Network Working Group T. Dierks Internet-Draft Independent Obsoletes: 3268, 4346, 4366, 5246 E. Rescorla (if approved) RTFM, Inc. Updates: 4492 (if approved) April 17, 2014 Intended status: Standards Track Expires: October 19, 2014 The Transport Layer Security (TLS) Protocol Version 1.3 draft-ietf-tls-rfc5246-bis-00 Abstract This document specifies Version 1.3 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 19, 2014. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must Dierks & Rescorla Expires October 19, 2014 [Page 1]

Internet-Draft TLS April 2014 include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Requirements Terminology . . . . . . . . . . . . . . . . 5 1.2. Major Differences from TLS 1.1 . . . . . . . . . . . . . 5 2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Goals of This Document . . . . . . . . . . . . . . . . . . . 7 4. Presentation Language . . . . . . . . . . . . . . . . . . . . 7 4.1. Basic Block Size . . . . . . . . . . . . . . . . . . . . 7 4.2. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . 8 4.3. Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.4. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.5. Enumerateds . . . . . . . . . . . . . . . . . . . . . . . 9 4.6. Constructed Types . . . . . . . . . . . . . . . . . . . . 10 4.6.1. Variants . . . . . . . . . . . . . . . . . . . . . . 11 4.7. Cryptographic Attributes . . . . . . . . . . . . . . . . 12 4.8. Constants . . . . . . . . . . . . . . . . . . . . . . . . 14 5. HMAC and the Pseudorandom Function . . . . . . . . . . . . . 14 6. The TLS Record Protocol . . . . . . . . . . . . . . . . . . . 16 6.1. Connection States . . . . . . . . . . . . . . . . . . . . 16 6.2. Record Layer . . . . . . . . . . . . . . . . . . . . . . 19 6.2.1. Fragmentation . . . . . . . . . . . . . . . . . . . . 19 6.2.2. Record Compression and Decompression . . . . . . . . 21 6.2.3. Record Payload Protection . . . . . . . . . . . . . . 21 6.3. Key Calculation . . . . . . . . . . . . . . . . . . . . . 26 7. The TLS Handshaking Protocols . . . . . . . . . . . . . . . . 27 7.1. Change Cipher Spec Protocol . . . . . . . . . . . . . . . 28 7.2. Alert Protocol . . . . . . . . . . . . . . . . . . . . . 28 7.2.1. Closure Alerts . . . . . . . . . . . . . . . . . . . 29 7.2.2. Error Alerts . . . . . . . . . . . . . . . . . . . . 30 7.3. Handshake Protocol Overview . . . . . . . . . . . . . . . 34 7.4. Handshake Protocol . . . . . . . . . . . . . . . . . . . 37 7.4.1. Hello Messages . . . . . . . . . . . . . . . . . . . 38 7.4.2. Server Certificate . . . . . . . . . . . . . . . . . 48 7.4.3. Server Key Exchange Message . . . . . . . . . . . . . 50 7.4.4. Certificate Request . . . . . . . . . . . . . . . . . 53 7.4.5. Server Hello Done . . . . . . . . . . . . . . . . . . 55 7.4.6. Client Certificate . . . . . . . . . . . . . . . . . 56 7.4.7. Client Key Exchange Message . . . . . . . . . . . . . 57 7.4.8. Certificate Verify . . . . . . . . . . . . . . . . . 62 7.4.9. Finished . . . . . . . . . . . . . . . . . . . . . . 63 8. Cryptographic Computations . . . . . . . . . . . . . . . . . 65 8.1. Computing the Master Secret . . . . . . . . . . . . . . . 65 8.1.1. RSA . . . . . . . . . . . . . . . . . . . . . . . . . 65 Dierks & Rescorla Expires October 19, 2014 [Page 2]

Internet-Draft TLS April 2014 1 . Introduction RFC 5246 translated into markdown format with no intentional technical or editorial changes beyond updating the references and minor reformatting introduced by the translation. It is being submitted as-is to create a clearer revision history for future versions. Any errata in TLS 1.2 remain in this version. Thanks to Mark Nottingham for doing the markdown translation. The primary goal of the TLS protocol is to provide privacy and data integrity between two communicating applications. The protocol is composed of two layers: the TLS Record Protocol and the TLS Handshake Protocol. At the lowest level, layered on top of some reliable transport protocol (e.g., TCP [RFC0793]), 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 [AES], RC4 [SCH], etc.). The keys for this symmetric encryption are generated uniquely for each connection and are based on a secret negotiated by another protocol (such as the TLS Handshake Protocol). The Record Protocol can also be used without encryption. - The connection is reliable. Message transport includes a message integrity check using a keyed MAC. Secure hash functions (e.g., SHA-1, etc.) are used for MAC computations. The Record Protocol can operate without a MAC, but is generally only used in this mode while another protocol is using the Record Protocol as a transport for negotiating security parameters. The TLS Record Protocol is used for encapsulation of various higher- level protocols. One such encapsulated protocol, the TLS Handshake Protocol, allows the server and client to authenticate each other and to negotiate an encryption algorithm and cryptographic keys before the application protocol transmits or receives its first byte of data. The TLS Handshake Protocol provides connection security that has three basic properties: - The peer's identity can be authenticated using asymmetric, or public key, cryptography (e.g., RSA [RSA], DSA [DSS], 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 Dierks & Rescorla Expires October 19, 2014 [Page 4]

Internet-Draft TLS April 2014 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. 1.1 . Requirements Terminology RFC 2119 [RFC2119]. 1.2 . Major Differences from TLS 1.1 RFC4346] protocol which contains improved flexibility, particularly for negotiation of cryptographic algorithms. The major changes are: - The MD5/SHA-1 combination in the pseudorandom function (PRF) has been replaced with cipher-suite-specified PRFs. All cipher suites in this document use P_SHA256. - The MD5/SHA-1 combination in the digitally-signed element has been replaced with a single hash. Signed elements now include a field that explicitly specifies the hash algorithm used. - Substantial cleanup to the client's and server's ability to specify which hash and signature algorithms they will accept. Note that this also relaxes some of the constraints on signature and hash algorithms from previous versions of TLS. - Addition of support for authenticated encryption with additional data modes. - TLS Extensions definition and AES Cipher Suites were merged in from external [TLSEXT] and [RFC3268]. - Tighter checking of EncryptedPreMasterSecret version numbers. Dierks & Rescorla Expires October 19, 2014 [Page 5]

Internet-Draft TLS April 2014 - Tightened up a number of requirements. - Verify_data length now depends on the cipher suite (default is still 12). - Cleaned up description of Bleichenbacher/Klima attack defenses. - Alerts MUST now be sent in many cases. - After a certificate_request, if no certificates are available, clients now MUST send an empty certificate list. - TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement cipher suite. - Added HMAC-SHA256 cipher suites. - Removed IDEA and DES cipher suites. They are now deprecated and will be documented in a separate document. - Support for the SSLv2 backward-compatible hello is now a MAY, not a SHOULD, with sending it a SHOULD NOT. Support will probably become a SHOULD NOT in the future. - Added limited "fall-through" to the presentation language to allow multiple case arms to have the same encoding. - Added an Implementation Pitfalls sections - The usual clarifications and editorial work. 2 . Goals Dierks & Rescorla Expires October 19, 2014 [Page 6]

Internet-Draft TLS April 2014 4. 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. 3 . Goals of This Document 4 . Presentation Language RFC4506] 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. 4.1 . Basic Block Size Dierks & Rescorla Expires October 19, 2014 [Page 7]

Internet-Draft TLS April 2014 ... | byte[n-1]; This byte ordering for multi-byte values is the commonplace network byte order or big-endian format. 4.2 . Miscellaneous 4.3 . Vectors Dierks & Rescorla Expires October 19, 2014 [Page 8]

Internet-Draft TLS April 2014 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 Section 4.4). 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). opaque mandatory<300..400>; /* length field is 2 bytes, cannot be empty */ uint16 longer<0..800>; /* zero to 400 16-bit unsigned integers */ 4.4 . Numbers Section 4.1 and are also unsigned. The following numeric types are predefined. uint8 uint16[2]; uint8 uint24[3]; uint8 uint32[4]; uint8 uint64[8]; 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). 4.5 . Enumerateds Dierks & Rescorla Expires October 19, 2014 [Page 9]

Internet-Draft TLS April 2014 maximal defined ordinal value. The following definition would cause one byte to be used to carry fields of type Color. enum { red(3), blue(5), white(7) } 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. enum { sweet(1), sour(2), bitter(4), (32000) } Taste; 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. Color color = Color.blue; /* overspecified, legal */ Color color = blue; /* correct, type implicit */ For enumerateds that are never converted to external representation, the numerical information may be omitted. enum { low, medium, high } Amount; 4.6 . Constructed Types Dierks & Rescorla Expires October 19, 2014 [Page 10]

Internet-Draft TLS April 2014 4.6.1 . Variants Dierks & Rescorla Expires October 19, 2014 [Page 11]

Internet-Draft TLS April 2014 enum { apple, orange, banana } VariantTag; struct { uint16 number; opaque string<0..10>; /* variable length */ } V1; struct { uint32 number; opaque string[10]; /* fixed length */ } V2; struct { select (VariantTag) { /* value of selector is implicit */ case apple: V1; /* VariantBody, tag = apple */ case orange: case banana: V2; /* VariantBody, tag = orange or banana */ } variant_body; /* optional label on variant */ } VariantRecord; 4.7 . Cryptographic Attributes Section 6.1). A digitally-signed element is encoded as a struct DigitallySigned: struct { SignatureAndHashAlgorithm algorithm; opaque signature<0..2^16-1>; } DigitallySigned; The algorithm field specifies the algorithm used (see Section 7.4.1.4.1 for the definition of this field). Note that the introduction of the algorithm field is a change from previous versions. The signature is a digital signature using those algorithms over the contents of the element. The contents themselves do not appear on the wire but are simply calculated. The length of the signature is specified by the signing algorithm and key. Dierks & Rescorla Expires October 19, 2014 [Page 12]

