Network Working Group                                           S. Kelly
Internet-Draft                                            Aruba Wireless Networks
Intended status: Standards Track                              S. Frankel
Expires: April 1, July 9, 2007                                               NIST
                                                      September 28, 2006
                                                         January 5, 2007

     Using HMAC-SHA-256 HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 With IPsec
                     draft-kelly-ipsec-ciph-sha2-00
                     draft-kelly-ipsec-ciph-sha2-01

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Copyright Notice

   Copyright (C) The Internet Society (2006). (2007).

Abstract

   This specification describes the use of the SHA-256 algorithm HMAC in conjunction with HMAC the
   SHA-256, SHA-384, and SHA-512 algorithms in IPsec.  These algorithms
   may be used as a the basis for data origin authentication and integrity
   verification mechanism mechanisms for the IPsec AH, ESP, IKE and IKEv2 protocols, and
   also as a PRF Pseudo-Random Functions (PRFs) for IKE and IKEv2.  Two  Truncated
   output truncation lengths are specified for data origin authentication and integrity verification
   usage, the authentication-related variants,
   with the corresponding algorithms designated as HMAC-SHA-256-
   128 HMAC-SHA-256-128,
   HMAC-SHA-384-192, and HMAC-SHA-256-192.  No truncation is specified for HMAC-SHA-512-256.  The PRF usage, variants are not
   truncated, and the resulting algorithm is designated as HMAC-SHA-PRF-256. are called HMAC-SHA-PRF-256, HMAC-SHA-PRF-384, and
   HMAC-SHA-PRF-512.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The HMAC-SHA-256 HMAC-SHA-256+ Algorithms . . . . . . . . . . . . . . . . .  3
     2.1.  Keying Material  . . . . . . . . . . . . . . . . . . . . .  3
       2.1.1.  Data Origin Authentication and Integrity
               Verification Usage . . . . . . . . . . . . . . . . . .  3  4
       2.1.2.  Pseudo-Random Function (PRF) Usage . . . . . . . . . .  4
       2.1.3.  Randomness and Key Strength  . . . . . . . . . . . . .  4  5
       2.1.4.  Key Distribution . . . . . . . . . . . . . . . . . . .  4  5
       2.1.5.  Refreshing Keys  . . . . . . . . . . . . . . . . . . .  4  5
     2.2.  Padding  . . . . . . . . . . . . . . . . . . . . . . . . .  5  6
     2.3.  Truncation . . . . . . . . . . . . . . . . . . . . . . . .  5  6
     2.4.  Using HMAC-SHA-256 HMAC-SHA-256+ As a PRF PRFs in IKE and IKEv2 . . . . . . . . . . .  6
     2.5.  Interactions with the ESP ESP, IKE, or IKEv2 Cipher
           Mechanisms . . .  6
     2.6.  Test Vectors . . . . . . . . . . . . . . . . . . . . .  7
     2.6.  HMAC-SHA-256+ Parameter Summary  . .  6
   3.  Security Considerations . . . . . . . . . . .  7
     2.7.  Test Vectors . . . . . . . . 10
     3.1.  HMAC Key Length vs Truncation Length . . . . . . . . . . . 10
   4.  IANA Considerations . . . .  7
       2.7.1.  PRF Test Vectors . . . . . . . . . . . . . . . . . . .  7
       2.7.2.  Authenticator Test Vectors . . . . . . . . . . . . . . 11
   5.  Acknowledgements
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     3.1.  HMAC Key Length vs Truncation Length . . . . . . . . . . 12
   6.  References . 17
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   5.  Acknowledgements . . . . 12
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     6.2.  Informative 18
   6.  Normative References . . . . . . . . . . . . . . . . . . 12 . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 14 21

1.  Introduction

   This document specifies the use of SHA-256 SHA-256, SHA-384, and SHA-512
   [SHA2-1] combined with HMAC [HMAC] as a data origin authentication and
   integrity verification mechanism mechanisms for the IPsec AH [AH], ESP [ESP],
   IKE [IKE], and IKEv2 [IKEv2] protocols.  For flexibility, two output  Output truncation lengths
   are is
   specified for the authentication-related these variants, with the corresponding algorithms
   designated as HMAC-SHA-256-128 HMAC-SHA-256-128, HMAC-SHA-384-192, and HMAC-SHA-
   256-192.  In addition, this document specifies use of HMAC-SHA-512-
   256.  These truncation lengths are chosen in accordance with the underlying
   HMAC-SHA-256 algorithm (without truncation)
   birthday bound for each algorithm.

   This specification also describes untruncated variants of these
   algorithms as a PRF within PRFs for use with IKE and IKEv2, and that variant is designated as HMAC-SHA-PRF-256.  These those algorithms
   are called HMAC-SHA-PRF-256, HMAC-SHA-PRF-384, and HMAC-SHA-PRF-512.
   For ease of reference, these PRF algorithms and the authentication
   variants described above are collectively referred to below as "the HMAC-SHA-256
   HMAC-SHA-256+ algorithms."

   The goal of the PRF variant variants is to provide a secure pseudo-random
   function
   functions suitable for generation of keying material and other
   protocol-specific numeric quantities, while the goal of the
   authentication variants is to ensure that packets are authentic and
   cannot be modified in transit.  The relative security of HMAC-SHA-256 HMAC-SHA-
   256+ when used in either case is dependent on the scope of distribution scope
   and the unpredictability of the associated secret key.  If the key is
   not predictable
   unpredictable and known only by the source sender and destination, recipient, these
   algorithms ensure that only parties holding an identical key can
   derive the associated values.

