send
a fatal “bad_certificate” alert message before closing the connection.
All servers are RECOMMENDED to transition to SHA-256 or better as soon
as possible to maintain interoperability with implementations
currently in the process of phasing out SHA-1 support.Note 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 ECDSA 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.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.New ClientCertificateType values are assigned by IANA as described in
.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 a server configuration which
the client can use in future to skip handshake negotiation and
(optionally) to allow 0-RTT handshakes. The ServerConfiguration
message is sent as the last message before the CertificateVerify.Structure of this Message:
The configuration identifier to be used in 0-RTT mode.
The group for the long-term DH key that is being established
for this configuration.
The last time when this configuration is expected to be valid
(in seconds since the Unix epoch). Servers MUST NOT use any value
more than 604800 seconds (7 days) in the future. Clients MUST
not cache configurations for longer than 7 days, regardless of
the expiration_date. [[OPEN ISSUE: Is this the right value?
The idea is just to minimize exposure.]]
The long-term DH key that is being established for this configuration.
The type of 0-RTT handshake that this configuration is to be used
for (see ). If “client_authentication”
or “client_authentication_and_data”, then the client should select
the certificate for future handshakes based on the CertificateRequest
parameters supplied in this handshake. The server MUST NOT send
either of these two options unless it also requested a certificate
on this handshake.
[[OPEN ISSUE: Should we relax this?]]
This field is a placeholder for future extensions to the
ServerConfiguration format.The semantics of this message are to establish a shared state between
the client and server for use with the “known_configuration” extension
with the key specified in key and with the handshake parameters negotiated
by this handshake.When the ServerConfiguration message is sent, the server MUST also
send a Certificate message and a CertificateVerify message, even
if the “known_configuration” extension was used for this handshake,
thus requiring a signature over the configuration before it can
be used by the client. Clients MUST not rely on the
ServerConfiguration message until successfully receiving and
processing the server’s Certificate, CertificateVerify, and
Finished. If there is a failure in processing those messages, the
client MUST discard the ServerConfiguration.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 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:Where session_hash is as described in and
includes the 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. The digest MUST be the
Hash used as the basis for HKDF.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.The signature algorithm and hash algorithm MUST be a pair offered in the
client’s “signature_algorithms” extension unless no valid certificate chain can be
produced without unsupported algorithms (see ). Note that
there is a possibility for inconsistencies here. For instance, the client might
offer ECDHE_ECDSA key exchange but omit any ECDSA 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.
RSA signatures MUST be based on RSASSA-PSS, regardless of whether
RSASSA-PKCS-v1_5 appears in “signature_algorithms”.
SHA-1 MUST NOT be used in any signatures in CertificateVerify,
regardless of whether SHA-1 appears in “signature_algorithms”.When this message will be sent:The Server’s Finished message is the final message sent by the
server and is essential for providing authentication of the server
side of the handshake and computed keys.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 ephemeral secret (see
).Structure of this message:The verify_data value is computed as follows:
HMAC(finished_secret, finished_label + ‘\0’ + handshake_hash)
where HMAC uses the Hash algorithm for the handshake.
See for the definition of handshake_hash.
For Finished messages sent by the client, the string
“client finished”. For Finished messages sent by the server,
the string “server finished”.In previous versions of TLS, the verify_data was always 12 octets long. In
the current version of TLS, it is the size of the HMAC output for the
Hash used for the handshake.Note: Alerts and any other record types are not handshake messages
and are not included in the hash computations. Also, HelloRequest
messages and the Finished message 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). 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.
RSA signatures MUST be based on RSASSA-PSS, regardless of whether
RSASSA-PKCS-v1_5 appears in “signature_algorithms”. SHA-1 MUST NOT be used
in any signatures in CertificateVerify, regardless of whether
SHA-1 appears in “signature_algorithms”.After the server has received the client Finished message, it MAY send
a NewSessionTicket message. This message MUST be sent before the server
sends any application data traffic, and is encrypted under the application
traffic key. This message creates a pre-shared key
(PSK) binding between the resumption master secret and the ticket
label. The client MAY use this PSK for future handshakes by including
it in the “pre_shared_key” extension in its ClientHello
() and supplying a suitable PSK cipher
suite.
