RE: a lengthy analysis of counter mode and ESP

[Chris Trobridge <CTrobridge@baltimore.com>]

I had a look at the paper and, assuming I interpreted it correctly, the gist of it is that you can differentiate between counter-mode key stream and a purely random sequence because a counter-mode key doesn't exhibit the birthday paradox.  This assumes that you have a counter-mode oracle that can provide the key stream.
 
Does anyone have any idea of the signficance of this property?
 
Chris  
-----Original Message----- 
From: Stephen Kent [mailto:kent@bbn.com] 
Sent: 16 August 2002 19:23 
To: ipsec@lists.tislabs.com 
Subject: a lengthy analysis of counter mode and ESP

We have been discussing how to make use of AES in counter mode with ESP.  Several folks have told me that the discussion is not generally understandable, because of the many subtle issues involved and because when the discussion moved to the general IPsec list, from the design team list, we failed to do an adequate job of establishing context. So, this very lengthy message is an attempt to provide background and a more thorough discussion of the various technical issues associated with this complex topic.  Hopefully, the WG as a whole will better understand the issues and be better positioned to make choices as a result of this analysis.

Steve 
----------

To put this issue in perspective, the current ID warns implementers against generating keystream for more than 2^64 blocks (not packets), even though we do not know of any specific vulnerabilities that could be exploited if this limit is exceeded. Yet we understand precisely what attacks could result if keystream is reused. Thus it makes sense to adopt approaches to keystream management that do not make it harder to do this task in a very secure fashion. 
 





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