2.  The HMAC-SHA-256 HMAC-SHA-256+ Algorithms

   [SHA2-1] and [SHA2-2] describe the underlying SHA-256 algorithm, SHA-256, SHA-384, and
   SHA-512 algorithms, while [HMAC] describes the HMAC algorithm.  The
   HMAC algorithm provides a framework for inserting various hashing
   algorithms such as
   SHA-256. SHA-256, and [SHA256+] describes combined usage of
   these algorithms.  The following sections describe the various
   characteristics and requirements of the HMAC-SHA-256 algorithms. HMAC-SHA-256+ algorithms when
   used with IPsec.

2.1.  Keying Material

   Requirements for keying material vary depending on usage. whether the
   algorithm is functioning as a PRF or as an authentication/integrity
   mechanism.  In the case of authentication/integrity, key lengths are
   fixed according to the output length of the algorithm in use.  In the
   case of PRFs, key lengths are variable, but guidance is given to
   ensure interoperability.  These distinctions are described further
   below.

   Before describing key requirements for each usage, it is important to
   clarify some terms we use below:

   Block size:  the size of the data block the underlying hash algorithm
      operates upon; for SHA-256, this is 512 bits.  For SHA-384 and
      SHA-512, this is 1024 bits.

   Output length:  the size of the hash value produced by the underlying
      hash algorithm.  For SHA-256, this is 256 bits, for SHA-384 this
      is 384 bits, and for SHA-512, this is 512 bits.

   Authenticator length:  the size of the "authenticator" in bits.  This
      only applies to authentication/integrity related algorithms, and
      refers to the following sections. bit length remaining after truncation.  In this
      specification, this is always half the output length of the
      underlying hash algorithm.

2.1.1.  Data Origin Authentication and Integrity Verification Usage

   HMAC-SHA-256 is a

   HMAC-SHA-256+ are secret key algorithm. algorithms.  While no fixed key length
   is specified in [HMAC], this specification requires that for use when used as
   an
   integrity integrity/authentication algorithm, a fixed key length equal to
   the output length of 256-bits the hash functions MUST be supported, and key
   lengths other than 256-bits the output length of the associated hash function
   MUST NOT be supported.
   The 256-bit

   These key length is chosen restrictions are based in part on the
   recommendations in [HMAC] (i.e. key (key lengths less than the authenticator output length
   decrease security strength strength, and keys longer than the authenticator output length do
   not significantly increase security strength). strength), and in part because
   allowing variable length keys for IPsec authenticator functions would
   create interoperability issues.

2.1.2.  Pseudo-Random Function (PRF) Usage

   IKE and IKEv2 uses use PRFs for multiple purposes, most notably for generating keying material and for
   authentication of the IKE_SA.  The IKEv2 specification differentiates
   between PRFs with fixed key sizes and those with variable key sizes. sizes,
   and so we give some special guidance for this below.

   When the a PRF described in this document is used with IKE or IKEv2, it
   is considered to have a variable key length, and keys are derived in
   the following way (as ways (note that we simply reiterate that which is
   specified in [HMAC]):

   o  If the length of the key is exactly 256 bits long, the algorithm block size, use
      it as-is.

   o  If the key has fewer is shorter than 256 bits, the block size, lengthen it to exactly 256
      bits
      the block size by padding it on the right with zero bits.
      However, note that [HMAC] strongly discourages a key length less
      than 256 bits. the output length.  Nonetheless, we describe handling of
      shorter lengths here in recognition of shorter lengths typically
      chosen for IKE or IKEv2 preshared keys.

   o  If the key is 257 bits or longer, longer than the block size, shorten it to exactly 256 bits by computing
      the SHA2-256 corresponding hash of algorithm output over the entire key value,
      and use treat the resulting output value as your HMAC key.  Note that
      this will always result in a key that is less than the block size
      in length, and this key value will therefore require 0-padding (as
      described above) prior to use.

2.1.3.  Randomness and Key Strength

   [HMAC] discusses requirements for key material, including a
   requirement for strong randomness.  Therefore, a strong pseudo-random
   function MUST be used to generate the required 256-bit key for use with either HMAC-SHA-256-128 or HMAC-SHA-256-192. HMAC-
   SHA-256+.  At the time of this writing there are no published weak
   keys for use with HMAC-SHA-
   256. any HMAC-SHA-256+ algorithms.

2.1.4.  Key Distribution

   [ARCH] describes the general mechanism for obtaining keying material
   when multiple keys are required for a single SA (e.g. when an ESP SA
   requires a key for confidentiality and a key for authentication).  In
   order to provide data origin authentication and integrity
   verification, the key distribution mechanism must ensure that unique
   keys are allocated and that they are distributed only to the parties
   participating in the communication.

2.1.5.  Refreshing Keys

   [HMAC] makes the following recommendation with regard to rekeying:
   "Current

   There are no currently practical attacks do not indicate a specific against the algorithms
   recommended frequency for here, and especially against the key changes ... sizes recommended
   here.  However, periodic as noted in [HMAC] "...periodic key refreshment is a
   fundamental security practice that helps against potential weaknesses
   of the function and keys, and limits the damage of an exposed key."

