Subject: [IPsec] Question on RFC 4109

From: Christopher Meyer <[email-masked]>
Date: Tue, 27 Jan 2009 10:28:44 -0500

I have a question about RFC 4109.  In section 3, it says that 

 "AES-128 in XCBC mode for PRF functions ([RFC3566] and [RFC3664]) SHOULD 
be supported." 

However, it's not clear to me how to accomplish this with IKEv1 since the 
PRF is implied by the authentication algorithm and AES-XCBC-MAC-96 is not 
included in this section of the RFC (although it is listed in section 4 as 
described below). 

Also, section 4  (the summary section) says that "AES-XCBC-MAC-96 for PRF" 
SHOULD be supported, but I don't see it listed anywhere else in this RFC.  
This is also strange because AES-XCBC-MAC-96 is an authentication 
algorithm - not a PRF.    Furthermore, AES-XCBC-PRF-128 is NOT listed in 
this section at all. 

Can someone please clarify the SHOULD for AES-XCBC based algorithms for 
IKEv1?    Was the intent to list both AES-XCBC-MAC-96 and AES-XCBC-PRF-128 
as SHOULDs and then have the MAC algorithm imply this PRF?   If not, how 
is the AES-XCBC-PRF-128 algorithm supposed to be specified during 
negotiation?

Thanks,

Chris Meyer

IBM z/OS Communications Server

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