Internet-Draft TLS April 2014 In RSA signing, the opaque vector contains the signature generated using the RSASSA-PKCS1-v1_5 signature scheme defined in [RFC3447]. As discussed in [RFC3447], the DigestInfo MUST be DER-encoded [X680] [X690]. For hash algorithms without parameters (which includes SHA-1), the DigestInfo.AlgorithmIdentifier.parameters field MUST be NULL, but implementations MUST accept both without parameters and with NULL parameters. Note that earlier versions of TLS used a different RSA signature scheme that did not include a DigestInfo encoding. In DSA, the 20 bytes of the SHA-1 hash are run directly through the Digital Signing Algorithm with no additional hashing. This produces two values, r and s. The DSA signature is an opaque vector, as above, the contents of which are the DER encoding of: Dss-Sig-Value ::= SEQUENCE { r INTEGER, s INTEGER } Note: In current terminology, DSA refers to the Digital Signature Algorithm and DSS refers to the NIST standard. In the original SSL and TLS specs, "DSS" was used universally. This document uses "DSA" to refer to the algorithm, "DSS" to refer to the standard, and it uses "DSS" in the code point definitions for historical continuity. In stream cipher encryption, the plaintext is exclusive-ORed with an identical amount of output generated from a cryptographically secure keyed pseudorandom number generator. In block cipher encryption, every block of plaintext encrypts to a block of ciphertext. All block cipher encryption is done in CBC (Cipher Block Chaining) mode, and all items that are block-ciphered will be an exact multiple of the cipher block length. In AEAD encryption, the plaintext is simultaneously encrypted and integrity protected. The input may be of any length, and aead- ciphered output is generally larger than the input in order to accommodate the integrity check value. In public key encryption, a public key algorithm is used to encrypt data in such a way that it can be decrypted only with the matching private key. A public-key-encrypted element is encoded as an opaque vector <0..2^16-1>, where the length is specified by the encryption algorithm and key. RSA encryption is done using the RSAES-PKCS1-v1_5 encryption scheme defined in [RFC3447]. Dierks & Rescorla Expires October 19, 2014 [Page 13]

Internet-Draft TLS April 2014 In the following example stream-ciphered struct { uint8 field1; uint8 field2; digitally-signed opaque { uint8 field3<0..255>; uint8 field4; }; } UserType; The contents of the inner struct (field3 and field4) are used as input for the signature/hash algorithm, and then the entire structure is encrypted with a stream cipher. The length of this structure, in bytes, would be equal to two bytes for field1 and field2, plus two bytes for the signature and hash algorithm, plus two bytes for the length of the signature, plus the length of the output of the signing algorithm. The length of the signature is known because the algorithm and key used for the signing are known prior to encoding or decoding this structure. 4.8 . Constants 5 . HMAC and the Pseudorandom Function RFC2104], which is based on a hash function. Other cipher suites MAY define their own MAC constructions, if needed. In addition, a construction is required to do expansion of secrets Dierks & Rescorla Expires October 19, 2014 [Page 14]

Internet-Draft TLS April 2014 into blocks of data for the purposes of key generation or validation. This pseudorandom function (PRF) takes as input a secret, a seed, and an identifying label and produces an output of arbitrary length. In this section, we define one PRF, based on HMAC. This PRF with the SHA-256 hash function is used for all cipher suites defined in this document and in TLS documents published prior to this document when TLS 1.2 is negotiated. New cipher suites MUST explicitly specify a PRF and, in general, SHOULD use the TLS PRF with SHA-256 or a stronger standard hash function. First, we define a data expansion function, P_hash(secret, data), that uses a single hash function to expand a secret and seed into an arbitrary quantity of output: P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) + HMAC_hash(secret, A(2) + seed) + HMAC_hash(secret, A(3) + seed) + ... where + indicates concatenation. A() is defined as: A(0) = seed A(i) = HMAC_hash(secret, A(i-1)) P_hash can be iterated as many times as necessary to produce the required quantity of data. For example, if P_SHA256 is being used to create 80 bytes of data, it will have to be iterated three times (through A(3)), creating 96 bytes of output data; the last 16 bytes of the final iteration will then be discarded, leaving 80 bytes of output data. TLS's PRF is created by applying P_hash to the secret as: PRF(secret, label, seed) = P_<hash>(secret, label + seed) The label is an ASCII string. It should be included in the exact form it is given without a length byte or trailing null character. For example, the label "slithy toves" would be processed by hashing the following bytes: 73 6C 69 74 68 79 20 74 6F 76 65 73 Dierks & Rescorla Expires October 19, 2014 [Page 15]

Internet-Draft TLS April 2014 6 . The TLS Record Protocol Section 12. Implementations MUST NOT send record types not defined in this document unless negotiated by some extension. If a TLS implementation receives an unexpected record type, it MUST send an unexpected_message alert. Any protocol designed for use over TLS must be carefully designed to deal with all possible attacks against it. As a practical matter, this means that the protocol designer must be aware of what security properties TLS does and does not provide and cannot safely rely on the latter. Note in particular that type and length of a record are not protected by encryption. If this information is itself sensitive, application designers may wish to take steps (padding, cover traffic) to minimize information leakage. 6.1 . Connection States Dierks & Rescorla Expires October 19, 2014 [Page 16]

Internet-Draft TLS April 2014 pending state is then reinitialized to an empty state. It is illegal to make a state that has not been initialized with security parameters a current state. The initial current state always specifies that no encryption, compression, or MAC will be used. The security parameters for a TLS Connection read and write state are set by providing the following values: connection end Whether this entity is considered the "client" or the "server" in this connection. PRF algorithm An algorithm used to generate keys from the master secret (see Section 5 and Section 6.3). bulk encryption algorithm An algorithm to be used for bulk encryption. This specification includes the key size of this algorithm, whether it is a block, stream, or AEAD cipher, the block size of the cipher (if appropriate), and the lengths of explicit and implicit initialization vectors (or nonces). MAC algorithm An algorithm to be used for message authentication. This specification includes the size of the value returned by the MAC algorithm. compression algorithm An algorithm to be used for data compression. This specification must include all information the algorithm requires to do compression. master secret A 48-byte secret shared between the two peers in the connection. client random A 32-byte value provided by the client. server random A 32-byte value provided by the server. These parameters are defined in the presentation language as: Dierks & Rescorla Expires October 19, 2014 [Page 17]

Internet-Draft TLS April 2014 enum { server, client } ConnectionEnd; enum { tls_prf_sha256 } PRFAlgorithm; enum { null, rc4, 3des, aes } BulkCipherAlgorithm; enum { stream, block, aead } CipherType; enum { null, hmac_md5, hmac_sha1, hmac_sha256, hmac_sha384, hmac_sha512} MACAlgorithm; enum { null(0), (255) } CompressionMethod; /* The algorithms specified in CompressionMethod, PRFAlgorithm, BulkCipherAlgorithm, and MACAlgorithm may be added to. */ struct { ConnectionEnd entity; PRFAlgorithm prf_algorithm; BulkCipherAlgorithm bulk_cipher_algorithm; CipherType cipher_type; uint8 enc_key_length; uint8 block_length; uint8 fixed_iv_length; uint8 record_iv_length; MACAlgorithm mac_algorithm; uint8 mac_length; uint8 mac_key_length; CompressionMethod compression_algorithm; opaque master_secret[48]; opaque client_random[32]; opaque server_random[32]; } SecurityParameters; The record layer will use the security parameters to generate the following six items (some of which are not required by all ciphers, and are thus empty): client write MAC key server write MAC key client write encryption key server write encryption key client write IV server write IV The client write parameters are used by the server when receiving and processing records and vice versa. The algorithm used for generating Dierks & Rescorla Expires October 19, 2014 [Page 18]

Internet-Draft TLS April 2014 these items from the security parameters is described in Section 6.3 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: compression state The current state of the compression algorithm. cipher state The current state of the encryption algorithm. This will consist of the scheduled key for that connection. For stream ciphers, this will also contain whatever state information is necessary to allow the stream to continue to encrypt or decrypt data. MAC key The MAC key for this connection, as generated above. sequence number Each connection state contains a sequence number, which is maintained separately for read and write states. The sequence number MUST be set to zero whenever a connection state is made the active state. Sequence numbers are of type uint64 and may not exceed 2^64-1. Sequence numbers do not wrap. If a TLS implementation would need to wrap a sequence number, it must renegotiate instead. A sequence number is incremented after each record: specifically, the first record transmitted under a particular connection state MUST use sequence number 0. 6.2 . Record Layer 6.2.1 . Fragmentation Dierks & Rescorla Expires October 19, 2014 [Page 19]

Internet-Draft TLS April 2014 struct { uint8 major; uint8 minor; } ProtocolVersion; enum { change_cipher_spec(20), alert(21), handshake(22), application_data(23), (255) } ContentType; struct { ContentType type; ProtocolVersion version; uint16 length; opaque fragment[TLSPlaintext.length]; } TLSPlaintext; type The higher-level protocol used to process the enclosed fragment. version The version of the protocol being employed. This document describes TLS Version 1.2, which uses the version { 3, 3 }. The version value 3.3 is historical, deriving from the use of {3, 1} for TLS 1.0. (See Appendix A.1.) Note that a client that supports multiple versions of TLS may not know what version will be employed before it receives the ServerHello. See Appendix E for discussion about what record layer version number should be employed for ClientHello. length The length (in bytes) of the following TLSPlaintext.fragment. The length MUST NOT exceed 2^14. fragment The application data. This data is transparent and treated as an independent block to be dealt with by the higher-level protocol specified by the type field. Implementations MUST NOT send zero-length fragments of Handshake, Alert, or ChangeCipherSpec content types. Zero-length fragments of Application data MAY be sent as they are potentially useful as a traffic analysis countermeasure. Note: Data of different TLS record layer content types MAY be interleaved. Application data is generally of lower precedence for transmission than other content types. However, records MUST be delivered to the network in the same order as they are protected by Dierks & Rescorla Expires October 19, 2014 [Page 20]