   Putting this into perspective, this specification requires 256-bit 256, 384,
   or 512-bit keys produced by a strong PRF for use as a MAC.  A brute
   force attack on such keys would take more years longer to mount than the
   universe has been in existence.  On the other hand, weak keys (e.g.
   dictionary words) would be dramatically less resistant to attack.  It
   is important to take this, these points, along with the specific threat
   model for your particular application and the current state of the
   art with respect to attacks on SHA-256, SHA-384, and SHA-512 into
   account when determining an appropriate upper bound for HMAC key
   lifetimes

2.2.  Padding

   The HMAC-SHA-256 algorithms operate on 512-bit blocks of data, while
   the HMAC-SHA-384 and HMAC-SHA-512 algorithms operate on 1024-bit
   blocks of data.  Padding requirements are specified in [SHA2-1] and are as
   part of the
   SHA-256 algorithm, underlying SHA-256, SHA-384, and SHA-512 algorithms, so
   if you build SHA-256 implement according to [SHA2-1], you do not need to add any
   additional padding as far as the HMAC-SHA-256 HMAC-SHA-256+ algorithms specified
   here are concerned.  With regard to "implicit packet padding" as
   defined in [AH], no implicit packet padding is required.

2.3.  Truncation

   The HMAC-SHA-256 HMAC-SHA-256+ algorithms each produce a 256-bit authenticator nnn-bit value,
   and where nnn
   corresponds to the output bit length of the algorithm, e.g.  HMAC-
   SHA-nnn.  For use as an authenticator, this 256-bit nnn-bit value can be
   truncated as described in [HMAC].  When used as a data origin
   authentication and integrity verification algorithm in ESP, AH, IKE,
   or IKEv2, a truncated value using the first 128
   or 192 nnn/2 bits -- exactly
   half the algorithm output size -- MUST be supported.  No other
   authenticator value lengths are supported by this specification.

   Upon sending, the truncated value is stored within the authenticator
   field.  Upon receipt, the entire 256-bit nnn-bit value is computed and the
   first 128 or 192 nnn/2 bits are compared to the value stored in the
   authenticator field, with the value of 'nnn' depending on whether the
   negotiated algorithm is
   HMAC-SHA-256-128 or HMAC-SHA-256-192. algorithm.

   [HMAC] discusses potential security benefits resulting from
   truncation of the output MAC value, and in general, encourages HMAC
   users to perform MAC truncation.  In the context of IPsec, a minimum
   truncation length of 128 nnn/2 bits is selected because it corresponds to
   the birthday attack bound, bound for each of the HMAC-SHA-256+ algorithms,
   and it simultaneously serves to minimize the additional bits on the
   wire resulting from use of this facility.
   This specification also defines a truncation length of 192 in order
   to provide an alternative to those whose security needs outweigh
   their concern for minimizing bits on the wire.

2.4.  Using HMAC-SHA-256 HMAC-SHA-256+ As a PRF PRFs in IKE and IKEv2

   The HMAC-SHA-PRF-256 algorithm is identical to HMAC-SHA-256-128 and
   HMAC-SHA-256-192 HMAC-SHA-256-128,
   except that variable-length keys are permitted, and the truncation
   step is NOT performed.  The test vectors below which  Likewise, the implementations of HMAC-SHA-
   PRF-384 and HMAC-SHA-PRF-512 are simply labeled HMAC-SHA-256 may be used identical to validate
   implementations those of HMAC-SHA-PRF-256. HMAC-SHA-384-
   192 and HMAC-SHA-512-256 respectively, except that again, truncation
   is NOT performed.

2.5.  Interactions with the ESP ESP, IKE, or IKEv2 Cipher Mechanisms

   As of this writing, there are no known issues which preclude the use
   of the HMAC-SHA-256 HMAC-SHA-256+ algorithms with any specific cipher algorithm.

2.6.  HMAC-SHA-256+ Parameter Summary

   The following table serves to summarize the various quantities
   associated with the HMAC-SHA-256+ algorithms.

   +------------------+--------+--------+--------+----------+------------+
   |    Algorithm     | Block  | Output | Trunc. |   Key    | Algorithm  |
   |       ID         |  Size  | Length | Length |  Length  |   Type     |
   +==================+========+========+========+==========+============+
   | HMAC-SHA-256-128 |   512  |   256  |  128   |   256    | auth/integ |
   +------------------+--------+--------+--------+----------+------------+
   | HMAC-SHA-384-192 |  1024  |   384  |  192   |   384    | auth/integ |
   +------------------+--------+--------+--------+----------+------------+
   | HMAC-SHA-512-256 |  1024  |   512  |  256   |   512    | auth/integ |
   +------------------+--------+--------+--------+----------+------------+
   | HMAC-SHA-256-PRF |   512  |   256  | (none) | variable |     PRF    |
   +------------------+--------+--------+--------+----------+------------+
   | HMAC-SHA-384-PRF |  1024  |   384  | (none) | variable |     PRF    |
   +------------------+--------+--------+--------+----------+------------+
   | HMAC-SHA-512-PRF |  1024  |   512  | (none) | variable |     PRF    |
   +------------------+--------+--------+--------+----------+------------+

2.7.  Test Vectors

   The following test cases for HMAC-SHA-256-192 and HMAC-SHA-256-128 include the key, the data, and the resulting HMAC.
   authenticator and/or PRF values for each algorithm.  The values of
   keys and data are either hexadecimal numbers (prefixed by "0x") or ASCII character strings (surrounded by
   double quotes). quotes) or hexadecimal numbers.  If a value is an ASCII
   character string, then the HMAC computation for the corresponding
   test case DOES NOT include the trailing null character ('\0') of the
   string.  The computed HMAC values are all hexadecimal numbers.

