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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.
Krawczyk, et. al. Informational [Page 1]
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.)
Krawczyk, et. al. Informational [Page 2]
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.
Krawczyk, et. al. Informational [Page 3]
RFC 2104 HMAC February 1997
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.
Krawczyk, et. al. Informational [Page 4]
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].)
Krawczyk, et. al. Informational [Page 5]
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
_same_ secret key K!) on about 2**64 known plaintexts. This would
require the processing of at least 2**64 blocks under H, an
impossible task in any realistic scenario (for a block length of 64
bytes this would take 250,000 years in a continuous 1Gbps link, and
without changing the secret key K during all this time). This attack
could become realistic only if serious flaws in the collision
behavior of the function H are discovered (e.g. collisions found
after 2**30 messages). Such a discovery would determine the immediate
replacement of the function H (the effects of such failure would be
far more severe for the traditional uses of H in the context of
digital signatures, public key certificates, etc.).
Note: this attack needs to be strongly contrasted with regular
collision attacks on cryptographic hash functions where no secret key
is involved and where 2**64 off-line parallelizable (!) operations
suffice to find collisions. The latter attack is approaching
feasibility [VW] while the birthday attack on HMAC is totally
impractical. (In the above examples, if one uses a hash function
with, say, 160 bit of output then 2**64 should be replaced by 2**80.)
Krawczyk, et. al. Informational [Page 6]
RFC 2104 HMAC February 1997
A correct implementation of the above construction, the choice of
random (or cryptographically pseudorandom) keys, a secure key
exchange mechanism, frequent key refreshments, and good secrecy
protection of keys are all essential ingredients for the security of
the integrity verification mechanism provided by HMAC.
Krawczyk, et. al. Informational [Page 7]
RFC 2104 HMAC February 1997
Appendix -- Sample Code
For the sake of illustration we provide the following sample code for
the implementation of HMAC-MD5 as well as some corresponding test
vectors (the code is based on MD5 code as described in [MD5]).
/*