Subject: Re: [MSEC] GDOI fix

From: Ran Canetti <[email-masked]>
Date: Wed, 19 Dec 2001 22:18:34 -0500

Brian, 

Prpending is just an easy way to be certain that there are no
decoding ambiguities: If the signed text contains variable-length fields 
and the encoding is stupid then an attacker may be able to play with the 
boundaries of fields to fool the signer. With prepending this is not possible.

As Cathy said, the reserved strings need only be prepended to the signed
text, not to the message itself. (BTW, this means that the bandwidth 
overhead is zero.) But if this is still a problem then 
sticking the reserved strings in a place with fixed offset from the 
beginning of the signed text is just as good.

Ran

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From: Brian Weis <[email-masked]>
Date: Thu, 20 Dec 2001 09:19:27 -0800

Thanks Ran and Cathy for both your replies. 

I had also thought of the logical prepending of the string, but
initially disliked it due to the complexity added to preparing the data
for the signing process. However, I think it might be less messy overall
than the KD payload append trick I mentioned earlier. I'll take a look
at the GDOI implementations to see how much of a problem it really is.

Ran, thanks for the alternate idea of using a fixed offset too.

Brian

-- 
Brian Weis
Strategic Cryptographic Development, ITD, Cisco Systems
Telephone: +1 408 526 4796
Email: [email-masked]