Indicates the lifetime
in seconds as a 32-bit unsigned integer in network byte order from
the time of ticket issuance. A value of zero is reserved to indicate
that the lifetime of the ticket is unspecified.
The value of the ticket to be used as the PSK identifier.The ticket lifetime hint is informative only.
A client SHOULD delete the ticket and associated
state when the time expires. It MAY delete the ticket earlier based
on local policy. A server MAY treat a ticket as valid for a shorter
or longer period of time than what is stated in the
ticket_lifetime_hint.The ticket itself is an opaque label. It MAY either be a database
lookup key or a self-encrypted and self-authenticated value. Section
4 of describes a recommended ticket construction mechanism.[[TODO: Should we require that tickets be bound to the existing
symmetric cipher suite. See the TODO above about early_data and
PSK.??]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
exchange, 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 key schedule.The TLS handshake establishes secret keying material which is then used
to protect traffic. This keying material is derived from the two
input secret values: Static Secret (SS) and Ephemeral Secret (ES).The exact source of each of these secrets depends on the operational
mode (DHE, ECDHE, PSK, etc.) and is summarized in the table below:These shared secret values are used to generate cryptographic keys as
shown below.The derivation process is as follows, where L denotes the length of
the underlying hash function for HKDF . SS and ES denote
the sources from the table above. Whilst SS and ES may be the same
in some cases, the extracted xSS and xES will not.The traffic keys are computed from xSS, xES, and the master_secret
as described in below.Note: although the steps above are phrased as individual HKDF-Extract
and HKDF-Expand operations, because each HKDF-Expand operation is
paired with an HKDF-Extract, it is possible to implement this key
schedule with a black-box HKDF API, albeit at some loss of efficiency
as some HKDF-Extract operations will be repeated.[[OPEN ISSUE: This needs to be revised. Most likely we’ll extract each
key component separately. 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 traffic key computation takes four input values and returns a key block
of sufficient size to produce the needed traffic keys:A secret valueA string label that indicates the purpose of keys being generated.The current handshake hash.The total length in octets of the key block.The keying material is computed using:The key_block is partitioned as follows:The following table describes the inputs to the key calculation for
each class of traffic keys:
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 in plaintext form (even if they
were encrypted on the wire).
When 0-RTT is in use ()
this contains the concatenation of the ServerConfiguration and Certificate
messages from the handshake where the configuration was established (including the
type and length fields). Note that
this requires the client and server to memorize these values.This final value of the handshake hash is referred to as the “session
hash” because it contains all the handshake messages required to
establish the session. 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 shared secret, and is used in the key schedule as
specified above. Leading bytes of Z that contain all zero bits are stripped
before it is used as the input to HKDF.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 shared 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 this secret for anything
other than for computing other secrets.)In the absence of an application profile standard specifying otherwise, a
TLS-compliant application MUST implement the following cipher suites:These cipher suites MUST support both digital signatures and key exchange
with secp256r1 (NIST P-256) and SHOULD support key exchange with X25519
.A TLS-compliant application SHOULD implement the following cipher suites:In the absence of an application profile standard specifying otherwise, a
TLS-compliant application MUST implement the following TLS extensions:Signature Algorithms (“signature_algorithms”; )Negotiated Groups (“supported_groups”; )Client Key Share (“client_key_share”; )Pre-Shared Key Extension (“pre_shared_key”; )Server Name Indication (“server_name”; Section 3 of )All implementations MUST send and use these extensions when offering
applicable cipher suites:“signature_algorithms” is REQUIRED for certificate authenticated cipher suites“supported_groups” and “client_key_share” are REQUIRED for DHE or ECDHE cipher suites“pre_shared_key” is REQUIRED for PSK cipher suitesWhen negotiating use of applicable cipher suites, endpoints MUST abort the
connection with a “missing_extension” alert if the required extension was
not provided. Any endpoint that receives any invalid combination of cipher
suites and extensions MAY abort the connection with a “missing_extension”
alert, regardless of negotiated parameters.Additionally, all implementations MUST support use of the “server_name”
extension with applications capable of using it.