2.7.1.  PRF Test Vectors

   These test cases were verified using 3 independent implementations:
   an HMAC wrapper on top borrowed from RFC 4231 [HMAC-TEST].  For
   reference implementations of Aaron Gifford's SHA256 implementation
   (http://www.adg.us/computers/sha.html), the BeeCrypt crypto library
   (http://beecrypt.sourceforge.net/) and underlying hash algorithms, see
   [SHA256+].  Note that for testing purposes, PRF output is considered
   to be simply the Nettle cryptographic
   library (www.lysator.liu.se/~nisse/nettle).  Partial blocks were
   padded as specified in [SHA2-1]. untruncated algorithm output.

      Test cases 1 and 2 were taken from the SHA-2 FIPS [SHA2-1] and Case PRF-1:
      Key =          0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
                     0b0b0b0b                          (20 bytes)

      Data =         4869205468657265                  ("Hi There")

      HMAC-SHA-256-PRF = b0344c61d8db38535ca8afceaf0bf12b
                         881dc200c9833da726e9376c2e32cff7

      HMAC-SHA-384-PRF = afd03944d84895626b0825f4ab46907f
                         15f9dadbe4101ec682aa034c7cebc59c
                         faea9ea9076ede7f4af152e8b2fa9cb6

      HMAC-SHA-512-PRF = 87aa7cdea5ef619d4ff0b4241a1d6cb0
                         2379f4e2ce4ec2787ad0b30545e17cde
                         daa833b7d6b8a702038b274eaea3f4e4
                         be9d914eeb61f1702e696c203a126854

   Test Case PRF-2:
   Key =          4a656665                          ("Jefe")

   Data =         7768617420646f2079612077616e7420  ("what do ya want ")
                  666f72206e6f7468696e673f          ("for nothing?")

   HMAC-SHA-256-PRF = 5bdcc146bf60754e6a042426089575c7
                      5a003f089d2739839dec58b964ec3843

   HMAC-SHA-384-PRF = af45d2e376484031617f78d2b58a6b1b
                      9c7ef464f5a01b47e42ec3736322445e
                      8e2240ca5e69e2c78b3239ecfab21649

   HMAC-SHA-512-PRF = 164b7a7bfcf819e2e395fbe73b56e0a3
                      87bd64222e831fd610270cd7ea250554
                      9758bf75c05a994a6d034f65f8f0e6fd
                      caeab1a34d4a6b4b636e070a38bce737

      Test Case PRF-3:
      Key            aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                     aaaaaaaa                          (20 bytes)

      Data =         dddddddddddddddddddddddddddddddd
                     dddddddddddddddddddddddddddddddd
                     dddddddddddddddddddddddddddddddd
                     dddd                              (50 bytes)

      HMAC-SHA-256-PRF = 773ea91e36800e46854db8ebd09181a7
                         2959098b3ef8c122d9635514ced565fe

      HMAC-SHA-384-PRF = 88062608d3e6ad8a0aa2ace014c8a86f
                         0aa635d947ac9febe83ef4e55966144b
                         2a5ab39dc13814b94e3ab6e101a34f27

      HMAC-SHA-512-PRF = fa73b0089d56a284efb0f0756c890be9
                         b1b5dbdd8ee81a3655f83e33b2279d39
                         bf3e848279a722c806b485a47e67c807
                         b946a337bee8942674278859e13292fb

      Test Case PRF-4:
      Key =          0102030405060708090a0b0c0d0e0f10
                     111213141516171819                (25 bytes)

      Data =         cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcd                              (50 bytes)

      HMAC-SHA-256-PRF = 82558a389a443c0ea4cc819899f2083a
                         85f0faa3e578f8077a2e3ff46729665b

      HMAC-SHA-384-PRF = 3e8a69b7783c25851933ab6290af6ca7
                         7a9981480850009cc5577c6e1f573b4e
                         6801dd23c4a7d679ccf8a386c674cffb

      HMAC-SHA-512-PRF = b0ba465637458c6990e5a8c5f61d4af7
                         e576d97ff94b872de76f8050361ee3db
                         a91ca5c11aa25eb4d679275cc5788063
                         a5f19741120c4f2de2adebeb10a298dd

   Test Case PRF-5:
   Key =          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaa                            (131 bytes)

   Data =         54657374205573696e67204c61726765  ("Test Using Large")
                  72205468616e20426c6f636b2d53697a  ("r Than Block-Siz")
                  65204b6579202d2048617368204b6579  ("e Key - Hash Key")
                  204669727374                      (" First")

   HMAC-SHA-256-PRF = 60e431591ee0b67f0d8a26aacbf5b77f
                      8e0bc6213728c5140546040f0ee37f54

   HMAC-SHA-384-PRF = 4ece084485813e9088d2c63a041bc5b4
                      4f9ef1012a2b588f3cd11f05033ac4c6
                      0c2ef6ab4030fe8296248df163f44952

   HMAC-SHA-512-PRF = 80b24263c7c1a3ebb71493c1dd7be8b4
                      9b46d1f41b4aeec1121b013783f8f352
                      6b56d037e05f2598bd0fd2215d6a1e52
                      95e64f73f63f0aec8b915a985d786598

   Test Case PRF-6:

   Key =          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  aaaaaa                            (131 bytes)