Internet-Draft TLS April 2014 the record layer. Recipients MUST receive and process interleaved application layer traffic during handshakes subsequent to the first one on a connection. 6.2.2 . Record Compression and Decompression RFC3749] describes compression algorithms for TLS. Compression must be lossless and may not increase the content length by more than 1024 bytes. If the decompression function encounters a TLSCompressed.fragment that would decompress to a length in excess of 2^14 bytes, it MUST report a fatal decompression failure error. struct { ContentType type; /* same as TLSPlaintext.type */ ProtocolVersion version;/* same as TLSPlaintext.version */ uint16 length; opaque fragment[TLSCompressed.length]; } TLSCompressed; length The length (in bytes) of the following TLSCompressed.fragment. The length MUST NOT exceed 2^14 + 1024. fragment The compressed form of TLSPlaintext.fragment. Note: A CompressionMethod.null operation is an identity operation; no fields are altered. Implementation note: Decompression functions are responsible for ensuring that messages cannot cause internal buffer overflows. 6.2.3 . Record Payload Protection Dierks & Rescorla Expires October 19, 2014 [Page 21]

Internet-Draft TLS April 2014 struct { ContentType type; ProtocolVersion version; uint16 length; select (SecurityParameters.cipher_type) { case stream: GenericStreamCipher; case block: GenericBlockCipher; case aead: GenericAEADCipher; } fragment; } TLSCiphertext; type The type field is identical to TLSCompressed.type. version The version field is identical to TLSCompressed.version. length The length (in bytes) of the following TLSCiphertext.fragment. The length MUST NOT exceed 2^14 + 2048. fragment The encrypted form of TLSCompressed.fragment, with the MAC. 6.2.3.1 . Null or Standard Stream Cipher Appendix A.6) convert TLSCompressed.fragment structures to and from stream TLSCiphertext.fragment structures. stream-ciphered struct { opaque content[TLSCompressed.length]; opaque MAC[SecurityParameters.mac_length]; } GenericStreamCipher; The MAC is generated as: MAC(MAC_write_key, seq_num + TLSCompressed.type + TLSCompressed.version + TLSCompressed.length + TLSCompressed.fragment); where "+" denotes concatenation. Dierks & Rescorla Expires October 19, 2014 [Page 22]

Internet-Draft TLS April 2014 seq_num The sequence number for this record. MAC The MAC algorithm specified by SecurityParameters.mac_algorithm. Note that the MAC is computed before encryption. The stream cipher encrypts the entire block, including the MAC. For stream ciphers that do not use a synchronization vector (such as RC4), the stream cipher state from the end of one record is simply used on the subsequent packet. If the cipher suite is TLS_NULL_WITH_NULL_NULL, encryption consists of the identity operation (i.e., the data is not encrypted, and the MAC size is zero, implying that no MAC is used). For both null and stream ciphers, TLSCiphertext.length is TLSCompressed.length plus SecurityParameters.mac_length. 6.2.3.2 . CBC Block Cipher Section 6.2.3.1. IV The Initialization Vector (IV) SHOULD be chosen at random, and MUST be unpredictable. Note that in versions of TLS prior to 1.1, there was no IV field, and the last ciphertext block of the previous record (the "CBC residue") was used as the IV. This was changed to prevent the attacks described in [CBCATT]. For block ciphers, the IV length is of length SecurityParameters.record_iv_length, which is equal to the SecurityParameters.block_size. padding Padding that is added to force the length of the plaintext to be an integral multiple of the block cipher's block length. The padding MAY be any length up to 255 bytes, as long as it results Dierks & Rescorla Expires October 19, 2014 [Page 23]

Internet-Draft TLS April 2014 in the TLSCiphertext.length being an integral multiple of the block length. Lengths longer than necessary might be desirable to frustrate attacks on a protocol that are based on analysis of the lengths of exchanged messages. Each uint8 in the padding data vector MUST be filled with the padding length value. The receiver MUST check this padding and MUST use the bad_record_mac alert to indicate padding errors. padding_length The padding length MUST be such that the total size of the GenericBlockCipher structure is a multiple of the cipher's block length. Legal values range from zero to 255, inclusive. This length specifies the length of the padding field exclusive of the padding_length field itself. The encrypted data length (TLSCiphertext.length) is one more than the sum of SecurityParameters.block_length, TLSCompressed.length, SecurityParameters.mac_length, and padding_length. Example: If the block length is 8 bytes, the content length (TLSCompressed.length) is 61 bytes, and the MAC length is 20 bytes, then the length before padding is 82 bytes (this does not include the IV. Thus, the padding length modulo 8 must be equal to 6 in order to make the total length an even multiple of 8 bytes (the block length). The padding length can be 6, 14, 22, and so on, through 254. If the padding length were the minimum necessary, 6, the padding would be 6 bytes, each containing the value 6. Thus, the last 8 octets of the GenericBlockCipher before block encryption would be xx 06 06 06 06 06 06 06, where xx is the last octet of the MAC. Note: With block ciphers in CBC mode (Cipher Block Chaining), it is critical that the entire plaintext of the record be known before any ciphertext is transmitted. Otherwise, it is possible for the attacker to mount the attack described in [CBCATT]. Implementation note: Canvel et al. [CBCTIME] have demonstrated a timing attack on CBC padding based on the time required to compute the MAC. In order to defend against this attack, implementations MUST ensure that record processing time is essentially the same whether or not the padding is correct. In general, the best way to do this is to compute the MAC even if the padding is incorrect, and only then reject the packet. For instance, if the pad appears to be incorrect, the implementation might assume a zero-length pad and then compute the MAC. This leaves a small timing channel, since MAC performance depends to some extent on the size of the data fragment, but it is not believed to be large enough to be exploitable, due to the large block size of existing MACs and the small size of the timing signal. Dierks & Rescorla Expires October 19, 2014 [Page 24]

Internet-Draft TLS April 2014 6.2.3.3 . AEAD Ciphers RFC5116] ciphers (such as [CCM] or [GCM]), the AEAD function converts TLSCompressed.fragment structures to and from AEAD TLSCiphertext.fragment structures. struct { opaque nonce_explicit[SecurityParameters.record_iv_length]; aead-ciphered struct { opaque content[TLSCompressed.length]; }; } GenericAEADCipher; 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 [RFC5116]. The key is either the client_write_key or the server_write_key. No MAC key is used. Each AEAD cipher suite MUST specify how the nonce supplied to the AEAD operation is constructed, and what is the length of the GenericAEADCipher.nonce_explicit part. In many cases, it is appropriate to use the partially implicit nonce technique described in Section 3.2.1 of [RFC5116]; with record_iv_length being the length of the explicit part. In this case, the implicit part SHOULD be derived from key_block as client_write_iv and server_write_iv (as described in Section 6.3), and the explicit part is included in GenericAEAEDCipher.nonce_explicit. The plaintext is the TLSCompressed.fragment. The additional authenticated data, which we denote as additional_data, is defined as follows: additional_data = seq_num + TLSCompressed.type + TLSCompressed.version + TLSCompressed.length; where "+" denotes concatenation. The aead_output consists of the ciphertext output by the AEAD encryption operation. The length will generally be larger than TLSCompressed.length, but by an amount that varies with the AEAD cipher. Since the ciphers might incorporate padding, the amount of overhead could vary with different TLSCompressed.length values. Each AEAD cipher MUST NOT produce an expansion of greater than 1024 bytes. Symbolically, AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext, additional_data) Dierks & Rescorla Expires October 19, 2014 [Page 25]

Internet-Draft TLS April 2014 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: TLSCompressed.fragment = AEAD-Decrypt(write_key, nonce, AEADEncrypted, additional_data) If the decryption fails, a fatal bad_record_mac alert MUST be generated. 6.3 . Key Calculation Appendix A.6) from the security parameters provided by the handshake protocol. The master secret is expanded into a sequence of secure bytes, which is then split to a client write MAC key, a server write MAC key, a client write encryption key, and a server write encryption key. Each of these is generated from the byte sequence in that order. Unused values are empty. Some AEAD ciphers may additionally require a client write IV and a server write IV (see Section 6.2.3.3). When keys and MAC keys are generated, the master secret is used as an entropy source. To generate the key material, compute key_block = PRF(SecurityParameters.master_secret, "key expansion", SecurityParameters.server_random + SecurityParameters.client_random); until enough output has been generated. Then, the key_block is partitioned as follows: client_write_MAC_key[SecurityParameters.mac_key_length] server_write_MAC_key[SecurityParameters.mac_key_length] client_write_key[SecurityParameters.enc_key_length] server_write_key[SecurityParameters.enc_key_length] client_write_IV[SecurityParameters.fixed_iv_length] server_write_IV[SecurityParameters.fixed_iv_length] Currently, the client_write_IV and server_write_IV are only generated for implicit nonce techniques as described in Section 3.2.1 of [RFC5116]. Dierks & Rescorla Expires October 19, 2014 [Page 26]

Internet-Draft TLS April 2014 Implementation note: The currently defined cipher suite which requires the most material is AES_256_CBC_SHA256. It requires 2 x 32 byte keys and 2 x 32 byte MAC keys, for a total 128 bytes of key material. 7 . The TLS Handshaking Protocols RFC3280] certificate of the peer. This element of the state may be null. compression method The algorithm used to compress data prior to encryption. cipher spec Specifies the pseudorandom function (PRF) used to generate keying material, the bulk data encryption algorithm (such as null, AES, etc.) and the MAC algorithm (such as HMAC-SHA1). It also defines cryptographic attributes such as the mac_length. (See Appendix A.6 for formal definition.) master secret 48-byte secret shared between the client and server. is resumable A flag indicating whether the session can be used to initiate new connections. These items are then used to create security parameters for use by the record layer when protecting application data. Many connections can be instantiated using the same session through the resumption feature of the TLS Handshake Protocol. Dierks & Rescorla Expires October 19, 2014 [Page 27]

