Network Working Group                                          T. Dierks
Request for Comments: 5246                                   Independent
Obsoletes: 3268, 4346, 4366                                  E. Rescorla
Updates: 4492                                                 RTFM, Inc.
Category: Standards Track                                    August 2008


              The Transport Layer Security (TLS) Protocol
                              Version 1.2

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document specifies Version 1.2 of the Transport Layer Security
   (TLS) protocol.  The TLS protocol provides communications security
   over the Internet.  The protocol allows client/server applications to
   communicate in a way that is designed to prevent eavesdropping,
   tampering, or message forgery.

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 ...........................................10
      4.7. Cryptographic Attributes ..................................12
      4.8. Constants .................................................14
   5. HMAC and the Pseudorandom Function .............................14
   6. The TLS Record Protocol ........................................15
      6.1. Connection States .........................................16
      6.2. Record Layer ..............................................19
           6.2.1. Fragmentation ......................................19

           6.2.2. Record Compression and Decompression ...............20
           6.2.3. Record Payload Protection ..........................21
                  6.2.3.1. Null or Standard Stream Cipher ............22
                  6.2.3.2. CBC Block Cipher ..........................22
                  6.2.3.3. AEAD Ciphers ..............................24
      6.3. Key Calculation ...........................................25
   7. The TLS Handshaking Protocols ..................................26
      7.1. Change Cipher Spec Protocol ...............................27
      7.2. Alert Protocol ............................................28
           7.2.1. Closure Alerts .....................................29
           7.2.2. Error Alerts .......................................30
      7.3. Handshake Protocol Overview ...............................33
      7.4. Handshake Protocol ........................................37
           7.4.1. Hello Messages .....................................38
                  7.4.1.1. Hello Request .............................38
                  7.4.1.2. Client Hello ..............................39
                  7.4.1.3. Server Hello ..............................42
                  7.4.1.4. Hello Extensions ..........................44
                           7.4.1.4.1. Signature Algorithms ...........45
           7.4.2. Server Certificate .................................47
           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 .................................55
           7.4.7. Client Key Exchange Message ........................57
                  7.4.7.1. RSA-Encrypted Premaster Secret Message ....58
                  7.4.7.2. Client Diffie-Hellman Public Value ........61
           7.4.8. Certificate Verify .................................62
           7.4.9. Finished ...........................................63
   8. Cryptographic Computations .....................................64
      8.1. Computing the Master Secret ...............................64
           8.1.1. RSA ................................................65
           8.1.2. Diffie-Hellman .....................................65
   9. Mandatory Cipher Suites ........................................65
   10. Application Data Protocol .....................................65
   11. Security Considerations .......................................65
   12. IANA Considerations ...........................................65
   Appendix A. Protocol Data Structures and Constant Values ..........68
      A.1. Record Layer ..............................................68
      A.2. Change Cipher Specs Message ...............................69
      A.3. Alert Messages ............................................69
      A.4. Handshake Protocol ........................................70
           A.4.1. Hello Messages .....................................71
           A.4.2. Server Authentication and Key Exchange Messages ....72
           A.4.3. Client Authentication and Key Exchange Messages ....74
           A.4.4. Handshake Finalization Message .....................74
      A.5. The Cipher Suite ..........................................75
      A.6. The Security Parameters ...................................77

      A.7. Changes to RFC 4492 .......................................78
   Appendix B. Glossary ..............................................78
   Appendix C. Cipher Suite Definitions ..............................83
   Appendix D. Implementation Notes ..................................85
      D.1. Random Number Generation and Seeding ......................85
      D.2. Certificates and Authentication ...........................85
      D.3. Cipher Suites .............................................85
      D.4. Implementation Pitfalls ...................................85
   Appendix E. Backward Compatibility ................................87
      E.1. Compatibility with TLS 1.0/1.1 and SSL 3.0 ................87
      E.2. Compatibility with SSL 2.0 ................................88
      E.3. Avoiding Man-in-the-Middle Version Rollback ...............90
   Appendix F. Security Analysis .....................................91
      F.1. Handshake Protocol ........................................91
           F.1.1. Authentication and Key Exchange ....................91
                  F.1.1.1. Anonymous Key Exchange ....................91
                  F.1.1.2. RSA Key Exchange and Authentication .......92
                  F.1.1.3. Diffie-Hellman Key Exchange with
                           Authentication ............................92
           F.1.2. Version Rollback Attacks ...........................93
           F.1.3. Detecting Attacks Against the Handshake Protocol ...94
           F.1.4. Resuming Sessions ..................................94
      F.2. Protecting Application Data ...............................94
      F.3. Explicit IVs ..............................................95
      F.4. Security of Composite Cipher Modes ........................95
      F.5. Denial of Service .........................................96
      F.6. Final Notes ...............................................96
   Normative References ..............................................97
   Informative References ............................................98
   Working Group Information ........................................101
   Contributors .....................................................101

1.  Introduction

   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 [TCP]), is the TLS Record Protocol.
   The TLS Record Protocol provides connection security that has two
   basic properties:

   -  The connection is private.  Symmetric cryptography is used for
      data encryption (e.g., AES [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
      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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [REQ].