   Data =         54686973206973206120746573742075  ("This is a test
   cases 4-10 were borrowed from [HMAC-TEST] with some u")
                  73696e672061206c6172676572207468  ("sing a larger th")
                  616e20626c6f636b2d73697a65206b65  ("an block-size ke")
                  7920616e642061206c61726765722074  ("y and a larger t")
                  68616e20626c6f636b2d73697a652064  ("han block-size d")
                  6174612e20546865206b6579206e6565  ("ata. The key sizes adjust-
   ed for HMAC-SHA-256.  These nee")
                  647320746f2062652068617368656420  ("ds to be hashed ")
                  6265666f7265206265696e6720757365  ("before being use")
                  642062792074686520484d414320616c  ("d by the HMAC al")
                  676f726974686d2e                  ("gorithm.")

   HMAC-SHA-256-PRF = 9b09ffa71b942fcb27635fbcd5b0e944
                      bfdc63644f0713938a7f51535c3a35e2

   HMAC-SHA-384-PRF = 6617178e941f020d351e2f254e8fd32c
                      602420feb0b8fb9adccebb82461e99c5
                      a678cc31e799176d3860e6110c46523e

   HMAC-SHA-512-PRF = e37b6a775dc87dbaa4dfa9f96e5e3ffd
                      debd71f8867289865df5a32d20cdc944
                      b6022cac3c4982b10d5eeb55c3e4de15
                      134676fb6de0446065c97440fa8c6a58

2.7.2.  Authenticator Test Vectors

   In the following sections are test cases illustrate HMAC-SHA-256 with
   various combinations of input for HMAC-SHA256-128, HMAC-
   SHA384-192, and keysize.  All HMAC-SHA512-256.  PRF outputs are also included for
   convenience.  These test cases include
   the computed HMAC-SHA-256; only those with a keysize of 32 bytes (256
   bits) also include were generated using the truncated SHA256+
   reference code provided in [SHA256+].

2.7.2.1.  SHA256 Authentication Test Vectors
      Test Case AUTH256-1:
      Key =          0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
                     0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b  (32 bytes)

      Data =         4869205468657265                  ("Hi There")

      HMAC-SHA-256-PRF = 198a607eb44bfbc69903a0f1cf2bbdc5
                         ba0aa3f3d9ae3c1c7a3b1696a0b68cf7

      HMAC-SHA-256-128 and HMAC-SHA-256-
   192. = 198a607eb44bfbc69903a0f1cf2bbdc5

   Test Case #1: HMAC-SHA-256 with 3-byte input and 32-byte key
          Key_len         : 32 AUTH256-2:
   Key             : 0x0102030405060708090a0b0c0d0e0f10
                            1112131415161718191a1b1c1d1e1f20
          Data_len        : 3 =          4a6566654a6566654a6566654a656665   ("JefeJefeJefeJefe")
                  4a6566654a6566654a6566654a656665   ("JefeJefeJefeJefe")

   Data            : "abc"

          HMAC-SHA-256    : 0xa21b1f5d4cf4f73a4dd939750f7a066a
                            7f98cc131cb16a6692759021cfab8181

          HMAC-SHA-256-128: 0xa21b1f5d4cf4f73a4dd939750f7a066a

          HMAC-SHA-256-192: 0xa21b1f5d4cf4f73a4dd939750f7a066a
                            7f98cc131cb16a66 =         7768617420646f2079612077616e7420  ("what do ya want ")
                  666f72206e6f7468696e673f          ("for nothing?")

   HMAC-SHA-256-PRF = 167f928588c5cc2eef8e3093caa0e87c
                      9ff566a14794aa61648d81621a2a40c6

   HMAC-SHA-256-128 = 167f928588c5cc2eef8e3093caa0e87c

      Test Case #2: HMAC-SHA-256 with 56-byte input and 32-byte key
          Key_len         : 32 AUTH256-3:
      Key             : 0x0102030405060708090a0b0c0d0e0f10
                            1112131415161718191a1b1c1d1e1f20
          Data_len        : 56 =          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa  (32 bytes)

      Data            : "abcdbcdecdefdefgefghfghighijhijk
                             ijkljklmklmnlmnomnopnopq"

          HMAC-SHA-256    : 0x104fdc1257328f08184ba73131c53cae
                              e698e36119421149ea8c712456697d30

          HMAC-SHA-256-128: 0x104fdc1257328f08184ba73131c53cae

          HMAC-SHA-256-192: 0x104fdc1257328f08184ba73131c53cae
                              e698e36119421149 =         dddddddddddddddddddddddddddddddd
                     dddddddddddddddddddddddddddddddd
                     dddddddddddddddddddddddddddddddd
                     dddd                              (50 bytes)

      HMAC-SHA-256-PRF = cdcb1220d1ecccea91e53aba3092f962
                         e549fe6ce9ed7fdc43191fbde45c30b0

      HMAC-SHA-256-128 = cdcb1220d1ecccea91e53aba3092f962
      Test Case #3: HMAC-SHA-256 with 112-byte (multi-block) input
                        and 32-byte key
          Key_len         : 32 AUTH256-4:
      Key             : 0x0102030405060708090a0b0c0d0e0f10 =          0102030405060708090a0b0c0d0e0f10
                     1112131415161718191a1b1c1d1e1f20
          Data_len        : 112  (32 bytes)