Internet-Draft TLS April 2014 7.1 . Change Cipher Spec Protocol Section 6.1.) The ChangeCipherSpec message is sent during the handshake after the security parameters have been agreed upon, but before the verifying Finished message is sent. Note: If a rehandshake occurs while data is flowing on a connection, the communicating parties may continue to send data using the old CipherSpec. However, once the ChangeCipherSpec has been sent, the new CipherSpec MUST be used. The first side to send the ChangeCipherSpec does not know that the other side has finished computing the new keying material (e.g., if it has to perform a time- consuming public key operation). Thus, a small window of time, during which the recipient must buffer the data, MAY exist. In practice, with modern machines this interval is likely to be fairly short. 7.2 . Alert Protocol Dierks & Rescorla Expires October 19, 2014 [Page 28]

Internet-Draft TLS April 2014 enum { warning(1), fatal(2), (255) } AlertLevel; enum { close_notify(0), unexpected_message(10), bad_record_mac(20), decryption_failed_RESERVED(21), record_overflow(22), decompression_failure(30), handshake_failure(40), no_certificate_RESERVED(41), bad_certificate(42), unsupported_certificate(43), certificate_revoked(44), certificate_expired(45), certificate_unknown(46), illegal_parameter(47), unknown_ca(48), access_denied(49), decode_error(50), decrypt_error(51), export_restriction_RESERVED(60), protocol_version(70), insufficient_security(71), internal_error(80), user_canceled(90), no_renegotiation(100), unsupported_extension(110), (255) } AlertDescription; struct { AlertLevel level; AlertDescription description; } Alert; 7.2.1 . Closure Alerts Dierks & Rescorla Expires October 19, 2014 [Page 29]

Internet-Draft TLS April 2014 Either party may initiate a close by sending a close_notify alert. Any data received after a closure alert is ignored. Unless some other fatal alert has been transmitted, each party is required to send a close_notify alert before closing the write side of the connection. The other party MUST respond with a close_notify alert of its own and close down the connection immediately, discarding any pending writes. It is not required for the initiator of the close to wait for the responding close_notify alert before closing the read side of the connection. If the application protocol using TLS provides that any data may be carried over the underlying transport after the TLS connection is closed, the TLS implementation must receive the responding close_notify alert before indicating to the application layer that the TLS connection has ended. If the application protocol will not transfer any additional data, but will only close the underlying transport connection, then the implementation MAY choose to close the transport without waiting for the responding close_notify. No part of this standard should be taken to dictate the manner in which a usage profile for TLS manages its data transport, including when connections are opened or closed. Note: It is assumed that closing a connection reliably delivers pending data before destroying the transport. 7.2.2 . Error Alerts Dierks & Rescorla Expires October 19, 2014 [Page 30]

Internet-Draft TLS April 2014 no_renegotiation alert that it is not willing to accept), it SHOULD send a fatal alert to terminate the connection. Given this, the sending party cannot, in general, know how the receiving party will behave. Therefore, warning alerts are not very useful when the sending party wants to continue the connection, and thus are sometimes omitted. For example, if a peer decides to accept an expired certificate (perhaps after confirming this with the user) and wants to continue the connection, it would not generally send a certificate_expired alert. The following error alerts are defined: unexpected_message An inappropriate message was received. This alert is always fatal and should never be observed in communication between proper implementations. bad_record_mac This alert is returned if a record is received with an incorrect MAC. This alert also MUST be returned if an alert is sent because a TLSCiphertext decrypted in an invalid way: either it wasn't an even multiple of the block length, or its padding values, when checked, weren't correct. This message is always fatal and should never be observed in communication between proper implementations (except when messages were corrupted in the network). decryption_failed_RESERVED This alert was used in some earlier versions of TLS, and may have permitted certain attacks against the CBC mode [CBCATT]. It MUST NOT be sent by compliant implementations. record_overflow A TLSCiphertext record was received that had a length more than 2^14+2048 bytes, or a record decrypted to a TLSCompressed record with more than 2^14+1024 bytes. This message is always fatal and should never be observed in communication between proper implementations (except when messages were corrupted in the network). decompression_failure The decompression function received improper input (e.g., data that would expand to excessive length). This message is always fatal and should never be observed in communication between proper implementations. Dierks & Rescorla Expires October 19, 2014 [Page 31]

Internet-Draft TLS April 2014 handshake_failure Reception of a handshake_failure alert message indicates that the sender was unable to negotiate an acceptable set of security parameters given the options available. This is a fatal error. no_certificate_RESERVED This alert was used in SSLv3 but not any version of TLS. It MUST NOT be sent by compliant implementations. bad_certificate A certificate was corrupt, contained signatures that did not verify correctly, etc. unsupported_certificate A certificate was of an unsupported type. certificate_revoked A certificate was revoked by its signer. certificate_expired A certificate has expired or is not currently valid. certificate_unknown Some other (unspecified) issue arose in processing the certificate, rendering it unacceptable. illegal_parameter A field in the handshake was out of range or inconsistent with other fields. This message is always fatal. unknown_ca A valid certificate chain or partial chain was received, but the certificate was not accepted because the CA certificate could not be located or couldn't be matched with a known, trusted CA. This message is always fatal. access_denied A valid certificate was received, but when access control was applied, the sender decided not to proceed with negotiation. This message is always fatal. decode_error A message could not be decoded because some field was out of the specified range or the length of the message was incorrect. This message is always fatal and should never be observed in communication between proper implementations (except when messages were corrupted in the network). Dierks & Rescorla Expires October 19, 2014 [Page 32]

Internet-Draft TLS April 2014 decrypt_error A handshake cryptographic operation failed, including being unable to correctly verify a signature or validate a Finished message. This message is always fatal. export_restriction_RESERVED This alert was used in some earlier versions of TLS. It MUST NOT be sent by compliant implementations. protocol_version The protocol version the client has attempted to negotiate is recognized but not supported. (For example, old protocol versions might be avoided for security reasons.) This message is always fatal. insufficient_security Returned instead of handshake_failure when a negotiation has failed specifically because the server requires ciphers more secure than those supported by the client. This message is always fatal. internal_error An internal error unrelated to the peer or the correctness of the protocol (such as a memory allocation failure) makes it impossible to continue. This message is always fatal. user_canceled This handshake is being canceled for some reason unrelated to a protocol failure. If the user cancels an operation after the handshake is complete, just closing the connection by sending a close_notify is more appropriate. This alert should be followed by a close_notify. This message is generally a warning. no_renegotiation Sent by the client in response to a hello request or by the server in response to a client hello after initial handshaking. Either of these would normally lead to renegotiation; when that is not appropriate, the recipient should respond with this alert. At that point, the original requester can decide whether to proceed with the connection. One case where this would be appropriate is where a server has spawned a process to satisfy a request; the process might receive security parameters (key length, authentication, etc.) at startup, and it might be difficult to communicate changes to these parameters after that point. This message is always a warning. Dierks & Rescorla Expires October 19, 2014 [Page 33]

Internet-Draft TLS April 2014 unsupported_extension sent by clients that receive an extended server hello containing an extension that they did not put in the corresponding client hello. This message is always fatal. New Alert values are assigned by IANA as described in Section 12. 7.3 . Handshake Protocol Overview Dierks & Rescorla Expires October 19, 2014 [Page 34]

Internet-Draft TLS April 2014 level of security: if you negotiate 3DES with a 1024-bit RSA key exchange with a host whose certificate you have verified, you can expect to be that secure. These goals are achieved by the handshake protocol, which can be summarized as follows: The client sends a ClientHello message to which the server must respond with a ServerHello message, or else a fatal error will occur and the connection will fail. The ClientHello and ServerHello are used to establish security enhancement capabilities between client and server. The ClientHello and ServerHello establish the following attributes: Protocol Version, Session ID, Cipher Suite, and Compression Method. Additionally, two random values are generated and exchanged: ClientHello.random and ServerHello.random. The actual key exchange uses up to four messages: the server Certificate, the ServerKeyExchange, the client Certificate, and the ClientKeyExchange. New key exchange methods can be created by specifying a format for these messages and by defining the use of the messages to allow the client and server to agree upon a shared secret. This secret MUST be quite long; currently defined key exchange methods exchange secrets that range from 46 bytes upwards. Following the hello messages, the server will send its certificate in a Certificate message if it is to be authenticated. Additionally, a ServerKeyExchange message may be sent, if it is required (e.g., if the server has no certificate, or if its certificate is for signing only). If the server is authenticated, it may request a certificate from the client, if that is appropriate to the cipher suite selected. Next, the server will send the ServerHelloDone message, indicating that the hello-message phase of the handshake is complete. The server will then wait for a client response. If the server has sent a CertificateRequest message, the client MUST send the Certificate message. The ClientKeyExchange message is now sent, and the content of that message will depend on the public key algorithm selected between the ClientHello and the ServerHello. If the client has sent a certificate with signing ability, a digitally-signed CertificateVerify message is sent to explicitly verify possession of the private key in the certificate. At this point, a ChangeCipherSpec message is sent by the client, and the client copies the pending Cipher Spec into the current Cipher Spec. The client then immediately sends the Finished message under the new algorithms, keys, and secrets. In response, the server will send its own ChangeCipherSpec message, transfer the pending to the current Cipher Spec, and send its Finished message under the new Cipher Spec. At this point, the handshake is complete, and the client and server may begin to exchange application layer data. (See Dierks & Rescorla Expires October 19, 2014 [Page 35]

Internet-Draft TLS April 2014 flow chart below.) Application data MUST NOT be sent prior to the completion of the first handshake (before a cipher suite other than TLS_NULL_WITH_NULL_NULL is established). Client Server ClientHello --------> ServerHello Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished --------> [ChangeCipherSpec] <-------- Finished Application Data <-------> Application Data Figure 1. Message flow for a full handshake * Indicates optional or situation-dependent messages that are not always sent. Note: To help avoid pipeline stalls, ChangeCipherSpec is an independent TLS protocol content type, and is not actually a TLS handshake message. When the client and server decide to resume a previous session or duplicate an existing session (instead of negotiating new security parameters), the message flow is as follows: The client sends a ClientHello using the Session ID of the session to be resumed. The server then checks its session cache for a match. If a match is found, and the server is willing to re-establish the connection under the specified session state, it will send a ServerHello with the same Session ID value. At this point, both client and server MUST send ChangeCipherSpec messages and proceed directly to Finished messages. Once the re-establishment is complete, the client and server MAY begin to exchange application layer data. (See flow chart below.) If a Session ID match is not found, the server generates a new session ID, and the TLS client and server perform a full handshake. Dierks & Rescorla Expires October 19, 2014 [Page 36]

