Subject: [AAA-WG]: Re: Resolution of Issue 235 - CMS replay attacks

From: Bernard Aboba <[email-masked]>
Date: Wed, 8 May 2002 08:19:34 -0700 (PDT)

OK. I think the approach below solves the problem. As I
understand it, you're suggesting including anti-replay AVPs in
all messages, and protecting them with CMS when the processing rules
indicate that. 

Bernard

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Stephen Farrell spake thusly:

"The E2E-Sequence AVP provides anti-replay protection for end to end
messages and is of type grouped. It contains a random value (an OctetString 
with a nonce) and counter (an Integer). For each end-to-end peer with which 
a node communicates (or remembers communicating) a different nonce value 
MUST be used and the counter is intitiated at zero and increases by one each 
time this AVP is emitted to that peer. This AVP MUST be included in all 
messages which use end-to-end protection (e.g. CMS signing or encryption)."

Assuming this only applies to CMS-protected stuff, I should have 
included a sentence like: "The recipient MUST reject the message if the 
E2E-Sequence and Event-Timestamp AVPs were not protected using end-to-end 
(e.g. CMS) protection that provides protection against substitution and
replacement attacks." 

I'm not stating anything about IPsec or TLS (though there was a recent
TLS thread with a hint that a relevant vulnerability could exist in
some TLS-like protocols). What I do mean is that if a message is being
send end to end, but without CMS protection, then there's still a
value in including the anti-replay AVPs since you then force a MITM 
to be an active attacker at the Diameter message level (i.e. they have
to decode the messages and tweak those AVP values), so as a result 
you're more likely to detect the replay (perhaps only later when an 
honest node re-uses a value via a different, honest, proxy).

Stephen.

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From: Stephen Farrell <[email-masked]>
Date: Wed, 08 May 2002 17:15:51 +0100

Exactly. 

I'll copy whatever text is final into the next CMS draft too.
 
Stephen.

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