      Data            : "abcdbcdecdefdefgefghfghighijhijk
                             ijkljklmklmnlmnomnopnopqabcdbcde
                             cdefdefgefghfghighijhijkijkljklm
                             klmnlmnomnopnopq"

          HMAC-SHA-256    : 0x470305fc7e40fe34d3eeb3e773d95aab
                              73acf0fd060447a5eb4595bf33a9d1a3

          HMAC-SHA-256-128: 0x470305fc7e40fe34d3eeb3e773d95aab

          HMAC-SHA-256-192: 0x470305fc7e40fe34d3eeb3e773d95aab
                              73acf0fd060447a5 =         cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcd                              (50 bytes)

      HMAC-SHA-256-PRF = 372efcf9b40b35c2115b1346903d2ef4
                         2fced46f0846e7257bb156d3d7b30d3f

      HMAC-SHA-256-128 = 372efcf9b40b35c2115b1346903d2ef4

2.7.2.2.  SHA384 Authentication Test Vectors

      Test Case #4:  HMAC-SHA-256 with 8-byte input and 32-byte key
          Key_len         : 32
          Key             : 0x0b repeated 32 times
          Data_len        : 8
          Data            : 0x4869205468657265 AUTH384-1:
      Key =          0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
                     0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
                     0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b  (48 bytes)

      Data            : "Hi There"

          HMAC-SHA-256    : 0x198a607eb44bfbc69903a0f1cf2bbdc5
                              ba0aa3f3d9ae3c1c7a3b1696a0b68cf7

          HMAC-SHA-256-128: 0x198a607eb44bfbc69903a0f1cf2bbdc5

          HMAC-SHA-256-192: 0x198a607eb44bfbc69903a0f1cf2bbdc5
                              ba0aa3f3d9ae3c1c =         4869205468657265                  ("Hi There")

      HMAC-SHA-384-PRF = b6a8d5636f5c6a7224f9977dcf7ee6c7
                         fb6d0c48cbdee9737a959796489bddbc
                         4c5df61d5b3297b4fb68dab9f1b582c2

      HMAC-SHA-384-128 = b6a8d5636f5c6a7224f9977dcf7ee6c7
                         fb6d0c48cbdee973

   Test Case #5:  HMAC-SHA-256 with 28-byte input and 4-byte key
          Key_len         : 4 AUTH384-2:
   Key             : "Jefe"
          Data_len        : 28 =          4a6566654a6566654a6566654a656665   ("JefeJefeJefeJefe")
                  4a6566654a6566654a6566654a656665   ("JefeJefeJefeJefe")
                  4a6566654a6566654a6566654a656665   ("JefeJefeJefeJefe")

   Data            : "what =         7768617420646f2079612077616e7420  ("what do ya want for nothing?"

          HMAC-SHA-256    : 0x5bdcc146bf60754e6a042426089575c7
                              5a003f089d2739839dec58b964ec3843 ")
                  666f72206e6f7468696e673f          ("for nothing?")

   HMAC-SHA-384-PRF = 2c7353974f1842fd66d53c452ca42122
                      b28c0b594cfb184da86a368e9b8e16f5
                      349524ca4e82400cbde0686d403371c9

   HMAC-SHA-384-192 = 2c7353974f1842fd66d53c452ca42122
                      b28c0b594cfb184d

      Test Case #6: HMAC-SHA-256 with 50-byte input and 32-byte key
          Key_len         : 32
          Key             : 0xaa repeated 32 times
          Data_len        : 50 AUTH384-3:
      Key =          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa  (48 bytes)

      Data            : 0xdd repeated 50 times

          HMAC-SHA-256    : 0xcdcb1220d1ecccea91e53aba3092f962
                              e549fe6ce9ed7fdc43191fbde45c30b0

          HMAC-SHA-256-128: 0xcdcb1220d1ecccea91e53aba3092f962

          HMAC-SHA-256-192: 0xcdcb1220d1ecccea91e53aba3092f962
                              e549fe6ce9ed7fdc =         dddddddddddddddddddddddddddddddd
                     dddddddddddddddddddddddddddddddd
                     dddddddddddddddddddddddddddddddd
                     dddd                              (50 bytes)

      HMAC-SHA-384-PRF = 809f439be00274321d4a538652164b53
                         554a508184a0c3160353e3428597003d
                         35914a18770f9443987054944b7c4b4a

      HMAC-SHA-384-192 = 809f439be00274321d4a538652164b53
                         554a508184a0c316

      Test Case #7: HMAC-SHA-256 with 50-byte input and 37-byte key
          Key_len         : 37 AUTH384-4:
      Key             : 0x0102030405060708090a0b0c0d0e0f10 =          0102030405060708090a0b0c0d0e0f10
                     1112131415161718191a1b1c1d1e1f20
                              2122232425
          Data_len        : 50
                     0a0b0c0d0e0f10111213141516171819  (48 bytes)

      Data            : 0xcd repeated 50 times

          HMAC-SHA-256    : 0xd4633c17f6fb8d744c66dee0f8f07455
                              6ec4af55ef07998541468eb49bd2e917 =         cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcd                              (50 bytes)

      HMAC-SHA-384-PRF = 5b540085c6e6358096532b2493609ed1
                         cb298f774f87bb5c2ebf182c83cc7428
                         707fb92eab2536a5812258228bc96687

      HMAC-SHA-384-192 = 5b540085c6e6358096532b2493609ed1
                         cb298f774f87bb5c

