RFC2404 - The Use of HMAC-SHA-1-96 within ESP and AH

Network Working Group C. Madson

Request for Comments: 2404 Cisco Systems Inc.

Category: Standards Track R. Glenn

NIST

November 1998

The Use of HMAC-SHA-1-96 within ESP and AH

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Abstract

This memo describes the use of the HMAC algorithm [RFC-2104] in

conjunction with the SHA-1 algorithm [FIPS-180-1] as an

authentication mechanism within the revised IPSEC Encapsulating

Security Payload [ESP] and the revised IPSEC Authentication Header

[AH]. HMAC with SHA-1 provides data origin authentication and

integrity protection.

Further information on the other components necessary for ESP and AH

implementations is provided by [Thayer97a].

1. IntrodUCtion

This memo specifies the use of SHA-1 [FIPS-180-1] combined with HMAC

[RFC-2104] as a keyed authentication mechanism within the context of

the Encapsulating Security Payload and the Authentication Header.

The goal of HMAC-SHA-1-96 is to ensure that the packet is authentic

and cannot be modified in transit.

HMAC is a secret key authentication algorithm. Data integrity and

data origin authentication as provided by HMAC are dependent upon the

scope of the distribution of the secret key. If only the source and

destination know the HMAC key, this provides both data origin

authentication and data integrity for packets sent between the two

parties; if the HMAC is correct, this proves that it must have been

added by the source.

In this memo, HMAC-SHA-1-96 is used within the context of ESP and AH.

For further information on how the various pieces of ESP - including

the confidentiality mechanism -- fit together to provide security

services, refer to [ESP] and [Thayer97a]. For further information on

AH, refer to [AH] and [Thayer97a].

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in [RFC2119].

2. Algorithm and Mode

[FIPS-180-1] describes the underlying SHA-1 algorithm, while [RFC-

2104] describes the HMAC algorithm. The HMAC algorithm provides a

framework for inserting various hashing algorithms such as SHA-1.

HMAC-SHA-1-96 operates on 64-byte blocks of data. Padding

requirements are specified in [FIPS-180-1] and are part of the SHA-1

algorithm. If you build SHA-1 according to [FIPS-180-1] you do not

need to add any additional padding as far as HMAC-SHA-1-96 is

concerned. With regard to "implicit packet padding" as defined in

[AH] no implicit packet padding is required.

HMAC-SHA-1-96 produces a 160-bit authenticator value. This 160-bit

value can be truncated as described in RFC2104. For use with either

ESP or AH, a truncated value using the first 96 bits MUST be

supported. Upon sending, the truncated value is stored within the

authenticator field. Upon receipt, the entire 160-bit value is

computed and the first 96 bits are compared to the value stored in

the authenticator field. No other authenticator value lengths are

supported by HMAC-SHA-1-96.

The length of 96 bits was selected because it is the default

authenticator length as specified in [AH] and meets the security

requirements described in [RFC-2104].

2.1 Performance

[Bellare96a] states that "(HMAC) performance is essentially that of

the underlying hash function". As of this writing no detailed

performance analysis has been done of SHA-1, HMAC or HMAC combined

with SHA-1.

[RFC-2104] outlines an implementation modification which can improve

per-packet performance without affecting interoperability.

3. Keying Material

HMAC-SHA-1-96 is a secret key algorithm. While no fixed key length is

specified in [RFC-2104], for use with either ESP or AH a fixed key

length of 160-bits MUST be supported. Key lengths other than 160-

bits MUST NOT be supported (i.e. only 160-bit keys are to be used by

HMAC-SHA-1-96). A key length of 160-bits was chosen based on the

recommendations in [RFC-2104] (i.e. key lengths less than the

authenticator length decrease security strength and keys longer than

the authenticator length do not significantly increase security

strength).

[RFC-2104] discusses requirements for key material, which includes a

discussion on requirements for strong randomness. A strong pseudo-

random function MUST be used to generate the required 160-bit key.

At the time of this writing there are no specified weak keys for use

with HMAC. This does not mean to imply that weak keys do not exist.