Servers MAY require clients to send a valid “server_name” extension.
Servers requiring this extension SHOULD respond to a ClientHello
lacking a “server_name” extension with a fatal “missing_extension” alert.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 B,
C, and D.[[TODO: Update https://github.com/tlswg/tls13-spec/issues/62]]
[[TODO: Rename “RSA” in TLS SignatureAlgorithm Registry
to RSASSA-PKCS1-v1_5 ]]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 AuthenticationThis 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. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kindKey words for use in RFCs to Indicate Requirement LevelsIn 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. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Guidelines for Writing an IANA Considerations Section in RFCsThis 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. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Public-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.HMAC-based Extract-and-Expand Key Derivation Function (HKDF)This document specifies a simple Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF), which can be used as a building block in various protocols and applications. The key derivation function (KDF) is intended to support a wide range of applications and requirements, and is conservative in its use of cryptographic hash functions. This document is not an Internet Standards Track specification; it is published for informational purposes.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]Addition of the ARIA Cipher Suites to Transport Layer Security (TLS)This document specifies a set of cipher suites for the Transport Layer Security (TLS) protocol to support the ARIA encryption algorithm as a block cipher. This document is not an Internet Standards Track specification; it is published for informational purposes.Addition of the Camellia Cipher Suites to Transport Layer Security (TLS)This document specifies forty-two cipher suites for the Transport Security Layer (TLS) protocol to support the Camellia encryption algorithm as a block cipher. This document is not an Internet Standards Track specification; it is published for informational purposes.AES-CCM Cipher Suites for Transport Layer Security (TLS)This memo describes the use of the Advanced Encryption Standard (AES) in the Counter with Cipher Block Chaining - Message Authentication Code (CBC-MAC) Mode (CCM) of operation within Transport Layer Security (TLS) and Datagram TLS (DTLS) to provide confidentiality and data origin authentication. The AES-CCM algorithm is amenable to compact implementations, making it suitable for constrained environments. [STANDARDS-TRACK]AES-CCM Elliptic Curve Cryptography (ECC) Cipher Suites for TLSThis memo describes the use of the Advanced Encryption Standard (AES) in the Counter and CBC-MAC Mode (CCM) of operation within Transport Layer Security (TLS) to provide confidentiality and data-origin authentication. The AES-CCM algorithm is amenable to compact implementations, making it suitable for constrained environments, while at the same time providing a high level of security. The cipher suites defined in this document use Elliptic Curve Cryptography (ECC) and are advantageous in networks with limited bandwidth.The ChaCha20-Poly1305 AEAD Cipher for Transport Layer SecurityThis document describes the use of the ChaCha stream cipher with Poly1305 in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) protocols.Elliptic Curves for SecurityThis memo specifies two elliptic curves over prime fields that offer high practical security in cryptographic applications, including Transport Layer Security (TLS). These curves are intended to operate at the ~128-bit and ~224-bit security level, respectively, and are generated deterministically based on a list of required properties.Specification for the Advanced Encryption Standard (AES)National Institute of Standards and TechnologySecure 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 ProtocolThe MD5 Message-Digest AlgorithmThis document describes the MD5 message-digest algorithm. The algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input. This memo provides information for the Internet community. It does not specify an Internet standard.Defending Against Sequence Number AttacksIP 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. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind.The TLS Protocol Version 1.0This 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. [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.Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)This document specifies three sets of new ciphersuites for the Transport Layer Security (TLS) protocol to support authentication based on pre-shared keys (PSKs). These pre-shared keys are symmetric keys, shared in advance among the communicating parties. The first set of ciphersuites uses only symmetric key operations for authentication. The second set uses a Diffie-Hellman exchange authenticated with a pre-shared key, and the third set combines public key authentication of the server with pre-shared key authentication of the client. [STANDARDS-TRACK]IP Authentication HeaderThis document describes an updated version of the IP Authentication Header (AH), which is designed to provide authentication services in IPv4 and IPv6. This document obsoletes RFC 2402 (November 1998). [STANDARDS-TRACK]IP Encapsulating Security Payload (ESP)This document describes an updated version of the Encapsulating Security Payload (ESP) protocol, which is designed to provide a mix of security services in IPv4 and IPv6. ESP is used to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality. This document obsoletes RFC 2406 (November 1998). [STANDARDS-TRACK]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.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]Transport Layer Security (TLS) Session Resumption without Server-Side StateThis document describes a mechanism that enables the Transport Layer Security (TLS) server to resume sessions and avoid keeping per-client session state. The TLS server encapsulates the session state into a ticket and forwards it to the client. The client can subsequently resume a session using the obtained ticket. This document obsoletes RFC 4507. [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]The Transport Layer Security (TLS) Protocol Version 1.2This document specifies Version 1.2 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]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]Framework for Establishing a Secure Real-time Transport Protocol (SRTP) Security Context Using Datagram Transport Layer Security (DTLS)This document specifies how to use the Session Initiation Protocol (SIP) to establish a Secure Real-time Transport Protocol (SRTP) security context using the Datagram Transport Layer Security (DTLS) protocol. It describes a mechanism of transporting a fingerprint attribute in the Session Description Protocol (SDP) that identifies the key that will be presented during the DTLS handshake. The key exchange travels along the media path as opposed to the signaling path. The SIP Identity mechanism can be used to protect the integrity of the fingerprint attribute from modification by intermediate proxies. [STANDARDS-TRACK]Channel Bindings for TLSThis document defines three channel binding types for Transport Layer Security (TLS), tls-unique, tls-server-end-point, and tls-unique-for-telnet, in accordance with RFC 5056 (On Channel Binding).Note that based on implementation experience, this document changes the original definition of 'tls-unique' channel binding type in the channel binding type IANA registry. [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]Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)This document specifies a new certificate type and two TLS extensions for exchanging raw public keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS). The new certificate type allows raw public keys to be used for authentication.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.TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade AttacksThis document defines a Signaling Cipher Suite Value (SCSV) that prevents protocol downgrade attacks on the Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) protocols. It updates RFCs 2246, 4346, 4347, 5246, and 6347. Server update considerations are included.Deprecating Secure Sockets Layer Version 3.0The Secure Sockets Layer version 3.0 (SSLv3), as specified in RFC 6101, is not sufficiently secure. This document requires that SSLv3 not be used. The replacement versions, in particular, Transport Layer Security (TLS) 1.2 (RFC 5246), 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.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.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].Security of CBC Ciphersuites in SSL/TLS: Problems and CountermeasuresDigital Signature Standard, version 4National Institute of Standards and Technology, U.S. Department of CommercePublic 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. Values listed as
_RESERVED were used in previous versions of TLS and are listed here
for completeness. TLS 1.3 implementations MUST NOT send them but
may receive them from older TLS implementations.Values within “obsolete_RESERVED” ranges were used in previous versions
of TLS and MUST NOT be offered or negotiated by TLS 1.3 implementations.
The obsolete curves have various known/theoretical weaknesses or have
had very little usage, in some cases only due to unintentional
server configuration issues. They are no longer considered appropriate
for general use and should be assumed to be potentially unsafe. The set
of curves specified here is sufficient for interoperability with all
currently deployed and properly configured TLS implementations.A cipher suite defines a cipher specification supported in TLS and negotiated
via hello messages in the TLS handshake.