Internet-Draft TLS April 2014 Client Server ClientHello --------> ServerHello [ChangeCipherSpec] <-------- Finished [ChangeCipherSpec] Finished --------> Application Data <-------> Application Data Figure 2. Message flow for an abbreviated handshake The contents and significance of each message will be presented in detail in the following sections. 7.4 . Handshake Protocol Dierks & Rescorla Expires October 19, 2014 [Page 37]

Internet-Draft TLS April 2014 The handshake protocol messages are presented below in the order they MUST be sent; sending handshake messages in an unexpected order results in a fatal error. Unneeded handshake messages can be omitted, however. Note one exception to the ordering: the Certificate message is used twice in the handshake (from server to client, then from client to server), but described only in its first position. The one message that is not bound by these ordering rules is the HelloRequest message, which can be sent at any time, but which SHOULD be ignored by the client if it arrives in the middle of a handshake. New handshake message types are assigned by IANA as described in Section 12. 7.4.1 . Hello Messages 7.4.1.1 . Hello Request Dierks & Rescorla Expires October 19, 2014 [Page 38]

Internet-Draft TLS April 2014 After sending a HelloRequest, servers SHOULD NOT repeat the request until the subsequent handshake negotiation is complete. Structure of this message: struct { } HelloRequest; This message MUST NOT be included in the message hashes that are maintained throughout the handshake and used in the Finished messages and the certificate verify message. 7.4.1.2 . Client Hello Dierks & Rescorla Expires October 19, 2014 [Page 39]

Internet-Draft TLS April 2014 this connection, or from another currently active connection. The second option is useful if the client only wishes to update the random structures and derived values of a connection, and the third option makes it possible to establish several independent secure connections without repeating the full handshake protocol. These independent connections may occur sequentially or simultaneously; a SessionID becomes valid when the handshake negotiating it completes with the exchange of Finished messages and persists until it is removed due to aging or because a fatal error was encountered on a connection associated with the session. The actual contents of the SessionID are defined by the server. opaque SessionID<0..32>; Warning: Because the SessionID is transmitted without encryption or immediate MAC protection, servers MUST NOT place confidential information in session identifiers or let the contents of fake session identifiers cause any breach of security. (Note that the content of the handshake as a whole, including the SessionID, is protected by the Finished messages exchanged at the end of the handshake.) The cipher suite list, passed from the client to the server in the ClientHello message, contains the combinations of cryptographic algorithms supported by the client in order of the client's preference (favorite choice first). Each cipher suite defines a key exchange algorithm, a bulk encryption algorithm (including secret key length), a MAC algorithm, and a PRF. The server will select a cipher suite or, if no acceptable choices are presented, return a handshake failure alert and close the connection. If the list contains cipher suites the server does not recognize, support, or wish to use, the server MUST ignore those cipher suites, and process the remaining ones as usual. uint8 CipherSuite[2]; /* Cryptographic suite selector */ The ClientHello includes a list of compression algorithms supported by the client, ordered according to the client's preference. Dierks & Rescorla Expires October 19, 2014 [Page 40]

Internet-Draft TLS April 2014 enum { null(0), (255) } CompressionMethod; struct { ProtocolVersion client_version; Random random; SessionID session_id; CipherSuite cipher_suites<2..2^16-2>; CompressionMethod compression_methods<1..2^8-1>; select (extensions_present) { case false: struct {}; case true: Extension extensions<0..2^16-1>; }; } ClientHello; 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. client_version The version of the TLS protocol by which the client wishes to communicate during this session. This SHOULD be the latest (highest valued) version supported by the client. For this version of the specification, the version will be 3.3 (see Appendix E for details about backward compatibility). random A client-generated random structure. session_id The ID of a session the client wishes to use for this connection. This field is empty if no session_id is available, or if the client wishes to generate new security parameters. cipher_suites This is a list of the cryptographic options supported by the client, with the client's first preference first. If the session_id field is not empty (implying a session resumption request), this vector MUST include at least the cipher_suite from that session. Values are defined in Appendix A.5. Dierks & Rescorla Expires October 19, 2014 [Page 41]

Internet-Draft TLS April 2014 compression_methods This is a list of the compression methods supported by the client, sorted by client preference. If the session_id field is not empty (implying a session resumption request), it MUST include the compression_method from that session. This vector MUST contain, and all implementations MUST support, CompressionMethod.null. Thus, a client and server will always be able to agree on a compression method. extensions Clients MAY request extended functionality from servers by sending data in the extensions field. The actual "Extension" format is defined in Section 7.4.1.4. In the event that a client requests additional functionality using extensions, and this functionality is not supplied by the server, the client MAY abort the handshake. A server MUST accept ClientHello messages both with and without the extensions field, and (as for all other messages) it MUST check that the amount of data in the message precisely matches one of these formats; if not, then it MUST send a fatal "decode_error" alert. After sending the ClientHello message, the client waits for a ServerHello message. Any handshake message returned by the server, except for a HelloRequest, is treated as a fatal error. 7.4.1.3 . Server Hello Dierks & Rescorla Expires October 19, 2014 [Page 42]

Internet-Draft TLS April 2014 struct { ProtocolVersion server_version; Random random; SessionID session_id; CipherSuite cipher_suite; CompressionMethod compression_method; select (extensions_present) { case false: struct {}; case true: Extension extensions<0..2^16-1>; }; } ServerHello; The presence of extensions can be detected by determining whether there are bytes following the compression_method field at the end of the ServerHello. server_version This field will contain the lower of that suggested by the client in the client hello and the highest supported by the server. For this version of the specification, the version is 3.3. (See Appendix E for details about backward compatibility.) random This structure is generated by the server and MUST be independently generated from the ClientHello.random. session_id This is the identity of the session corresponding to this connection. If the ClientHello.session_id was non-empty, the server will look in its session cache for a match. If a match is found and the server is willing to establish the new connection using the specified session state, the server will respond with the same value as was supplied by the client. This indicates a resumed session and dictates that the parties must proceed directly to the Finished messages. Otherwise, this field will contain a different value identifying the new session. The server may return an empty session_id to indicate that the session will not be cached and therefore cannot be resumed. If a session is resumed, it must be resumed using the same cipher suite it was originally negotiated with. Note that there is no requirement that the server resume any session even if it had formerly provided a session_id. Clients MUST be prepared to do a full negotiation -- including negotiating new cipher suites -- during any handshake. Dierks & Rescorla Expires October 19, 2014 [Page 43]

Internet-Draft TLS April 2014 cipher_suite 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. compression_method The single compression algorithm selected by the server from the list in ClientHello.compression_methods. For resumed sessions, this field is the value from the resumed session state. extensions A list of extensions. Note that only extensions offered by the client can appear in the server's list. 7.4.1.4 . Hello Extensions TLSEXT]. The list of extension types is maintained by IANA as described in Section 12. An extension type MUST NOT appear in the ServerHello unless the same extension type appeared in the corresponding ClientHello. If a client receives an extension type in ServerHello that it did not request in the associated ClientHello, it MUST abort the handshake with an unsupported_extension fatal alert. Nonetheless, "server-oriented" extensions may be provided in the future within this framework. Such an extension (say, of type x) would require the client to first send an extension of type x in a ClientHello with empty extension_data to indicate that it supports Dierks & Rescorla Expires October 19, 2014 [Page 44]

Internet-Draft TLS April 2014 the extension type. In this case, the client is offering the capability to understand the extension type, and the server is taking the client up on its offer. When multiple extensions of different types are present in the ClientHello or ServerHello messages, the extensions MAY appear in any order. There MUST NOT be more than one extension of the same type. Finally, note that extensions can be sent both when starting a new session and when requesting session resumption. Indeed, a client that requests session resumption does not in general know whether the server will accept this request, and therefore it SHOULD send the same extensions as it would send if it were not attempting resumption. In general, the specification of each extension type needs to describe the effect of the extension both during full handshake and session resumption. Most current TLS extensions are relevant only when a session is initiated: when an older session is resumed, the server does not process these extensions in Client Hello, and does not include them in Server Hello. However, some extensions may specify different behavior during session resumption. There are subtle (and not so subtle) interactions that may occur in this protocol between new features and existing features which may result in a significant reduction in overall security. The following considerations should be taken into account when designing new extensions: - Some cases where a server does not agree to an extension are error conditions, and some are simply refusals to support particular features. In general, error alerts should be used for the former, and a field in the server extension response for the latter. - Extensions should, as far as possible, be designed to prevent any attack that forces use (or non-use) of a particular feature by manipulation of handshake messages. This principle should be followed regardless of whether the feature is believed to cause a security problem. Often the fact that the extension fields are included in the inputs to the Finished message hashes will be sufficient, but extreme care is needed when the extension changes the meaning of messages sent in the handshake phase. Designers and implementors should be aware of the fact that until the handshake has been authenticated, active attackers can modify messages and insert, remove, or replace extensions. Dierks & Rescorla Expires October 19, 2014 [Page 45]

Internet-Draft TLS April 2014 - 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. 7.4.1.4.1 . Signature Algorithms RFC1321], SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512 [SHS], respectively. The "none" value is provided for future extensibility, in case of a signature algorithm which does not require hashing before signing. Dierks & Rescorla Expires October 19, 2014 [Page 46]