If, at some point, a set of weak keys for HMAC are identified, the

use of these weak keys must be rejected followed by a request for

replacement keys or a newly negotiated Security Association.

[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, the key distribution

mechanism must ensure that unique keys are allocated and that they

are distributed only to the parties participating in the

communication.

[RFC-2104] makes the following recommendation with regard to

rekeying. 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, reduces the information avaliable to a

cryptanalyst, and limits the damage of an eXPosed key.

4. Interaction with the ESP Cipher Mechanism

As of this writing, there are no known issues which preclude the use

of the HMAC-SHA-1-96 algorithm with any specific cipher algorithm.

5. Security Considerations

The security provided by HMAC-SHA-1-96 is based upon the strength of

HMAC, and to a lesser degree, the strength of SHA-1. At the time of

this writing there are no practical cryptographic attacks against

HMAC-SHA-1-96.

[RFC-2104] states that for "minimally reasonable hash functions" the

"birthday attack" is impractical. For a 64-byte block hash such as

HMAC-SHA-1-96, an attack involving the successful processing of 2**80

blocks would be infeasible unless it were discovered that the

underlying hash had collisions after processing 2**30 blocks. A hash

with such weak collision-resistance characteristics would generally

be considered to be unusable.

It is also important to consider that while SHA-1 was never developed

to be used as a keyed hash algorithm, HMAC had that criteria from the

onset.

[RFC-2104] also discusses the potential additional security which is

provided by the truncation of the resulting hash. Specifications

which include HMAC are strongly encouraged to perform this hash

truncation.

As [RFC-2104] provides a framework for incorporating various hash

algorithms with HMAC, it is possible to replace SHA-1 with other

algorithms such as MD5. [RFC-2104] contains a detailed discussion on

the strengths and weaknesses of HMAC algorithms.

As is true with any cryptographic algorithm, part of its strength

lies in the correctness of the algorithm implementation, the security

of the key management mechanism and its implementation, the strength

of the associated secret key, and upon the correctness of the

implementation in all of the participating systems. [RFC-2202]

contains test vectors and example code to assist in verifying the

correctness of HMAC-SHA-1-96 code.

6. Acknowledgments

This document is derived in part from previous works by Jim Hughes,

those people that worked with Jim on the combined DES/CBC+HMAC-MD5

ESP transforms, the ANX bakeoff participants, and the members of the

IPsec working group.

We would also like to thank Hugo Krawczyk for his comments and

recommendations regarding some of the cryptographic specific text in

this document.

7. References

[FIPS-180-1] NIST, FIPS PUB 180-1: Secure Hash Standard,

April 1995.

http://csrc.nist.gov/fips/fip180-1.txt (ascii)

http://csrc.nist.gov/fips/fip180-1.ps (postscript)

[RFC-2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-

Hashing for Message Authentication", RFC2104, February

1997.

[Bellare96a] Bellare, M., Canetti, R., and H. Krawczyk, "Keying Hash

Functions for Message Authentication", Advances in

Cryptography, Crypto96 Proceeding, June 1996.

[ARCH] Kent, S., and R. Atkinson, "Security Architecture for

the Internet Protocol", RFC2401, November 1998.

[ESP] Kent, S., and R. Atkinson, "IP Encapsulating Security

Payload", RFC2406, November 1998.

[AH] Kent, S., and R. Atkinson, "IP Authentication Header",

RFC2402, November 1998.

[Thayer97a] Thayer, R., Doraswamy, N., and R. Glenn, "IP Security

Document Roadmap", RFC2411, November 1998.

[RFC-2202] Cheng, P., and R. Glenn, "Test Cases for HMAC-MD5 and

HMAC-SHA-1", RFC2202, March 1997.

[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

8. Editors' Address

Cheryl Madson

Cisco Systems, Inc.

EMail: cmadson@cisco.com

Rob Glenn

NIST

EMail: rob.glenn@nist.gov

The IPsec working group can be contacted through the chairs:

Robert Moskowitz

ICSA

EMail: rgm@icsa.net

Ted T'so

Massachusetts Institute of Technology

EMail: tytso@mit.edu

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