1.2.  Major Differences from TLS 1.1

   This document is a revision of the TLS 1.1 [TLS1.1] protocol which
   contains improved flexibility, particularly for negotiation of
   cryptographic algorithms.  The major changes are:

   -  The MD5/SHA-1 combination in the pseudorandom function (PRF) has
      been replaced with cipher-suite-specified PRFs.  All cipher suites
      in this document use P_SHA256.

   -  The MD5/SHA-1 combination in the digitally-signed element has been
      replaced with a single hash.  Signed elements now include a field
      that explicitly specifies the hash algorithm used.

   -  Substantial cleanup to the client's and server's ability to
      specify which hash and signature algorithms they will accept.
      Note that this also relaxes some of the constraints on signature
      and hash algorithms from previous versions of TLS.

   -  Addition of support for authenticated encryption with additional
      data modes.

   -  TLS Extensions definition and AES Cipher Suites were merged in
      from external [TLSEXT] and [TLSAES].

   -  Tighter checking of EncryptedPreMasterSecret version numbers.

   -  Tightened up a number of requirements.

   -  Verify_data length now depends on the cipher suite (default is
      still 12).

   -  Cleaned up description of Bleichenbacher/Klima attack defenses.

   -  Alerts MUST now be sent in many cases.

   -  After a certificate_request, if no certificates are available,
      clients now MUST send an empty certificate list.

   -  TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement
      cipher suite.

   -  Added HMAC-SHA256 cipher suites.

   -  Removed IDEA and DES cipher suites.  They are now deprecated and
      will be documented in a separate document.

   -  Support for the SSLv2 backward-compatible hello is now a MAY, not
      a SHOULD, with sending it a SHOULD NOT.  Support will probably
      become a SHOULD NOT in the future.

   -  Added limited "fall-through" to the presentation language to allow
      multiple case arms to have the same encoding.

   -  Added an Implementation Pitfalls sections

   -  The usual clarifications and editorial work.

2.  Goals

   The goals of the TLS protocol, in order of priority, are as follows:

   1. Cryptographic security: TLS should be used to establish a secure
      connection between two parties.

   2. Interoperability: Independent programmers should be able to
      develop applications utilizing TLS that can successfully exchange
      cryptographic parameters without knowledge of one another's code.

   3. Extensibility: TLS seeks to provide a framework into which new
      public key and bulk encryption methods can be incorporated as
      necessary.  This will also accomplish two sub-goals: preventing
      the need to create a new protocol (and risking the introduction of
      possible new weaknesses) and avoiding the need to implement an
      entire new security library.

   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

   This document and the TLS protocol itself are based on the SSL 3.0
   Protocol Specification as published by Netscape.  The differences
   between this protocol and SSL 3.0 are not dramatic, but they are
   significant enough that the various versions of TLS and SSL 3.0 do
   not interoperate (although each protocol incorporates a mechanism by
   which an implementation can back down to prior versions).  This
   document is intended primarily for readers who will be implementing
   the protocol and for those doing cryptographic analysis of it.  The
   specification has been written with this in mind, and it is intended
   to reflect the needs of those two groups.  For that reason, many of
   the algorithm-dependent data structures and rules are included in the
   body of the text (as opposed to in an appendix), providing easier
   access to them.

   This document is not intended to supply any details of service
   definition or of interface definition, although it does cover select
   areas of policy as they are required for the maintenance of solid
   security.

4.  Presentation Language

   This document deals with the formatting of data in an external
   representation.  The following very basic and somewhat casually
   defined presentation syntax will be used.  The syntax draws from
   several sources in its structure.  Although it resembles the
   programming language "C" in its syntax and XDR [XDR] in both its
   syntax and intent, it would be risky to draw too many parallels.  The
   purpose of this presentation language is to document TLS only; it has
   no general application beyond that particular goal.

4.1.  Basic Block Size

   The representation of all data items is explicitly specified.  The
   basic data block size is one byte (i.e., 8 bits).  Multiple byte data
   items are concatenations of bytes, from left to right, from top to
   bottom.  From the byte stream, a multi-byte item (a numeric in the
   example) is formed (using C notation) by:

      value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
              ... | byte[n-1];

   This byte ordering for multi-byte values is the commonplace network
   byte order or big-endian format.

4.2.  Miscellaneous

   Comments begin with "/*" and end with "*/".

   Optional components are denoted by enclosing them in "[[ ]]" double
   brackets.

   Single-byte entities containing uninterpreted data are of type
   opaque.

4.3.  Vectors

   A vector (single-dimensioned array) is a stream of homogeneous data
   elements.  The size of the vector may be specified at documentation
   time or left unspecified until runtime.  In either case, the length
   declares the number of bytes, not the number of elements, in the
   vector.  The syntax for specifying a new type, T', that is a fixed-
   length vector of type T is

      T T'[n];

   Here, T' occupies n bytes in the data stream, where n is a multiple
   of the size of T.  The length of the vector is not included in the
   encoded stream.