Internet-Draft TLS April 2014 signature This field indicates the signature algorithm that may be used. The values indicate anonymous signatures, RSASSA-PKCS1-v1_5 [RFC3447] and DSA [DSS], and ECDSA [ECDSA], respectively. The "anonymous" value is meaningless in this context but used in Section 7.4.3. It MUST NOT appear in this extension. The semantics of this extension are somewhat complicated because the cipher suite indicates permissible signature algorithms but not hash algorithms. Section 7.4.2 and Section 7.4.3 describe the appropriate rules. If the client supports only the default hash and signature algorithms (listed in this section), it MAY omit the signature_algorithms extension. If the client does not support the default algorithms, or supports other hash and signature algorithms (and it is willing to use them for verifying messages sent by the server, i.e., server certificates and server key exchange), it MUST send the signature_algorithms extension, listing the algorithms it is willing to accept. If the client does not send the signature_algorithms extension, the server MUST do the following: - If the negotiated key exchange algorithm is one of (RSA, DHE_RSA, DH_RSA, RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had sent the value {sha1,rsa}. - If the negotiated key exchange algorithm is one of (DHE_DSS, DH_DSS), behave as if the client had sent the value {sha1,dsa}. - If the negotiated key exchange algorithm is one of (ECDH_ECDSA, ECDHE_ECDSA), behave as if the client had sent value {sha1,ecdsa}. Note: this is a change from TLS 1.1 where there are no explicit rules, but as a practical matter one can assume that the peer supports MD5 and SHA-1. Note: this extension is not meaningful for TLS versions prior to 1.2. Clients MUST NOT offer it if they are offering prior versions. However, even if clients do offer it, the rules specified in [TLSEXT] require servers to ignore extensions they do not understand. Servers MUST NOT send this extension. TLS servers MUST support receiving this extension. When performing session resumption, this extension is not included in Server Hello, and the server ignores the extension in Client Hello Dierks & Rescorla Expires October 19, 2014 [Page 47]

Internet-Draft TLS April 2014 (if present). 7.4.2 . Server Certificate PKCS7] is not used as the format for the certificate vector because PKCS #6 [PKCS6] 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: Dierks & Rescorla Expires October 19, 2014 [Page 48]

Internet-Draft TLS April 2014 - The certificate type MUST be X.509v3, unless explicitly negotiated otherwise (e.g., [RFC5081]). - The end entity certificate's public key (and associated restrictions) MUST be compatible with the selected key exchange algorithm. Key Exchange Alg. Certificate Key Type RSA RSA public key; the certificate MUST allow the RSA_PSK key to be used for encryption (the keyEncipherment bit MUST be set if the key usage extension is present). Note: RSA_PSK is defined in [RFC4279]. DHE_RSA RSA public key; the certificate MUST allow the ECDHE_RSA key to be used for signing (the digitalSignature bit MUST be set if the key usage extension is present) with the signature scheme and hash algorithm that will be employed in the server key exchange message. Note: ECDHE_RSA is defined in [RFC4492]. DHE_DSS DSA public key; the certificate MUST allow the key to be used for signing with the hash algorithm that will be employed in the server key exchange message. DH_DSS Diffie-Hellman public key; the keyAgreement bit DH_RSA MUST be set if the key usage extension is present. ECDH_ECDSA ECDH-capable public key; the public key MUST ECDH_RSA use a curve and point format supported by the client, as described in [RFC4492]. ECDHE_ECDSA ECDSA-capable public key; the certificate MUST allow the key to be used for signing with the hash algorithm that will be employed in the server key exchange message. The public key MUST use a curve and point format supported by the client, as described in [RFC4492]. - The "server_name" and "trusted_ca_keys" extensions [TLSEXT] are used to guide certificate selection. If the client provided a "signature_algorithms" extension, then all Dierks & Rescorla Expires October 19, 2014 [Page 49]

Internet-Draft TLS April 2014 certificates provided by the server MUST be signed by a hash/ signature algorithm pair that appears in that extension. Note that this implies that a certificate containing a key for one signature algorithm MAY be signed using a different signature algorithm (for instance, an RSA key signed with a DSA key). This is a departure from TLS 1.1, which required that the algorithms be the same. Note that this also implies that the DH_DSS, DH_RSA, ECDH_ECDSA, and ECDH_RSA key exchange algorithms do not restrict the algorithm used to sign the certificate. Fixed DH certificates MAY be signed with any hash/signature algorithm pair appearing in the extension. The names DH_DSS, DH_RSA, ECDH_ECDSA, and ECDH_RSA are historical. If the server has multiple certificates, it chooses one of them based on the above-mentioned criteria (in addition to other criteria, such as transport layer endpoint, local configuration and preferences, etc.). If the server has a single certificate, it SHOULD attempt to validate that it meets these criteria. Note that there are certificates that use algorithms and/or algorithm combinations that cannot be currently used with TLS. For example, a certificate with RSASSA-PSS signature key (id-RSASSA-PSS OID in SubjectPublicKeyInfo) cannot be used because TLS defines no corresponding signature algorithm. As cipher suites that specify new key exchange methods are specified for the TLS protocol, they will imply the certificate format and the required encoded keying information. 7.4.3 . Server Key Exchange Message Dierks & Rescorla Expires October 19, 2014 [Page 50]

Internet-Draft TLS April 2014 It is not legal to send the ServerKeyExchange message for the following key exchange methods: RSA DH_DSS DH_RSA Other key exchange algorithms, such as those defined in [RFC4492], MUST specify whether the ServerKeyExchange message is sent or not; and if the message is sent, its contents. Meaning of this message: This message conveys cryptographic information to allow the client to communicate the premaster secret: a Diffie-Hellman public key with which the client can complete a key exchange (with the result being the premaster secret) or a public key for some other algorithm. Structure of this message: Dierks & Rescorla Expires October 19, 2014 [Page 51]

Internet-Draft TLS April 2014 enum { dhe_dss, dhe_rsa, dh_anon, rsa, dh_dss, dh_rsa /* may be extended, e.g., for ECDH -- see [RFC4492] */ } KeyExchangeAlgorithm; struct { opaque dh_p<1..2^16-1>; opaque dh_g<1..2^16-1>; opaque dh_Ys<1..2^16-1>; } ServerDHParams; /* Ephemeral DH parameters */ dh_p The prime modulus used for the Diffie-Hellman operation. dh_g The generator used for the Diffie-Hellman operation. dh_Ys The server's Diffie-Hellman public value (g^X mod p). struct { select (KeyExchangeAlgorithm) { case dh_anon: ServerDHParams params; case dhe_dss: case dhe_rsa: ServerDHParams params; digitally-signed struct { opaque client_random[32]; opaque server_random[32]; ServerDHParams params; } signed_params; case rsa: case dh_dss: case dh_rsa: struct {} ; /* message is omitted for rsa, dh_dss, and dh_rsa */ /* may be extended, e.g., for ECDH -- see [RFC4492] */ }; } ServerKeyExchange; params The server's key exchange parameters. signed_params For non-anonymous key exchanges, a signature over the server's key exchange parameters. If the client has offered the "signature_algorithms" extension, the Dierks & Rescorla Expires October 19, 2014 [Page 52]

Internet-Draft TLS April 2014 signature algorithm and hash algorithm MUST be a pair listed in that extension. Note that there is a possibility for inconsistencies here. For instance, the client might offer DHE_DSS key exchange but omit any DSA pairs from its "signature_algorithms" extension. In order to negotiate correctly, the server MUST check any candidate cipher suites against the "signature_algorithms" extension before selecting them. This is somewhat inelegant but is a compromise designed to minimize changes to the original cipher suite design. In addition, the hash and signature algorithms MUST be compatible with the key in the server's end-entity certificate. RSA keys MAY be used with any permitted hash algorithm, subject to restrictions in the certificate, if any. Because DSA signatures do not contain any secure indication of hash algorithm, there is a risk of hash substitution if multiple hashes may be used with any key. Currently, DSA [DSS] may only be used with SHA-1. Future revisions of DSS [DSS-3] are expected to allow the use of other digest algorithms with DSA, as well as guidance as to which digest algorithms should be used with each key size. In addition, future revisions of [RFC3280] may specify mechanisms for certificates to indicate which digest algorithms are to be used with DSA. As additional cipher suites are defined for TLS that include new key exchange algorithms, the server key exchange message will be sent if and only if the certificate type associated with the key exchange algorithm does not provide enough information for the client to exchange a premaster secret. 7.4.4 . Certificate Request Dierks & Rescorla Expires October 19, 2014 [Page 53]

Internet-Draft TLS April 2014 enum { rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6), fortezza_dms_RESERVED(20), (255) } ClientCertificateType; opaque DistinguishedName<1..2^16-1>; struct { ClientCertificateType certificate_types<1..2^8-1>; SignatureAndHashAlgorithm supported_signature_algorithms<2^16-1>; DistinguishedName certificate_authorities<0..2^16-1>; } CertificateRequest; certificate_types A list of the types of certificate types that the client may offer. rsa_sign a certificate containing an RSA key dss_sign a certificate containing a DSA key rsa_fixed_dh a certificate containing a static DH key. dss_fixed_dh a certificate containing a static DH key supported_signature_algorithms A list of the hash/signature algorithm pairs that the server is able to verify, listed in descending order of preference. certificate_authorities A list of the distinguished names [X501] of acceptable certificate_authorities, represented in DER-encoded format. These distinguished names may specify a desired distinguished name for a root CA or for a subordinate CA; thus, this message can be used to describe known roots as well as a desired authorization space. If the certificate_authorities list is empty, then the client MAY send any certificate of the appropriate ClientCertificateType, unless there is some external arrangement to the contrary. The interaction of the certificate_types and supported_signature_algorithms fields is somewhat complicated. certificate_types has been present in TLS since SSLv3, but was somewhat underspecified. Much of its functionality is superseded by supported_signature_algorithms. The following rules apply: - Any certificates provided by the client MUST be signed using a hash/signature algorithm pair found in supported_signature_algorithms. Dierks & Rescorla Expires October 19, 2014 [Page 54]

