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Using HMACSHA256, HMACSHA384, and HMACSHA512 with IPsec. S. Kelly, S. Frankel. May 2007.

Network Working Group S. Kelly
Request for Comments: 4868 Aruba Networks
Category: Standards Track S. Frankel
NIST
May 2007
Using HMACSHA256, HMACSHA384, and HMACSHA512 with IPsec
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.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This specification describes the use of Hashed Message Authentication
Mode (HMAC) in conjunction with the SHA256, SHA384, and SHA512
algorithms in IPsec. These algorithms may be used as the basis for
data origin authentication and integrity verification mechanisms for
the Authentication Header (AH), Encapsulating Security Payload (ESP),
Internet Key Exchange Protocol (IKE), and IKEv2 protocols, and also
as PseudoRandom Functions (PRFs) for IKE and IKEv2. Truncated
output lengths are specified for the authenticationrelated variants,
with the corresponding algorithms designated as HMACSHA256128,
HMACSHA384192, and HMACSHA512256. The PRF variants are not
truncated, and are called PRFHMACSHA256, PRFHMACSHA384, and
PRFHMACSHA512.
Kelly & Frankel Standards Track [Page 1]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The HMACSHA256+ Algorithms . . . . . . . . . . . . . . . . . 3
2.1. Keying Material . . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Data Origin Authentication and Integrity
Verification Usage . . . . . . . . . . . . . . . . . . 4
2.1.2. PseudoRandom Function (PRF) Usage . . . . . . . . . . 4
2.1.3. Randomness and Key Strength . . . . . . . . . . . . . 5
2.1.4. Key Distribution . . . . . . . . . . . . . . . . . . . 5
2.1.5. Refreshing Keys . . . . . . . . . . . . . . . . . . . 5
2.2. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Truncation . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Using HMACSHA256+ as PRFs in IKE and IKEv2 . . . . . . . 7
2.5. Interactions with the ESP, IKE, or IKEv2 Cipher
Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . 7
2.6. HMACSHA256+ Parameter Summary . . . . . . . . . . . . . 7
2.7. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . 7
2.7.1. PRF Test Vectors . . . . . . . . . . . . . . . . . . . 8
2.7.2. Authenticator Test Vectors . . . . . . . . . . . . . . 11
3. Security Considerations . . . . . . . . . . . . . . . . . . . 17
3.1. HMAC Key Length vs Truncation Length . . . . . . . . . . . 17
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1. Normative References . . . . . . . . . . . . . . . . . . . 19
6.2. Informative References . . . . . . . . . . . . . . . . . . 20
Kelly & Frankel Standards Track [Page 2]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
1. Introduction
This document specifies the use of SHA256, SHA384, and SHA512
[SHA21] combined with HMAC [HMAC] as data origin authentication and
integrity verification mechanisms for the IPsec AH [AH], ESP [ESP],
IKE [IKE], and IKEv2 [IKEv2] protocol. Output truncation is
specified for these variants, with the corresponding algorithms
designated as HMACSHA256128, HMACSHA384192, and HMACSHA512
256. These truncation lengths are chosen in accordance with the
birthday bound for each algorithm.
This specification also describes untruncated variants of these
algorithms as PseudoRandom Functions (PRFs) for use with IKE and
IKEv2, and those algorithms are called PRFHMACSHA256, PRFHMAC
SHA384, and PRFHMACSHA512. For ease of reference, these PRF
algorithms and the authentication variants described above are
collectively referred to below as "the HMACSHA256+ algorithms".
The goal of the PRF variants are to provide secure pseudorandom
functions suitable for generation of keying material and other
protocolspecific 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 HMACSHA
256+ when used in either case is dependent on the distribution scope
and unpredictability of the associated secret key. If the key is
unpredictable and known only by the sender and recipient, these
algorithms ensure that only parties holding an identical key can
derive the associated values.
2. The HMACSHA256+ Algorithms
[SHA21] and [SHA22] describe the underlying SHA256, SHA384, and
SHA512 algorithms, while [HMAC] describes the HMAC algorithm. The
HMAC algorithm provides a framework for inserting various hashing
algorithms such as SHA256, and [SHA256+] describes combined usage of
these algorithms. The following sections describe the various
characteristics and requirements of the HMACSHA256+ algorithms when
used with IPsec.
