The Use of HMAC-RIPEMD-160-96 within ESP and AH
RFC 2857
Network Working Group A. Keromytis
Request for Comments: 2857 University of Pennsylvania
Category: Standards Track N. Provos
Center for Information Technology Integration
June 2000
The Use of HMAC-RIPEMD-160-96 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.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This memo describes the use of the HMAC algorithm [RFC 2104] in
conjunction with the RIPEMD-160 algorithm [RIPEMD-160] as an
authentication mechanism within the revised IPSEC Encapsulating
Security Payload [ESP] and the revised IPSEC Authentication Header
[AH]. HMAC with RIPEMD-160 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 RIPEMD-160 [RIPEMD-160] 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-RIPEMD-160-96 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.
Keromytis & Provos Standards Track [Page 1]
RFC 2857 HMAC-RIPEMD-160-96 within ESP and AH June 2000
In this memo, HMAC-RIPEMD-160-96 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 [RFC 2119].
2. Algorithm and Mode
[RIPEMD-160] describes the underlying RIPEMD-160 algorithm, while
[RFC 2104] describes the HMAC algorithm. The HMAC algorithm provides
a framework for inserting various hashing algorithms such as RIPEMD-
160.
HMAC-RIPEMD-160-96 operates on 64-byte blocks of data. Padding
requirements are specified in [RIPEMD-160] and are part of the
RIPEMD-160 algorithm. Padding bits are only necessary in computing
the HMAC-RIPEMD-160 authenticator value and MUST NOT be included in
the packet.
HMAC-RIPEMD-160-96 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-RIPEMD-160-96.
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". [RIPEMD-160] provides some
performance analysis. As of this writing no detailed performance
analysis has been done of HMAC or HMAC combined with RIPEMD-160.
[RFC 2104] outlines an implementation modification which can improve
per-packet performance without affecting interoperability.
Keromytis & Provos Standards Track [Page 2]
RFC 2857 HMAC-RIPEMD-160-96 within ESP and AH June 2000
3. Keying Material
HMAC-RIPEMD-160-96 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 SHALL NOT be supported. A key length of 160-bits was
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