   In the following example, Datum is defined to be three consecutive
   bytes that the protocol does not interpret, while Data is three
   consecutive Datum, consuming a total of nine bytes.

      opaque Datum[3];      /* three uninterpreted bytes */
      Datum Data[9];        /* 3 consecutive 3 byte vectors */

   Variable-length vectors are defined by specifying a subrange of legal
   lengths, inclusively, using the notation <floor..ceiling>.  When
   these are encoded, the actual length precedes the vector's contents
   in the byte stream.  The length will be in the form of a number
   consuming as many bytes as required to hold the vector's specified
   maximum (ceiling) length.  A variable-length vector with an actual
   length field of zero is referred to as an empty vector.

      T T'<floor..ceiling>;

   In the following example, mandatory is a vector that must contain
   between 300 and 400 bytes of type opaque.  It can never be empty.
   The actual length field consumes two bytes, a uint16, which is
   sufficient to represent the value 400 (see 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

   The basic numeric data type is an unsigned byte (uint8).  All larger
   numeric data types are formed from fixed-length series of bytes
   concatenated as described in 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

   An additional sparse data type is available called enum.  A field of
   type enum can only assume the values declared in the definition.
   Each definition is a different type.  Only enumerateds of the same
   type may be assigned or compared.  Every element of an enumerated
   must be assigned a value, as demonstrated in the following example.
   Since the elements of the enumerated are not ordered, they can be
   assigned any unique value, in any order.

      enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;

   An enumerated occupies as much space in the byte stream as would its
   maximal defined ordinal value.  The following definition would cause
   one byte to be used to carry fields of type Color.

      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

   Structure types may be constructed from primitive types for
   convenience.  Each specification declares a new, unique type.  The
   syntax for definition is much like that of C.

      struct {
          T1 f1;
          T2 f2;
          ...
          Tn fn;
      } [[T]];

   The fields within a structure may be qualified using the type's name,
   with a syntax much like that available for enumerateds.  For example,
   T.f2 refers to the second field of the previous declaration.
   Structure definitions may be embedded.

4.6.1.  Variants

   Defined structures may have variants based on some knowledge that is
   available within the environment.  The selector must be an enumerated
   type that defines the possible variants the structure defines.  There
   must be a case arm for every element of the enumeration declared in
   the select.  Case arms have limited fall-through: if two case arms
   follow in immediate succession with no fields in between, then they

   both contain the same fields.  Thus, in the example below, "orange"
   and "banana" both contain V2.  Note that this is a new piece of
   syntax in TLS 1.2.

   The body of the variant structure may be given a label for reference.
   The mechanism by which the variant is selected at runtime is not
   prescribed by the presentation language.

      struct {
          T1 f1;
          T2 f2;
          ....
          Tn fn;
           select (E) {
               case e1: Te1;
               case e2: Te2;
               case e3: case e4: Te3;
               ....
               case en: Ten;
           } [[fv]];
      } [[Tv]];

   For example:

      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

   The five cryptographic operations -- digital signing, stream cipher
   encryption, block cipher encryption, authenticated encryption with
   additional data (AEAD) encryption, and public key encryption -- are
   designated digitally-signed, stream-ciphered, block-ciphered, aead-
   ciphered, and public-key-encrypted, respectively.  A field's
   cryptographic processing is specified by prepending an appropriate
   key word designation before the field's type specification.
   Cryptographic keys are implied by the current session state (see
   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.

   In RSA signing, the opaque vector contains the signature generated
   using the RSASSA-PKCS1-v1_5 signature scheme defined in [PKCS1].  As
   discussed in [PKCS1], 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 [PKCS1].

   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

   Typed constants can be defined for purposes of specification by
   declaring a symbol of the desired type and assigning values to it.

   Under-specified types (opaque, variable-length vectors, and
   structures that contain opaque) cannot be assigned values.  No fields
   of a multi-element structure or vector may be elided.

   For example:

      struct {
          uint8 f1;
          uint8 f2;
      } Example1;

      Example1 ex1 = {1, 4};  /* assigns f1 = 1, f2 = 4 */

5.  HMAC and the Pseudorandom Function

   The TLS record layer uses a keyed Message Authentication Code (MAC)
   to protect message integrity.  The cipher suites defined in this
   document use a construction known as HMAC, described in [HMAC], which
   is based on a hash function.  Other cipher suites MAY define their
   own MAC constructions, if needed.

   In addition, a construction is required to do expansion of secrets
   into blocks of data for the purposes of key generation or validation.
   This pseudorandom function (PRF) takes as input a secret, a seed, and
   an identifying label and produces an output of arbitrary length.

   In this section, we define one PRF, based on HMAC.  This PRF with the
   SHA-256 hash function is used for all cipher suites defined in this
   document and in TLS documents published prior to this document when
   TLS 1.2 is negotiated.  New cipher suites MUST explicitly specify a
   PRF and, in general, SHOULD use the TLS PRF with SHA-256 or a
   stronger standard hash function.

   First, we define a data expansion function, P_hash(secret, data),
   that uses a single hash function to expand a secret and seed into an
   arbitrary quantity of output:

      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