2.1. Keying Material
Requirements for keying material vary depending on 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
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RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
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 SHA256, this is 512 bits, for SHA384 and
SHA512, this is 1024 bits.
Output length: the size of the hash value produced by the underlying
hash algorithm. For SHA256, this is 256 bits, for SHA384 this
is 384 bits, and for SHA512, 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 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
HMACSHA256+ are secret key algorithms. While no fixed key length
is specified in [HMAC], this specification requires that when used as
an integrity/authentication algorithm, a fixed key length equal to
the output length of the hash functions MUST be supported, and key
lengths other than the output length of the associated hash function
MUST NOT be supported.
These key length restrictions are based in part on the
recommendations in [HMAC] (key lengths less than the output length
decrease security strength, and keys longer than the output length do
not significantly increase security strength), and in part because
allowing variable length keys for IPsec authenticator functions would
create interoperability issues.
2.1.2. PseudoRandom Function (PRF) Usage
IKE and IKEv2 use PRFs for generating keying material and for
authentication of the IKE Security Association. The IKEv2
specification differentiates between PRFs with fixed key sizes and
those with variable key sizes, and so we give some special guidance
for this below.
Kelly & Frankel Standards Track [Page 4]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
When 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 ways (note that we simply reiterate that which is
specified in [HMAC]):
o If the length of the key is exactly the algorithm block size, use
it asis.
o If the key is shorter than the block size, lengthen it to exactly
the block size by padding it on the right with zero bits.
However, note that [HMAC] strongly discourages a key length less
than 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 longer than the block size, shorten it by computing
the corresponding hash algorithm output over the entire key value,
and 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 zeropadding
(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 pseudorandom
function MUST be used to generate the required key for use with HMAC
SHA256+. At the time of this writing there are no published weak
keys for use with any HMACSHA256+ 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
Currently, there are no practical attacks against the algorithms
recommended here, and especially against the key sizes recommended
here. However, 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".
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RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
Putting this into perspective, this specification requires 256, 384,
or 512bit keys produced by a strong PRF for use as a MAC. A brute
force attack on such keys would take 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 these points, along with the specific threat
model for your particular application and the current state of the
art with respect to attacks on SHA256, SHA384, and SHA512 into
account when determining an appropriate upper bound for HMAC key
lifetimes.
2.2. Padding
The HMACSHA256 algorithms operate on 512bit blocks of data, while
the HMACSHA384 and HMACSHA512 algorithms operate on 1024bit
blocks of data. Padding requirements are specified in [SHA21] as
part of the underlying SHA256, SHA384, and SHA512 algorithms, so
if you implement according to [SHA21], you do not need to add any
additional padding as far as the HMACSHA256+ 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 HMACSHA256+ algorithms each produce an nnnbit value, where nnn
corresponds to the output bit length of the algorithm, e.g., HMAC
SHAnnn. For use as an authenticator, this nnnbit 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 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 nnnbit value is computed and the
first nnn/2 bits are compared to the value stored in the
authenticator field, with the value of 'nnn' depending on the
negotiated 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
truncation length of nnn/2 bits is selected because it corresponds to
the birthday attack bound for each of the HMACSHA256+ algorithms,
and it simultaneously serves to minimize the additional bits on the
wire resulting from use of this facility.
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RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
2.4. Using HMACSHA256+ as PRFs in IKE and IKEv2
The PRFHMACSHA256 algorithm is identical to HMACSHA256128,
except that variablelength keys are permitted, and the truncation
step is NOT performed. Likewise, the implementations of PRFHMAC
SHA384 and PRFHMACSHA512 are identical to those of HMACSHA384
192 and HMACSHA512256 respectively, except that again, variable
length keys are permitted, and truncation is NOT performed.
2.5. Interactions with the ESP, IKE, or IKEv2 Cipher Mechanisms
As of this writing, there are no known issues that preclude the use
of the HMACSHA256+ algorithms with any specific cipher algorithm.
2.6. HMACSHA256+ Parameter Summary
The following table serves to summarize the various quantities
associated with the HMACSHA256+ algorithms. In this table, "var"
stands for "variable".