2.7.2.3.  SHA512 Authentication Test Vectors
      Test Case #8: HMAC-SHA-256 with 20-byte input and 32-byte key
          Key_len         : 32
          Key             : 0x0c repeated 32 times
          Data_len        : 20 AUTH512-1:
      Key =          0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
                     0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
                     0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
                     0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b  (64 bytes)

      Data            : "Test With Truncation"

          HMAC-SHA-256    : 0x7546af01841fc09b1ab9c3749a5f1c17
                              d4f589668a587b2700a9c97c1193cf42

          HMAC-SHA-256-128: 0x7546af01841fc09b1ab9c3749a5f1c17

          HMAC-SHA-256-192: 0x7546af01841fc09b1ab9c3749a5f1c17
                              d4f589668a587b27 =         4869205468657265                  ("Hi There")

      HMAC-SHA-512-PRF = 637edc6e01dce7e6742a99451aae82df
                         23da3e92439e590e43e761b33e910fb8
                         ac2878ebd5803f6f0b61dbce5e251ff8
                         789a4722c1be65aea45fd464e89f8f5b

      HMAC-SHA-512-256 = 637edc6e01dce7e6742a99451aae82df
                         23da3e92439e590e43e761b33e910fb8

   Test Case #9: HMAC-SHA-256 with 54-byte input and 80-byte key
          Key_len         : 80
          Key             : 0xaa repeated 80 times
          Data_len        : 54
          Data            : "Test Using Larger Than Block-Size Key -
                             Hash AUTH512-2:
   Key First"

          HMAC-SHA-256    : 0x6953025ed96f0c09f80a96f78e6538db
                              e2e7b820e3dd970e7ddd39091b32352f =          4a6566654a6566654a6566654a656665   ("JefeJefeJefeJefe")
                  4a6566654a6566654a6566654a656665   ("JefeJefeJefeJefe")
                  4a6566654a6566654a6566654a656665   ("JefeJefeJefeJefe")
                  4a6566654a6566654a6566654a656665   ("JefeJefeJefeJefe")

   Data =         7768617420646f2079612077616e7420  ("what do ya want ")
                  666f72206e6f7468696e673f          ("for nothing?")

   HMAC-SHA-512-PRF = cb370917ae8a7ce28cfd1d8f4705d614
                      1c173b2a9362c15df235dfb251b15454
                      6aa334ae9fb9afc2184932d8695e397b
                      fa0ffb93466cfcceaae38c833b7dba38

   HMAC-SHA-512-256 = cb370917ae8a7ce28cfd1d8f4705d614
                      1c173b2a9362c15df235dfb251b15454

      Test Case #10: HMAC-SHA-256 with 73-byte (multi-block) input
                         and 80-byte key
          Key_len         : 80
          Key             : 0xaa repeated 80 times
          Data_len        : 73 AUTH512-3:
      Key =          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa  (64 bytes)

      Data            : "Test Using Larger Than Block-Size =         dddddddddddddddddddddddddddddddd
                     dddddddddddddddddddddddddddddddd
                     dddddddddddddddddddddddddddddddd
                     dddd                              (50 bytes)

      HMAC-SHA-512-PRF = 2ee7acd783624ca9398710f3ee05ae41
                         b9f9b0510c87e49e586cc9bf961733d8
                         623c7b55cebefccf02d5581acc1c9d5f
                         b1ff68a1de45509fbe4da9a433922655

      HMAC-SHA-512-256 = 2ee7acd783624ca9398710f3ee05ae41
                         b9f9b0510c87e49e586cc9bf961733d8

      Test Case AUTH512-4:
      Key and
                             Larger Than One Block-Size Data"

          HMAC-SHA-256    : 0x6355ac22e890d0a3c8481a5ca4825bc8
                              84d3e7a1ff98a2fc2ac7d8e064c3b2e6 =          0a0b0c0d0e0f10111213141516171819
                     0102030405060708090a0b0c0d0e0f10
                     1112131415161718191a1b1c1d1e1f20
                     2122232425262728292a2b2c2d2e2f30
                     3132333435363738393a3b3c3d3e3f40  (64 bytes)

      Data =         cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
                     cdcd                              (50 bytes)

      HMAC-SHA-512-PRF = 5e6688e5a3daec826ca32eaea224eff5
                         e700628947470e13ad01302561bab108
                         b8c48cbc6b807dcfbd850521a685babc
                         7eae4a2a2e660dc0e86b931d65503fd2

      HMAC-SHA-512-256 = 5e6688e5a3daec826ca32eaea224eff5
                         e700628947470e13ad01302561bab108

3.  Security Considerations

   In a general sense, the security provided by the HMAC-SHA-256 HMAC-SHA-256+
   algorithms is based both upon the strength of SHA-256, the underlying hash
   algorithm, and upon the additional strength derived from the HMAC
   construct.  At the time of this writing there are no practical
   cryptographic attacks against
   either SHA-256 SHA-256, SHA-384, SHA-512 or HMAC.
   However, as with any cryptographic algorithm, an important component
   of HMAC-SHA-256's these algorithms' strength lies in the correctness of the
   algorithm implementation, the security of the key management
   mechanism, the strength of the associated secret key, and upon the
   correctness of the implementation in all of the participating
   systems.  This specification contains test vectors to assist in
   verifying the correctness of HMAC-SHA-256 code, the algorithm implementation, but these
   in no way verify the correctness (or security) of the surrounding
   security infrastructure.