Cipher suite names follow a general naming convention composed of a series
of component algorithm names separated by underscores:The “CIPHER” component commonly has sub-components used to designate
the cipher name, bits, and mode, if applicable. For example, “AES_256_GCM”
represents 256-bit AES in the GCM mode of operation. Cipher suite names that
lack a “HASH” value that are defined for use with TLS 1.2 or later use the
SHA-256 hash algorithm by default.The primary key exchange algorithm used in TLS is Ephemeral Diffie-Hellman
. The finite field based version is denoted “DHE” and the elliptic
curve based version is denoted “ECDHE”. Prior versions of TLS supported
non-ephemeral key exchanges, however these are not supported by TLS 1.3.See the definitions of each cipher suite in its specification document for
the full details of each combination of algorithms that is specified.The following is a list of standards track server-authenticated (and optionally
client-authenticated) cipher suites which are currently available in TLS 1.3:[[TODO: CHACHA20_POLY1305 cipher suite IDs are TBD.]]The following is a list of non-standards track server-authenticated (and optionally
client-authenticated) cipher suites which are currently available in TLS 1.3:ECDHE AES GCM is not yet standards track, however it is already widely deployed.Note: In the case of the CCM mode of AES, two variations exist: “CCM_8” which
uses an 8-bit authentication tag and “CCM” which uses a 16-bit authentication
tag. Both use the default hash, SHA-256.All cipher suites in this section are specified for use with both TLS 1.2
and TLS 1.3, as well as the corresponding versions of DTLS.
(see )New cipher suite values are assigned by IANA as described in
.Previous versions of TLS offered explicitly unauthenticated cipher suites
base on anonymous Diffie-Hellman. These cipher suites have been deprecated
in TLS 1.3. However, it is still possible to negotiate cipher suites
that do not provide verifiable server authentication by serveral methods,
including:Raw public keys .Using a public key contained in a certificate but without
validation of the certificate chain or any of its contents.Either technique used alone is are vulnerable to man-in-the-middle attacks
and therefore unsafe for general use. However, it is also possible to
bind such connections to an external authentication mechanism via
out-of-band validation of the server’s public key, trust on first
use, or channel bindings . [[NOTE: TLS 1.3 needs a new
channel binding definition that has not yet been defined.]]
If no such mechanism is used, then the connection has no protection
against active man-in-the-middle attack; applications MUST NOT use TLS
in such a way absent explicit configuration or a specific application
profile.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-256, 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. 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.
See also .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, EXPORT ciphers, and
MD5 (via the Signature Algorithms extension) 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
)?When processing the plaintext fragment produced by AEAD-Decrypt and
scanning from the end for the ContentType, do you avoid scanning
past the start of the cleartext in the event that the peer has sent
a malformed plaintext of all-zeros?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 ECDSA “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 use of TLS 1.0-1.2 SHOULD set the record layer
version number to the negotiated version for the ServerHello and all
records thereafter.For maximum compatibility with previously non-standard behavior and misconfigured
deployments, all implementations SHOULD support validation of certificate chains
based on the expectations in this document, even when handling prior TLS versions’
handshakes. (see )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 use 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 use 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 [RFC7568], 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.Implementations MUST NOT use the Truncated HMAC extension, defined in
Section 7 of [RFC6066], as it is not applicable to AEAD ciphers and has
been shown to be insecure in some scenarios.[[TODO: The entire security analysis needs a rewrite.]]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 TLS 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 TLS 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.[[TODO: Rewrite this because the master_secret is not used this way any
more after Hugo’s changes.]]
The general goal of the key exchange process is to create a master_secret
known to the communicating parties and not to attackers (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 master_secret.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. (See also .)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 static secret, the attacker cannot
repair the Finished messages, so the attack will be discovered.The shared secrets are hashed with the handshake transcript
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