+++++++
 Algorithm  Block  Output  Trunc.  Key  Algorithm 
 ID  Size  Length  Length  Length  Type 
+==================+========+========+========+========+============+
 HMACSHA256128  512  256  128  256  auth/integ 
+++++++
 HMACSHA384192  1024  384  192  384  auth/integ 
+++++++
 HMACSHA512256  1024  512  256  512  auth/integ 
+++++++
 PRFHMACSHA256  512  256  (none)  var  PRF 
+++++++
 PRFHMACSHA384  1024  384  (none)  var  PRF 
+++++++
 PRFHMACSHA512  1024  512  (none)  var  PRF 
+++++++
2.7. Test Vectors
The following test cases include the key, the data, and the resulting
authenticator, and/or PRF values for each algorithm. The values of
keys and data are either ASCII character strings (surrounded by
double 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.
Kelly & Frankel Standards Track [Page 7]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
2.7.1. PRF Test Vectors
These test cases were borrowed from RFC 4231 [HMACTEST]. For
reference implementations of the underlying hash algorithms, see
[SHA256+]. Note that for testing purposes, PRF output is considered
to be simply the untruncated algorithm output.
Test Case PRF1:
Key = 0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
0b0b0b0b (20 bytes)
Data = 4869205468657265 ("Hi There")
PRFHMACSHA256 = b0344c61d8db38535ca8afceaf0bf12b
881dc200c9833da726e9376c2e32cff7
PRFHMACSHA384 = afd03944d84895626b0825f4ab46907f
15f9dadbe4101ec682aa034c7cebc59c
faea9ea9076ede7f4af152e8b2fa9cb6
PRFHMACSHA512 = 87aa7cdea5ef619d4ff0b4241a1d6cb0
2379f4e2ce4ec2787ad0b30545e17cde
daa833b7d6b8a702038b274eaea3f4e4
be9d914eeb61f1702e696c203a126854
Test Case PRF2:
Key = 4a656665 ("Jefe")
Data = 7768617420646f2079612077616e7420 ("what do ya want ")
666f72206e6f7468696e673f ("for nothing?")
PRFHMACSHA256 = 5bdcc146bf60754e6a042426089575c7
5a003f089d2739839dec58b964ec3843
PRFHMACSHA384 = af45d2e376484031617f78d2b58a6b1b
9c7ef464f5a01b47e42ec3736322445e
8e2240ca5e69e2c78b3239ecfab21649
PRFHMACSHA512 = 164b7a7bfcf819e2e395fbe73b56e0a3
87bd64222e831fd610270cd7ea250554
9758bf75c05a994a6d034f65f8f0e6fd
caeab1a34d4a6b4b636e070a38bce737
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RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
Test Case PRF3:
Key aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaa (20 bytes)
Data = dddddddddddddddddddddddddddddddd
dddddddddddddddddddddddddddddddd
dddddddddddddddddddddddddddddddd
dddd (50 bytes)
PRFHMACSHA256 = 773ea91e36800e46854db8ebd09181a7
2959098b3ef8c122d9635514ced565fe
PRFHMACSHA384 = 88062608d3e6ad8a0aa2ace014c8a86f
0aa635d947ac9febe83ef4e55966144b
2a5ab39dc13814b94e3ab6e101a34f27
PRFHMACSHA512 = fa73b0089d56a284efb0f0756c890be9
b1b5dbdd8ee81a3655f83e33b2279d39
bf3e848279a722c806b485a47e67c807
b946a337bee8942674278859e13292fb
Test Case PRF4:
Key = 0102030405060708090a0b0c0d0e0f10
111213141516171819 (25 bytes)
Data = cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcd (50 bytes)
PRFHMACSHA256 = 82558a389a443c0ea4cc819899f2083a
85f0faa3e578f8077a2e3ff46729665b
PRFHMACSHA384 = 3e8a69b7783c25851933ab6290af6ca7
7a9981480850009cc5577c6e1f573b4e
6801dd23c4a7d679ccf8a386c674cffb
PRFHMACSHA512 = b0ba465637458c6990e5a8c5f61d4af7
e576d97ff94b872de76f8050361ee3db
a91ca5c11aa25eb4d679275cc5788063
a5f19741120c4f2de2adebeb10a298dd
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RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
Test Case PRF5:
Key = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaa (131 bytes)
Data = 54657374205573696e67204c61726765 ("Test Using Large")
72205468616e20426c6f636b2d53697a ("r Than BlockSiz")
65204b6579202d2048617368204b6579 ("e Key  Hash Key")
204669727374 (" First")
PRFHMACSHA256 = 60e431591ee0b67f0d8a26aacbf5b77f
8e0bc6213728c5140546040f0ee37f54
