rfc2104.txt

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Network Working Group                                       H. Krawczyk
Request for Comments: 2104                                          IBM
Category: Informational                                      M. Bellare
                                                                   UCSD
                                                             R. Canetti
                                                                    IBM
                                                          February 1997


             HMAC: Keyed-Hashing for Message Authentication

Status of This Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document describes HMAC, a mechanism for message authentication
   using cryptographic hash functions. HMAC can be used with any
   iterative cryptographic hash function, e.g., MD5, SHA-1, in
   combination with a secret shared key.  The cryptographic strength of
   HMAC depends on the properties of the underlying hash function.

1. Introduction

   Providing a way to check the integrity of information transmitted
   over or stored in an unreliable medium is a prime necessity in the
   world of open computing and communications. Mechanisms that provide
   such integrity check based on a secret key are usually called
   "message authentication codes" (MAC). Typically, message
   authentication codes are used between two parties that share a secret
   key in order to validate information transmitted between these
   parties. In this document we present such a MAC mechanism based on
   cryptographic hash functions. This mechanism, called HMAC, is based
   on work by the authors [BCK1] where the construction is presented and
   cryptographically analyzed. We refer to that work for the details on
   the rationale and security analysis of HMAC, and its comparison to
   other keyed-hash methods.











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RFC 2104                          HMAC                     February 1997


   HMAC can be used in combination with any iterated cryptographic hash
   function. MD5 and SHA-1 are examples of such hash functions. HMAC
   also uses a secret key for calculation and verification of the
   message authentication values. The main goals behind this
   construction are

   * To use, without modifications, available hash functions.
     In particular, hash functions that perform well in software,
     and for which code is freely and widely available.

   * To preserve the original performance of the hash function without
     incurring a significant degradation.

   * To use and handle keys in a simple way.

   * To have a well understood cryptographic analysis of the strength of
     the authentication mechanism based on reasonable assumptions on the
     underlying hash function.

   * To allow for easy replaceability of the underlying hash function in
     case that faster or more secure hash functions are found or
     required.

   This document specifies HMAC using a generic cryptographic hash
   function (denoted by H). Specific instantiations of HMAC need to
   define a particular hash function. Current candidates for such hash
   functions include SHA-1 [SHA], MD5 [MD5], RIPEMD-128/160 [RIPEMD].
   These different realizations of HMAC will be denoted by HMAC-SHA1,
   HMAC-MD5, HMAC-RIPEMD, etc.

   Note: To the date of writing of this document MD5 and SHA-1 are the
   most widely used cryptographic hash functions. MD5 has been recently
   shown to be vulnerable to collision search attacks [Dobb].  This
   attack and other currently known weaknesses of MD5 do not compromise
   the use of MD5 within HMAC as specified in this document (see
   [Dobb]); however, SHA-1 appears to be a cryptographically stronger
   function. To this date, MD5 can be considered for use in HMAC for
   applications where the superior performance of MD5 is critical.   In
   any case, implementers and users need to be aware of possible
   cryptanalytic developments regarding any of these cryptographic hash
   functions, and the eventual need to replace the underlying hash
   function. (See section 6 for more information on the security of
   HMAC.)








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RFC 2104                          HMAC                     February 1997


2. Definition of HMAC

   The definition of HMAC requires a cryptographic hash function, which
   we denote by H, and a secret key K. We assume H to be a cryptographic
   hash function where data is hashed by iterating a basic compression
   function on blocks of data.   We denote by B the byte-length of such
   blocks (B=64 for all the above mentioned examples of hash functions),
   and by L the byte-length of hash outputs (L=16 for MD5, L=20 for
   SHA-1).  The authentication key K can be of any length up to B, the
   block length of the hash function.  Applications that use keys longer
   than B bytes will first hash the key using H and then use the
   resultant L byte string as the actual key to HMAC. In any case the
   minimal recommended length for K is L bytes (as the hash output
   length). See section 3 for more information on keys.

   We define two fixed and different strings ipad and opad as follows
   (the 'i' and 'o' are mnemonics for inner and outer):

                   ipad = the byte 0x36 repeated B times
                  opad = the byte 0x5C repeated B times.

   To compute HMAC over the data `text' we perform

                    H(K XOR opad, H(K XOR ipad, text))

   Namely,

    (1) append zeros to the end of K to create a B byte string
        (e.g., if K is of length 20 bytes and B=64, then K will be
         appended with 44 zero bytes 0x00)
    (2) XOR (bitwise exclusive-OR) the B byte string computed in step
        (1) with ipad
    (3) append the stream of data 'text' to the B byte string resulting
        from step (2)
    (4) apply H to the stream generated in step (3)
    (5) XOR (bitwise exclusive-OR) the B byte string computed in
        step (1) with opad
    (6) append the H result from step (4) to the B byte string
        resulting from step (5)
    (7) apply H to the stream generated in step (6) and output
        the result

   For illustration purposes, sample code based on MD5 is provided as an
   appendix.







