Re: [TLS] Data volume limits

sneves@dei.uc.pt Fri, 01 January 2016 17:19 UTC

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Date: Fri, 01 Jan 2016 17:16:30 +0000
From: sneves@dei.uc.pt
To: Aaron Zauner <azet@azet.org>
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Subject: Re: [TLS] Data volume limits
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Quoting Aaron Zauner <azet@azet.org>rg>:

> * Samuel Neves <sneves@dei.uc.pt> [01/01/2016 12:19:36] wrote:
>> OCB is, if anything, worse than GCM when it comes to data volume  
>> limits. It has the same confidentiality bounds as GCM
>> (slightly worse, in fact), but once you hit a collision you also  
>> lose authenticity and enable simple forgeries [1].
>>
>
> If I understand correctly the same is true for GCM?
>

The contention with GCM in this thread has been, so far, focused on  
confidentiality. This is because, by a result of Bernstein [1] (see  
also Appendix C of [2]), after q = 2^60 messages sent, plus q' = 2^60  
attempted forgeries by an attacker with messages at most l_A = 2^16  
blocks long, the probability of an attacker to forge a message is  
still ~2^-52. This does not present a data volume problem at the  
moment for the authentication part of AES-GCM.

On the other hand, after 2^60 OCB messages of 2^16 blocks (and thus  
2^76 total blocks), a block collision is almost guaranteed to have  
happened, enabling the aforementioned forgeries.

What you may be thinking of is the GCM behavior on _nonce reuse_. In  
this case, we are able to recover the authentication key by root  
finding and forge messages at will. This is also the case with  
OCB---on nonce reuse, we can forge any message that has the same  
checksum as a valid message.

[1] http://cr.yp.to/antiforgery/securitywcs-20050227.pdf
[2] https://eprint.iacr.org/2012/438