PRFHMACSHA384 = 4ece084485813e9088d2c63a041bc5b4
4f9ef1012a2b588f3cd11f05033ac4c6
0c2ef6ab4030fe8296248df163f44952
PRFHMACSHA512 = 80b24263c7c1a3ebb71493c1dd7be8b4
9b46d1f41b4aeec1121b013783f8f352
6b56d037e05f2598bd0fd2215d6a1e52
95e64f73f63f0aec8b915a985d786598
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RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
Test Case PRF6:
Key = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaa (131 bytes)
Data = 54686973206973206120746573742075 ("This is a test u")
73696e672061206c6172676572207468 ("sing a larger th")
616e20626c6f636b2d73697a65206b65 ("an blocksize ke")
7920616e642061206c61726765722074 ("y and a larger t")
68616e20626c6f636b2d73697a652064 ("han blocksize d")
6174612e20546865206b6579206e6565 ("ata. The key nee")
647320746f2062652068617368656420 ("ds to be hashed ")
6265666f7265206265696e6720757365 ("before being use")
642062792074686520484d414320616c ("d by the HMAC al")
676f726974686d2e ("gorithm.")
PRFHMACSHA256 = 9b09ffa71b942fcb27635fbcd5b0e944
bfdc63644f0713938a7f51535c3a35e2
PRFHMACSHA384 = 6617178e941f020d351e2f254e8fd32c
602420feb0b8fb9adccebb82461e99c5
a678cc31e799176d3860e6110c46523e
PRFHMACSHA512 = e37b6a775dc87dbaa4dfa9f96e5e3ffd
debd71f8867289865df5a32d20cdc944
b6022cac3c4982b10d5eeb55c3e4de15
134676fb6de0446065c97440fa8c6a58
2.7.2. Authenticator Test Vectors
The following sections are test cases for HMACSHA256128, HMAC
SHA384192, and HMACSHA512256. PRF outputs are also included for
convenience. These test cases were generated using the SHA256+
reference code provided in [SHA256+].
Kelly & Frankel Standards Track [Page 11]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
2.7.2.1. SHA256 Authentication Test Vectors
Test Case AUTH2561:
Key = 0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b (32 bytes)
Data = 4869205468657265 ("Hi There")
PRFHMACSHA256 = 198a607eb44bfbc69903a0f1cf2bbdc5
ba0aa3f3d9ae3c1c7a3b1696a0b68cf7
HMACSHA256128 = 198a607eb44bfbc69903a0f1cf2bbdc5
Test Case AUTH2562:
Key = 4a6566654a6566654a6566654a656665 ("JefeJefeJefeJefe")
4a6566654a6566654a6566654a656665 ("JefeJefeJefeJefe")
Data = 7768617420646f2079612077616e7420 ("what do ya want ")
666f72206e6f7468696e673f ("for nothing?")
PRFHMACSHA256 = 167f928588c5cc2eef8e3093caa0e87c
9ff566a14794aa61648d81621a2a40c6
HMACSHA256128 = 167f928588c5cc2eef8e3093caa0e87c
Test Case AUTH2563:
Key = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa (32 bytes)
Data = dddddddddddddddddddddddddddddddd
dddddddddddddddddddddddddddddddd
dddddddddddddddddddddddddddddddd
dddd (50 bytes)
PRFHMACSHA256 = cdcb1220d1ecccea91e53aba3092f962
e549fe6ce9ed7fdc43191fbde45c30b0
HMACSHA256128 = cdcb1220d1ecccea91e53aba3092f962
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RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
Test Case AUTH2564:
Key = 0102030405060708090a0b0c0d0e0f10
1112131415161718191a1b1c1d1e1f20 (32 bytes)
Data = cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcd (50 bytes)
PRFHMACSHA256 = 372efcf9b40b35c2115b1346903d2ef4
2fced46f0846e7257bb156d3d7b30d3f
HMACSHA256128 = 372efcf9b40b35c2115b1346903d2ef4
2.7.2.2. SHA384 Authentication Test Vectors
Test Case AUTH3841:
Key = 0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b (48 bytes)
Data = 4869205468657265 ("Hi There")
PRFHMACSHA384 = b6a8d5636f5c6a7224f9977dcf7ee6c7
fb6d0c48cbdee9737a959796489bddbc
4c5df61d5b3297b4fb68dab9f1b582c2
HMACSHA384128 = b6a8d5636f5c6a7224f9977dcf7ee6c7
fb6d0c48cbdee973
Test Case AUTH3842:
Key = 4a6566654a6566654a6566654a656665 ("JefeJefeJefeJefe")
4a6566654a6566654a6566654a656665 ("JefeJefeJefeJefe")
4a6566654a6566654a6566654a656665 ("JefeJefeJefeJefe")
Data = 7768617420646f2079612077616e7420 ("what do ya want ")
666f72206e6f7468696e673f ("for nothing?")