3.1.  HMAC Key Length vs Truncation Length

   There are important differences between the security levels afforded
   by HMAC-SHA1-96 and the HMAC-SHA-256-128 and HMAC-SHA-256-192 HMAC-SHA-256+ algorithms, but there are also
   considerations which are somewhat counter-intuitive.  There are two
   different axes along which we gauge the security of these algorithms:
   HMAC output length and HMAC key length.  If we assume the HMAC key is
   a well-guarded secret which can only be determined through offline
   attacks on observed values, and that its length is less than or equal
   to the output length of the underlying hash algorithm, then the key's
   strength is directly proportional to its length.  And if we assume an
   adversary has no knowledge of the HMAC key, then the probability of
   guessing a correct MAC value for any given packet is directly
   proportional to the HMAC output length.

   This specification defines truncation to output lengths of either 128
   192, or 192 256 bits.  It is important to note that at this time, it is
   not clear that HMAC-SHA-256 with a truncation length of 128 bits is
   any more secure than HMAC-SHA1 with the same truncation length,
   assuming the adversary has no knowledge of the HMAC key.  This is
   because in such cases, the adversary must predict only those bits
   which remain after truncation.  Since in both cases that output
   length is the same (128 bits), the adversary's odds of correctly
   guessing the value are also the same in either case: 1 in 2^128.
   Again, if we assume the HMAC key remains unknown to the attacker,
   then only a bias in one of the algorithms would distinguish one from
   the other.  Currently, no such bias is known to exist in either HMAC-SHA1 HMAC-
   SHA1 or HMAC-SHA-256. HMAC-SHA-256+.

   If, on the other hand, the attacker is focused on guessing the HMAC
   key, and we assume that the hash algorithms are indistinguishable
   when viewed as PRF's, then the HMAC key length provides a direct
   measure of the underlying security: the longer the key, the harder it
   is to guess.  This means that with respect to passive attacks on the
   HMAC key, size matters - and the HMAC-SHA-256 algorithms, with their
   256-bit key lengths, HMAC-SHA-256+ algorithms provide
   more security in this regard than HMAC-
   SHA1 (with its 160-bit key length). HMAC-SHA1-96.

4.  IANA Considerations

   This document does not specify the conventions for using SHA256+ for
   IKE Phase 1 negotiations.  For IKE Phase 2 negotiations, IANA has
   assigned the following authentication algorithm identifiers:

   HMAC-SHA2-256:  5

   HMAC-SHA2-384:  6

   HMAC-SHA2-512:  7

   For use of HMAC-SHA-256 HMAC-SHA-256+ as a PRF in IKEv2, IANA has assigned the
   following IKEv2 Pseudo-random function (type 2) transform identifier:

   [TBA-1]  for
   identifiers:

   PRF_HMAC_SHA2_256  [TBA-1]

   PRF_HMAC_SHA2_384  [TBA-2]

   PRF_HMAC_SHA2_512  [TBA-3]

   For the use of the HMAC-SHA-256 HMAC-SHA-256+ algorithms for data origin
   authentication and integrity verification in IKEv2, ESP or AH, IANA
   has assigned the following IKEv2 integrity (type 3) transform
   identifiers:

   [TBA-2]  for

   AUTH_HMAC_SHA2_256_128

   [TBA-3]  for AUTH_HMAC_SHA2_256_192  [TBA-4]

   AUTH_HMAC_SHA2_384_192  [TBA-5]

   AUTH_HMAC_SHA2_512_256  [TBA-6]

5.  Acknowledgements

   Portions of this text were unabashedly borrowed from [HMAC-SHA1], and
   also
   from [XCBC-PRF]. [HMAC-TEST].  Thanks to Hugo Krawczyk for comments and
   recommendations on early revisions of this document, and thanks also
   to Russ Housley and Steve Bellovin for various security-related
   comments and recommendations.

6.  References

6.1.  Normative References

   [AH]       Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [ARCH]     Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [ESP]      Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [HMAC]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [HMAC-SHA1]
              Madsen, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
              ESP and AH", RFC 2404, November 1998.

   [HMAC-TEST]
              Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
              224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
              RFC 4231, December 2005.

   [IKE]      Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [IKEv2]    Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [SHA2-1]   NIST, "Draft FIPS "FIPS PUB 180-2 'Specifications for the Secure Hash
              Standard'", 2001 MAY, <http://csrc.nist.gov/
              publications/fips/fips180-2/
              fips180-2withchangenotice.pdf>. 2004 FEB, <http://csrc.nist.gov/publications/
              fips/fips180-2/fips180-2withchangenotice.pdf>.

   [SHA2-2]   NIST, "Descriptions of SHA-256, SHA-384, and SHA-512",
              2001 MAY,
              <http://csrc.nist.gov/cryptval/shs/sha256-384-512.pdf>.

6.2.  Informative References

   [HMAC-TEST]
              Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5

   [SHA256+]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-
              SHA-1", RFC 2202, September 1997.

   [XCBC-PRF]
              Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the
              Internet Key Exchange Protocol (IKE)", HMAC-SHA)", RFC 4434,
              February 4634, July 2006.

Authors' Addresses

   Scott G. Kelly
   Aruba Wireless Networks
   1322 Crossman Ave
   Sunnyvale, CA  94089
   US

   Email: scott@hyperthought.com
   Sheila Frankel
   NIST
   Bldg. 222 Room B264
   Gaithersburg, MD  20899
   US

   Email: sheila.frankel@nist.gov

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