Internet-Draft TLS April 2014 - The end-entity certificate provided by the client MUST contain a key that is compatible with certificate_types. If the key is a signature key, it MUST be usable with some hash/signature algorithm pair in supported_signature_algorithms. - For historical reasons, the names of some client certificate types include the algorithm used to sign the certificate. For example, in earlier versions of TLS, rsa_fixed_dh meant a certificate signed with RSA and containing a static DH key. In TLS 1.2, this functionality has been obsoleted by the supported_signature_algorithms, and the certificate type no longer restricts the algorithm used to sign the certificate. For example, if the server sends dss_fixed_dh certificate type and {{sha1, dsa}, {sha1, rsa}} signature types, the client MAY reply with a certificate containing a static DH key, signed with RSA- SHA1. New ClientCertificateType values are assigned by IANA as described in Section 12. Note: Values listed as RESERVED may not be used. They were used in SSLv3. Note: It is a fatal handshake_failure alert for an anonymous server to request client authentication. 7.4.5 . Server Hello Done Dierks & Rescorla Expires October 19, 2014 [Page 55]

Internet-Draft TLS April 2014 7.4.6 . Client Certificate Section 7.4.2. Meaning of this message: This message conveys the client's certificate chain to the server; the server will use it when verifying the CertificateVerify message (when the client authentication is based on signing) or calculating the premaster secret (for non-ephemeral Diffie- Hellman). The certificate MUST be appropriate for the negotiated cipher suite's key exchange algorithm, and any negotiated extensions. In particular: - The certificate type MUST be X.509v3, unless explicitly negotiated otherwise (e.g., [RFC5081]). - The end-entity certificate's public key (and associated restrictions) has to be compatible with the certificate types listed in CertificateRequest: Dierks & Rescorla Expires October 19, 2014 [Page 56]

Internet-Draft TLS April 2014 Client Cert. Type Certificate Key Type rsa_sign RSA public key; the certificate MUST allow the key to be used for signing with the signature scheme and hash algorithm that will be employed in the certificate verify message. dss_sign DSA public key; the certificate MUST allow the key to be used for signing with the hash algorithm that will be employed in the certificate verify message. ecdsa_sign ECDSA-capable public key; the certificate MUST allow the key to be used for signing with the hash algorithm that will be employed in the certificate verify message; the public key MUST use a curve and point format supported by the server. rsa_fixed_dh Diffie-Hellman public key; MUST use the same dss_fixed_dh parameters as server's key. rsa_fixed_ecdh ECDH-capable public key; MUST use the ecdsa_fixed_ecdh same curve as the server's key, and MUST use a point format supported by the server. - 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 Section 7.4.4. 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. 7.4.7 . Client Key Exchange Message Dierks & Rescorla Expires October 19, 2014 [Page 57]

Internet-Draft TLS April 2014 Meaning of this message: With this message, the premaster secret is set, either by direct transmission of the RSA-encrypted secret or by the transmission of Diffie-Hellman parameters that will allow each side to agree upon the same premaster secret. When the client is using an ephemeral Diffie-Hellman exponent, then this message contains the client's Diffie-Hellman public value. If the client is sending a certificate containing a static DH exponent (i.e., it is doing fixed_dh client authentication), then this message MUST be sent but MUST be empty. Structure of this message: The choice of messages depends on which key exchange method has been selected. See Section 7.4.3 for the KeyExchangeAlgorithm definition. struct { select (KeyExchangeAlgorithm) { case rsa: EncryptedPreMasterSecret; case dhe_dss: case dhe_rsa: case dh_dss: case dh_rsa: case dh_anon: ClientDiffieHellmanPublic; } exchange_keys; } ClientKeyExchange; 7.4.7.1 . RSA-Encrypted Premaster Secret Message Dierks & Rescorla Expires October 19, 2014 [Page 58]

Internet-Draft TLS April 2014 struct { ProtocolVersion client_version; opaque random[46]; } PreMasterSecret; client_version The latest (newest) version supported by the client. This is used to detect version rollback attacks. random 46 securely-generated random bytes. struct { public-key-encrypted PreMasterSecret pre_master_secret; } EncryptedPreMasterSecret; pre_master_secret This random value is generated by the client and is used to generate the master secret, as specified in [Section 8.1]. Note: The version number in the PreMasterSecret is the version offered by the client in the ClientHello.client_version, not the version negotiated for the connection. This feature is designed to prevent rollback attacks. Unfortunately, some old implementations use the negotiated version instead, and therefore checking the version number may lead to failure to interoperate with such incorrect client implementations. Client implementations MUST always send the correct version number in PreMasterSecret. If ClientHello.client_version is TLS 1.1 or higher, server implementations MUST check the version number as described in the note below. If the version number is TLS 1.0 or earlier, server implementations SHOULD check the version number, but MAY have a configuration option to disable the check. Note that if the check fails, the PreMasterSecret SHOULD be randomized as described below. Note: Attacks discovered by Bleichenbacher [BLEI] and Klima et al. [KPR03] can be used to attack a TLS server that reveals whether a particular message, when decrypted, is properly PKCS#1 formatted, contains a valid PreMasterSecret structure, or has the correct version number. As described by Klima [KPR03], these vulnerabilities can be avoided by treating incorrectly formatted message blocks and/or mismatched version numbers in a manner indistinguishable from correctly formatted RSA blocks. In other words: Dierks & Rescorla Expires October 19, 2014 [Page 59]

Internet-Draft TLS April 2014 1. Generate a string R of 46 random bytes 2. Decrypt the message to recover the plaintext M 3. If the PKCS#1 padding is not correct, or the length of message M is not exactly 48 bytes: pre_master_secret = ClientHello.client_version || R else If ClientHello.client_version <= TLS 1.0, and version number check is explicitly disabled: pre_master_secret = M else: pre_master_secret = ClientHello.client_version || M[2..47] Note that explicitly constructing the pre_master_secret with the ClientHello.client_version produces an invalid master_secret if the client has sent the wrong version in the original pre_master_secret. An alternative approach is to treat a version number mismatch as a PKCS-1 formatting error and randomize the premaster secret completely: 1. Generate a string R of 48 random bytes 2. Decrypt the message to recover the plaintext M 3. If the PKCS#1 padding is not correct, or the length of message M is not exactly 48 bytes: pre_master_secret = R else If ClientHello.client_version <= TLS 1.0, and version number check is explicitly disabled: premaster secret = M Dierks & Rescorla Expires October 19, 2014 [Page 60]

Internet-Draft TLS April 2014 else If M[0..1] != ClientHello.client_version: premaster secret = R else: premaster secret = M Although no practical attacks against this construction are known, Klima et al. [KPR03] describe some theoretical attacks, and therefore the first construction described is RECOMMENDED. In any case, a TLS server MUST NOT generate an alert if processing an RSA-encrypted premaster secret message fails, or the version number is not as expected. Instead, it MUST continue the handshake with a randomly generated premaster secret. It may be useful to log the real cause of failure for troubleshooting purposes; however, care must be taken to avoid leaking the information to an attacker (through, e.g., timing, log files, or other channels.) The RSAES-OAEP encryption scheme defined in [RFC3447] is more secure against the Bleichenbacher attack. However, for maximal compatibility with earlier versions of TLS, this specification uses the RSAES-PKCS1-v1_5 scheme. No variants of the Bleichenbacher attack are known to exist provided that the above recommendations are followed. Implementation note: Public-key-encrypted data is represented as an opaque vector <0..2^16-1> (see Section 4.7). Thus, the RSA-encrypted PreMasterSecret in a ClientKeyExchange is preceded by two length bytes. These bytes are redundant in the case of RSA because the EncryptedPreMasterSecret is the only data in the ClientKeyExchange and its length can therefore be unambiguously determined. The SSLv3 specification was not clear about the encoding of public-key- encrypted data, and therefore many SSLv3 implementations do not include the length bytes -- they encode the RSA-encrypted data directly in the ClientKeyExchange message. This specification requires correct encoding of the EncryptedPreMasterSecret complete with length bytes. The resulting PDU is incompatible with many SSLv3 implementations. Implementors upgrading from SSLv3 MUST modify their implementations to generate and accept the correct encoding. Implementors who wish to be compatible with both SSLv3 and TLS should make their implementation's behavior dependent on the protocol version. Dierks & Rescorla Expires October 19, 2014 [Page 61]

Internet-Draft TLS April 2014 Implementation note: It is now known that remote timing-based attacks on TLS are possible, at least when the client and server are on the same LAN. Accordingly, implementations that use static RSA keys MUST use RSA blinding or some other anti-timing technique, as described in [TIMING]. 7.4.7.2 . Client Diffie-Hellman Public Value 7.4.8 . Certificate Verify Dierks & Rescorla Expires October 19, 2014 [Page 62]

Internet-Draft TLS April 2014 sent, it MUST immediately follow the client key exchange message. Structure of this message: struct { digitally-signed struct { opaque handshake_messages[handshake_messages_length]; } } CertificateVerify; Here handshake_messages refers to all handshake messages sent or received, starting at client hello and up to, but not including, this message, including the type and length fields of the handshake messages. This is the concatenation of all the Handshake structures (as defined in Section 7.4) exchanged thus far. Note that this requires both sides to either buffer the messages or compute running hashes for all potential hash algorithms up to the time of the CertificateVerify computation. Servers can minimize this computation cost by offering a restricted set of digest algorithms in the CertificateRequest message. The hash and signature algorithms used in the signature MUST be one of those present in the supported_signature_algorithms field of the CertificateRequest message. In addition, the hash and signature algorithms MUST be compatible with the key in the client's end-entity certificate. RSA keys MAY be used with any permitted hash algorithm, subject to restrictions in the certificate, if any. Because DSA signatures do not contain any secure indication of hash algorithm, there is a risk of hash substitution if multiple hashes may be used with any key. Currently, DSA [DSS] may only be used with SHA-1. Future revisions of DSS [DSS-3] are expected to allow the use of other digest algorithms with DSA, as well as guidance as to which digest algorithms should be used with each key size. In addition, future revisions of [RFC3280] may specify mechanisms for certificates to indicate which digest algorithms are to be used with DSA. 7.4.9 . Finished Dierks & Rescorla Expires October 19, 2014 [Page 63]