PRFHMACSHA384 = 2c7353974f1842fd66d53c452ca42122
b28c0b594cfb184da86a368e9b8e16f5
349524ca4e82400cbde0686d403371c9
HMACSHA384192 = 2c7353974f1842fd66d53c452ca42122
b28c0b594cfb184d
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RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
Test Case AUTH3843:
Key = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa (48 bytes)
Data = dddddddddddddddddddddddddddddddd
dddddddddddddddddddddddddddddddd
dddddddddddddddddddddddddddddddd
dddd (50 bytes)
PRFHMACSHA384 = 809f439be00274321d4a538652164b53
554a508184a0c3160353e3428597003d
35914a18770f9443987054944b7c4b4a
HMACSHA384192 = 809f439be00274321d4a538652164b53
554a508184a0c316
Test Case AUTH3844:
Key = 0102030405060708090a0b0c0d0e0f10
1112131415161718191a1b1c1d1e1f20
0a0b0c0d0e0f10111213141516171819 (48 bytes)
Data = cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcd (50 bytes)
PRFHMACSHA384 = 5b540085c6e6358096532b2493609ed1
cb298f774f87bb5c2ebf182c83cc7428
707fb92eab2536a5812258228bc96687
HMACSHA384192 = 5b540085c6e6358096532b2493609ed1
cb298f774f87bb5c
Kelly & Frankel Standards Track [Page 14]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
2.7.2.3. SHA512 Authentication Test Vectors
Test Case AUTH5121:
Key = 0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b
0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b (64 bytes)
Data = 4869205468657265 ("Hi There")
PRFHMACSHA512 = 637edc6e01dce7e6742a99451aae82df
23da3e92439e590e43e761b33e910fb8
ac2878ebd5803f6f0b61dbce5e251ff8
789a4722c1be65aea45fd464e89f8f5b
HMACSHA512256 = 637edc6e01dce7e6742a99451aae82df
23da3e92439e590e43e761b33e910fb8
Test Case AUTH5122:
Key = 4a6566654a6566654a6566654a656665 ("JefeJefeJefeJefe")
4a6566654a6566654a6566654a656665 ("JefeJefeJefeJefe")
4a6566654a6566654a6566654a656665 ("JefeJefeJefeJefe")
4a6566654a6566654a6566654a656665 ("JefeJefeJefeJefe")
Data = 7768617420646f2079612077616e7420 ("what do ya want ")
666f72206e6f7468696e673f ("for nothing?")
