Re: [IPsec] [ipsecme] #112: Truncation of SHA-1 ICVs

[Tero Kivinen <kivinen at iki.fi>]

Frankel, Sheila E. writes:
> Additional text:
>    Some of these algorithms generate a fixed-length ICV, which is truncated 
>    when it is included in an IPsec-protected packet. For example, standard 
>    HMAC-SHA-1 generates a 160-bit ICV, which is truncated to 96 bits when it 
>    is used to provide integrity-protection to an ESP or AH packet. The 
>    individual RFC descriptions mention those algorithms that are truncated. 
>    When these algorithms are used to protect IKEv1 SAs, they are not 
>    truncated. For HMAC-SHA-1 and HMAC-MD5, the IKEv2 IANA registry contains 
>    values for both the truncated version and the standard non-truncated 
>    version; thus, IKEv2 has the capability to negotiate either version to 
>    protect IKEv2 and/or IPsec-v3 SAs.

This is not completely correct. The non-truncated versions are not
meant to be used with normal IPsec-v2/v3. They are meant to be used
with Fibre Channel Security (RFC4595). The AUTH_HMAC_MD5_128 and
AUTH_HMAC_MSHA1_160 are really only specified when used with Fibre
Channel CT_Authentication format instead of ESP format. Even when
using fibre channel if ESP format is used then I think you must use
the truncated versions.

Here is some cut & paste parts of RFC4595 to explain situation:

----------------------------------------------------------------------
2. Overview

   Fibre Channel defines two security protocols that provide security
   services for different portions of Fibre Channel traffic: the
   ESP_Header defined in [FC-FS] and CT_Authentication defined in
   [FC-GS-4].

   The ESP_Header protocol is a transform applied to FC-2 Fibre Channel
   frames.  It is based on the IP Encapsulation Security Payload
   [RFC4303] to provide origin authentication, integrity, anti-replay
   protection, and optional confidentiality to generic fibre channel
   frames.  The CT_Authentication protocol is a transform that provides
   the same set of security services for Common Transport Information
   Units, which are used to convey control information.  As a result of
   the separation of Fibre Channel data traffic from control traffic,
   only one protocol (either ESP_Header or CT_Authentication) is
   applicable to any FC Security Association (SA).
...
   Since IP is transported over Fibre Channel [RFC4338] and Fibre
   Channel/SCSI are transported over IP [RFC3643], [RFC3821] there is
   the potential for confusion when IKEv2 is used for both IP and FC
   traffic.  This document specifies identifiers for IKEv2 over FC in a
   fashion that ensures that any mistaken usage of IKEv2/FC over IP will
   result in a negotiation failure due to the absence of an acceptable
   proposal (and likewise for IKEv2/IP over FC).  This document gives an
   overview of the security architecture defined by the FC-SP standard,
   including the security protocols used to protect frames and to
   negotiate SAs, and it specifies the entities for which new
   identifiers have been assigned.
...
3.2. CT_Authentication Protocol


   CT_Authentication is a security protocol for Common Transport FC-4
   Information Units that provides origin authentication, integrity, and
   anti-replay protection.  The CT_Authentication protocol is carried in
   the optional extended CT_IU preamble
...
   The Authentication Hash Block is computed as an HMAC keyed hash of
   the CT_IU, as defined in [RFC2104].  The entire output of the HMAC
   computation is included in the Authentication Hash Block, without any
   truncation.  Two transforms are defined: HMAC-SHA1-160 that is based
   on the cryptographic hash function SHA1 [NIST.180-1.1995], and
   HMAC-MD5-128 that is based on the cryptographic hash function MD5
   [RFC1321].
...
4.3. CT_Authentication Protocol Transform Identifiers


   The CT_Authentication Transform IDs defined for Transform Type 3
   (Integrity Algorithm) are:

           Name                   Number                    Defined in
           ----                   ------                    ----------
           AUTH_HMAC_MD5_128      6                         FC-SP

           AUTH_HMAC_SHA1_160     7                         FC-SP

   These transforms differ from the corresponding _96 transforms used in
   IPsec solely in the omission of the truncation of the HMAC output to
   96 bits; instead, the entire output (128 bits for MD5, 160 bits for
   SHA-1) is transmitted.  MD5 support is required due to existing usage
   of MD5 in CT_Authentication; SHA-1 is RECOMMENDED in all new
   implementations.
...
----------------------------------------------------------------------

So AUTH_HMAC_MD5_128 and AUTH_HMAC_SHA1_160 cannot be used in IPsec,
but they have the numbers in the IKEv2 registry, as they are
negotiated for their CT_Authentication use using IKEv2. 

> For the other algorithms (AES-XCBC, 
>    HMAC-SHA-256/384/512, AES-CMAC and HMAC-RIPEMD), only the truncated 
>    version can be used for both IKEv2 and IPsec-v3 SAs.
>  
> NOTE to Tero, Paul, Yaron: do we want to expand the IKEv2 IANA
> registry to include non-truncated AES-XCBC-MAC,
> HMAC-SHA-256/384/512, AES-CMAC and HMAC-RIPEMD? 

Not for IPsec use. I do not know if the Fibre Channel people want to
use non-truncated versions of them in their CT_Authentication format,
but for IPsec if you want to have longer MAC, use longer HMAC-SHA-2
variant... 
-- 
kivinen at iki.fi