Internet-Draft TLS April 2014 messages and the Finished message. Meaning of this message: The Finished message is the first one protected with the just negotiated algorithms, keys, and secrets. Recipients of Finished messages MUST verify that the contents are correct. Once a side has sent its Finished message and received and validated the Finished message from its peer, it may begin to send and receive application data over the connection. Structure of this message: struct { opaque verify_data[verify_data_length]; } Finished; verify_data PRF(master_secret, finished_label, Hash(handshake_messages)) [0..verify_data_length-1]; finished_label For Finished messages sent by the client, the string "client finished". For Finished messages sent by the server, the string "server finished". Hash denotes a Hash of the handshake messages. For the PRF defined in Section 5, the Hash MUST be the Hash used as the basis for the PRF. Any cipher suite which defines a different PRF MUST also define the Hash to use in the Finished computation. In previous versions of TLS, the verify_data was always 12 octets long. In the current version of TLS, it depends on the cipher suite. Any cipher suite which does not explicitly specify verify_data_length has a verify_data_length equal to 12. This includes all existing cipher suites. Note that this representation has the same encoding as with previous versions. Future cipher suites MAY specify other lengths but such length MUST be at least 12 bytes. handshake_messages All of the data from all messages in this handshake (not including any HelloRequest messages) up to, but not including, this message. This is only data visible at the handshake layer and does not include record layer headers. This is the concatenation of all the Handshake structures as defined in Section 7.4, exchanged thus far. Dierks & Rescorla Expires October 19, 2014 [Page 64]

Internet-Draft TLS April 2014 It is a fatal error if a Finished message is not preceded by a ChangeCipherSpec message at the appropriate point in the handshake. The value handshake_messages includes all handshake messages starting at ClientHello up to, but not including, this Finished message. This may be different from handshake_messages in Section 7.4.8 because it would include the CertificateVerify message (if sent). Also, the handshake_messages for the Finished message sent by the client will be different from that for the Finished message sent by the server, because the one that is sent second will include the prior one. Note: ChangeCipherSpec messages, alerts, and any other record types are not handshake messages and are not included in the hash computations. Also, HelloRequest messages are omitted from handshake hashes. 8 . Cryptographic Computations 8.1 . Computing the Master Secret 8.1.1 . RSA Dierks & Rescorla Expires October 19, 2014 [Page 65]

Internet-Draft TLS April 2014 above. 8.1.2 . Diffie-Hellman 9 . Mandatory Cipher Suites Appendix A.5 for the definition). 10 . Application Data Protocol 11 . Security Considerations 12 . IANA Considerations RFC4346]. IANA has updated these to reference this document. The registries and their allocation policies (unchanged from [RFC4346]) are listed below. - TLS ClientCertificateType Identifiers Registry: Future values in the range 0-63 (decimal) inclusive are assigned via Standards Action [RFC2434]. Values in the range 64-223 (decimal) inclusive are assigned via Specification Required [RFC2434]. Values from 224-255 (decimal) inclusive are reserved for Private Use [RFC2434]. - TLS Cipher Suite Registry: Future values with the first byte in the range 0-191 (decimal) inclusive are assigned via Standards Dierks & Rescorla Expires October 19, 2014 [Page 66]

Internet-Draft TLS April 2014 Action [RFC2434]. Values with the first byte in the range 192-254 (decimal) are assigned via Specification Required [RFC2434]. Values with the first byte 255 (decimal) are reserved for Private Use [RFC2434]. - This document defines several new HMAC-SHA256-based cipher suites, whose values (in Appendix A.5) have been allocated from the TLS Cipher Suite registry. - TLS ContentType Registry: Future values are allocated via Standards Action [RFC2434]. - TLS Alert Registry: Future values are allocated via Standards Action [RFC2434]. - TLS HandshakeType Registry: Future values are allocated via Standards Action [RFC2434]. This document also uses a registry originally created in [RFC4366]. IANA has updated it to reference this document. The registry and its allocation policy (unchanged from [RFC4366]) is listed below: - TLS ExtensionType Registry: Future values are allocated via IETF Consensus [RFC2434]. IANA has updated this registry to include the signature_algorithms extension and its corresponding value (see Section 7.4.1.4). 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 Section 7.4.1.4.1. Future values in the range 0-63 (decimal) inclusive are assigned via Standards Action [RFC2434]. Values in the range 64-223 (decimal) inclusive are assigned via Specification Required [RFC2434]. Values from 224-255 (decimal) inclusive are reserved for Private Use [RFC2434]. - TLS HashAlgorithm Registry: The registry has been initially populated with the values described in Section 7.4.1.4.1. Future values in the range 0-63 (decimal) inclusive are assigned via Standards Action [RFC2434]. Values in the range 64-223 (decimal) inclusive are assigned via Specification Required [RFC2434]. Values from 224-255 (decimal) inclusive are reserved for Private Use [RFC2434]. This document also uses the TLS Compression Method Identifiers Registry, defined in [RFC3749]. IANA has allocated value 0 for the Dierks & Rescorla Expires October 19, 2014 [Page 67]

Internet-Draft TLS April 2014 [X680] ITU-T, "Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation", ISO/IEC 8824-1:2002, 2002. [X690] ITU-T, "Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ISO/IEC 8825-1:2002, 2002. 13.2 . Informative References BLEI] Bleichenbacher, D., "Chosen Ciphertext Attacks against Protocols Based on RSA Encryption Standard PKCS", CRYPTO98 LNCS vol. 1462, pages: 1-12, 1998, Advances in Cryptology, 1998. [CBCATT] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures", May 2004, <http://www.openssl.org/~bodo/tls-cbc.txt>. [CBCTIME] Canvel, B., Hiltgen, A., Vaudenay, S., and M. Vuagnoux, "Password Interception in a SSL/TLS Channel", CRYPTO 2003 LNCS vol. 2729, 2003. [CCM] "NIST Special Publication 800-38C: The CCM Mode for Authentication and Confidentiality", May 2004, <http:// csrc.nist.gov/publications/nistpubs/800-38C/ SP800-38C.pdf>. [DES] "Data Encryption Standard (DES)", NIST FIPS PUB 46-3, October 1999. [DSS-3] National Institute of Standards and Technology, U.S., "Digital Signature Standard", NIST FIPS PUB 186-3 Draft, 2006. [ECDSA] American National Standards Institute, "Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI ANS X9.62-2005, November 2005. [ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication for Protecting Communications (Or: How Secure is SSL?)", 2001. [FI06] "Bleichenbacher's RSA signature forgery based on implementation error", August 2006, <http://www.imc.org/ ietf-openpgp/mail-archive/msg14307.html>. Dierks & Rescorla Expires October 19, 2014 [Page 69]

Internet-Draft TLS April 2014 [TLSEXT] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", February 2008. [X501] "Information Technology - Open Systems Interconnection - The Directory: Models", ITU-T X.501, 1993. URIs [1] <mailto:tls@ietf.org> Appendix A . Protocol Data Structures and Constant Values A.1 . Record Layer Dierks & Rescorla Expires October 19, 2014 [Page 72]

Internet-Draft TLS April 2014 case block: GenericBlockCipher; case aead: GenericAEADCipher; } fragment; } TLSCiphertext; stream-ciphered struct { opaque content[TLSCompressed.length]; opaque MAC[SecurityParameters.mac_length]; } GenericStreamCipher; struct { opaque IV[SecurityParameters.record_iv_length]; block-ciphered struct { opaque content[TLSCompressed.length]; opaque MAC[SecurityParameters.mac_length]; uint8 padding[GenericBlockCipher.padding_length]; uint8 padding_length; }; } GenericBlockCipher; struct { opaque nonce_explicit[SecurityParameters.record_iv_length]; aead-ciphered struct { opaque content[TLSCompressed.length]; }; } GenericAEADCipher; A.2 . Change Cipher Specs Message Dierks & Rescorla Expires October 19, 2014 [Page 73]

Internet-Draft TLS April 2014 A.3 . Alert Messages Dierks & Rescorla Expires October 19, 2014 [Page 74]

Internet-Draft TLS April 2014 A.4 . Handshake Protocol A.4.1 . Hello Messages Dierks & Rescorla Expires October 19, 2014 [Page 75]

Internet-Draft TLS April 2014 CompressionMethod compression_methods<1..2^8-1>; select (extensions_present) { case false: struct {}; case true: Extension extensions<0..2^16-1>; }; } ClientHello; struct { ProtocolVersion server_version; Random random; SessionID session_id; CipherSuite cipher_suite; CompressionMethod compression_method; select (extensions_present) { case false: struct {}; case true: Extension extensions<0..2^16-1>; }; } ServerHello; struct { ExtensionType extension_type; opaque extension_data<0..2^16-1>; } Extension; enum { signature_algorithms(13), (65535) } ExtensionType; enum{ none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5), sha512(6), (255) } HashAlgorithm; enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) } SignatureAlgorithm; struct { HashAlgorithm hash; SignatureAlgorithm signature; } SignatureAndHashAlgorithm; SignatureAndHashAlgorithm supported_signature_algorithms<2..2^16-1>; Dierks & Rescorla Expires October 19, 2014 [Page 76]

Internet-Draft TLS April 2014 A.4.2 . Server Authentication and Key Exchange Messages RFC4492] */ } ServerKeyExchange; enum { rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6), fortezza_dms_RESERVED(20), (255) } ClientCertificateType; opaque DistinguishedName<1..2^16-1>; struct { Dierks & Rescorla Expires October 19, 2014 [Page 77]

Internet-Draft TLS April 2014 ClientCertificateType certificate_types<1..2^8-1>; DistinguishedName certificate_authorities<0..2^16-1>; } CertificateRequest; struct { } ServerHelloDone; A.4.3 . Client Authentication and Key Exchange Messages Dierks & Rescorla Expires October 19, 2014 [Page 78]

Internet-Draft TLS April 2014 A.4.4 . Handshake Finalization Message A.5 . The Cipher Suite Dierks & Rescorla Expires October 19, 2014 [Page 79]

Internet-Draft TLS April 2014 must use the parameters (group and generator) described by the server. CipherSuite TLS_DH_DSS_WITH_3DES_ED