PRFHMACSHA512 = cb370917ae8a7ce28cfd1d8f4705d614
1c173b2a9362c15df235dfb251b15454
6aa334ae9fb9afc2184932d8695e397b
fa0ffb93466cfcceaae38c833b7dba38
HMACSHA512256 = cb370917ae8a7ce28cfd1d8f4705d614
1c173b2a9362c15df235dfb251b15454
Kelly & Frankel Standards Track [Page 15]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
Test Case AUTH5123:
Key = aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa (64 bytes)
Data = dddddddddddddddddddddddddddddddd
dddddddddddddddddddddddddddddddd
dddddddddddddddddddddddddddddddd
dddd (50 bytes)
PRFHMACSHA512 = 2ee7acd783624ca9398710f3ee05ae41
b9f9b0510c87e49e586cc9bf961733d8
623c7b55cebefccf02d5581acc1c9d5f
b1ff68a1de45509fbe4da9a433922655
HMACSHA512256 = 2ee7acd783624ca9398710f3ee05ae41
b9f9b0510c87e49e586cc9bf961733d8
Test Case AUTH5124:
Key = 0a0b0c0d0e0f10111213141516171819
0102030405060708090a0b0c0d0e0f10
1112131415161718191a1b1c1d1e1f20
2122232425262728292a2b2c2d2e2f30
3132333435363738393a3b3c3d3e3f40 (64 bytes)
Data = cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcdcdcdcdcdcdcdcdcdcdcdcdcdcdcd
cdcd (50 bytes)
PRFHMACSHA512 = 5e6688e5a3daec826ca32eaea224eff5
e700628947470e13ad01302561bab108
b8c48cbc6b807dcfbd850521a685babc
7eae4a2a2e660dc0e86b931d65503fd2
HMACSHA512256 = 5e6688e5a3daec826ca32eaea224eff5
e700628947470e13ad01302561bab108
Kelly & Frankel Standards Track [Page 16]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
3. Security Considerations
In a general sense, the security provided by the HMACSHA256+
algorithms is based both upon the strength of 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 SHA256, SHA384, SHA512, or HMAC.
However, as with any cryptographic algorithm, an important component
of 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 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 HMACSHA196 [HMACSHA1] and the HMACSHA256+ algorithms, but
there are also considerations that are somewhat counterintuitive.
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 wellguarded secret that 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 256 bits. It is important to note that at this time, it is
not clear that HMACSHA256 with a truncation length of 128 bits is
any more secure than HMACSHA1 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
that 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 HMACSHA1 or
HMACSHA256+.
If, on the other hand, the attacker is focused on guessing the HMAC
key, and we assume that the hash algorithms are indistinguishable
Kelly & Frankel Standards Track [Page 17]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
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 HMACSHA256+ algorithms provide
more security in this regard than HMACSHA196.
4. IANA Considerations
This document does not specify the conventions for using SHA256+ for
IKE Phase 1 negotiations, except to note that IANA has made the
following IKE hash algorithm attribute assignments:
SHA2256: 4
SHA2384: 5
SHA2512: 6
For IKE Phase 2 negotiations, IANA has assigned the following
authentication algorithm identifiers:
HMACSHA2256: 5
HMACSHA2384: 6
HMACSHA2512: 7
For use of HMACSHA256+ as a PRF in IKEv2, IANA has assigned the
following IKEv2 Pseudorandom function (type 2) transform
identifiers:
PRF_HMAC_SHA2_256 5
PRF_HMAC_SHA2_384 6
PRF_HMAC_SHA2_512 7
For the use of HMACSHA256+ algorithms for data origin
authentication and integrity verification in IKEv2, ESP, or AH, IANA
has assigned the following IKEv2 integrity (type 3) transform
identifiers:
AUTH_HMAC_SHA2_256_128 12
AUTH_HMAC_SHA2_384_192 13
AUTH_HMAC_SHA2_512_256 14
Kelly & Frankel Standards Track [Page 18]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
5. Acknowledgements
Portions of this text were unabashedly borrowed from [HMACSHA1] and
[HMACTEST]. 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 securityrelated
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.
[HMACSHA1] Madsen, C. and R. Glenn, "The Use of HMACSHA196
within ESP and AH", RFC 2404, November 1998.
[HMACTEST] Nystrom, M., "Identifiers and Test Vectors for HMACSHA
224, HMACSHA256, HMACSHA384, and HMACSHA512",
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.
[SHA21] NIST, "FIPS PUB 1802 'Specifications for the Secure
Hash Standard'", 2004 FEB, <http://csrc.nist.gov/
publications/fips/fips1802/
fips1802withchangenotice.pdf>.
[SHA256+] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and HMACSHA)", RFC 4634, July 2006.
Kelly & Frankel Standards Track [Page 19]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
6.2. Informative References
[SHA22] NIST, "Descriptions of SHA256, SHA384, and SHA512",
2001 MAY,
<http://csrc.nist.gov/cryptval/shs/sha256384512.pdf>.
Authors' Addresses
Scott G. Kelly
Aruba 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
Kelly & Frankel Standards Track [Page 20]
RFC 4868 HMACSHA256, SHA384, and SHA512 in IPsec May 2007
Full Copyright Statement
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contained in BCP 78, and except as set forth therein, the authors
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Kelly & Frankel Standards Track [Page 21]