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3. Keys

   The key for HMAC can be of any length (keys longer than B bytes are
   first hashed using H).  However, less than L bytes is strongly
   discouraged as it would decrease the security strength of the
   function.  Keys longer than L bytes are acceptable but the extra
   length would not significantly increase the function strength. (A
   longer key may be advisable if the randomness of the key is
   considered weak.)

   Keys need to be chosen at random (or using a cryptographically strong
   pseudo-random generator seeded with a random seed), and periodically
   refreshed.  (Current attacks do not indicate a specific recommended
   frequency for key changes as these attacks are practically
   infeasible.  However, 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.)

4. Implementation Note

   HMAC is defined in such a way that the underlying hash function H can
   be used with no modification to its code. In particular, it uses the
   function H with the pre-defined initial value IV (a fixed value
   specified by each iterative hash function to initialize its
   compression function).  However, if desired, a performance
   improvement can be achieved at the cost of (possibly) modifying the
   code of H to support variable IVs.

   The idea is that the intermediate results of the compression function
   on the B-byte blocks (K XOR ipad) and (K XOR opad) can be precomputed
   only once at the time of generation of the key K, or before its first
   use. These intermediate results are stored and then used to
   initialize the IV of H each time that a message needs to be
   authenticated.  This method saves, for each authenticated message,
   the application of the compression function of H on two B-byte blocks
   (i.e., on (K XOR ipad) and (K XOR opad)). Such a savings may be
   significant when authenticating short streams of data.  We stress
   that the stored intermediate values need to be treated and protected
   the same as secret keys.

   Choosing to implement HMAC in the above way is a decision of the
   local implementation and has no effect on inter-operability.









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RFC 2104                          HMAC                     February 1997


5. Truncated output

   A well-known practice with message authentication codes is to
   truncate the output of the MAC and output only part of the bits
   (e.g., [MM, ANSI]).  Preneel and van Oorschot [PV] show some
   analytical advantages of truncating the output of hash-based MAC
   functions. The results in this area are not absolute as for the
   overall security advantages of truncation. It has advantages (less
   information on the hash result available to an attacker) and
   disadvantages (less bits to predict for the attacker).  Applications
   of HMAC can choose to truncate the output of HMAC by outputting the t
   leftmost bits of the HMAC computation for some parameter t (namely,
   the computation is carried in the normal way as defined in section 2
   above but the end result is truncated to t bits). We recommend that
   the output length t be not less than half the length of the hash
   output (to match the birthday attack bound) and not less than 80 bits
   (a suitable lower bound on the number of bits that need to be
   predicted by an attacker).  We propose denoting a realization of HMAC
   that uses a hash function H with t bits of output as HMAC-H-t. For
   example, HMAC-SHA1-80 denotes HMAC computed using the SHA-1 function
   and with the output truncated to 80 bits.  (If the parameter t is not
   specified, e.g. HMAC-MD5, then it is assumed that all the bits of the
   hash are output.)

6. Security

   The security of the message authentication mechanism presented here
   depends on cryptographic properties of the hash function H: the
   resistance to collision finding (limited to the case where the
   initial value is secret and random, and where the output of the
   function is not explicitly available to the attacker), and the
   message authentication property of the compression function of H when
   applied to single blocks (in HMAC these blocks are partially unknown
   to an attacker as they contain the result of the inner H computation
   and, in particular, cannot be fully chosen by the attacker).

   These properties, and actually stronger ones, are commonly assumed
   for hash functions of the kind used with HMAC. In particular, a hash
   function for which the above properties do not hold would become
   unsuitable for most (probably, all) cryptographic applications,
   including alternative message authentication schemes based on such
   functions.  (For a complete analysis and rationale of the HMAC
   function the reader is referred to [BCK1].)








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RFC 2104                          HMAC                     February 1997


   Given the limited confidence gained so far as for the cryptographic
   strength of candidate hash functions, it is important to observe the
   following two properties of the HMAC construction and its secure use
   for message authentication:

   1. The construction is independent of the details of the particular
   hash function H in use and then the latter can be replaced by any
   other secure (iterative) cryptographic hash function.

   2. Message authentication, as opposed to encryption, has a
   "transient" effect. A published breaking of a message authentication
   scheme would lead to the replacement of that scheme, but would have
   no adversarial effect on information authenticated in the past.  This
   is in sharp contrast with encryption, where information encrypted
   today may suffer from exposure in the future if, and when, the
   encryption algorithm is broken.

   The strongest attack known against HMAC is based on the frequency of
   collisions for the hash function H ("birthday attack") [PV,BCK2], and
   is totally impractical for minimally reasonable hash functions.

   As an example, if we consider a hash function like MD5 where the
   output length equals L=16 bytes (128 bits) the attacker needs to
   acquire the correct message authentication tags computed (with the