Re: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis

"Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk> Tue, 06 January 2015 10:37 UTC

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From: "Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk>
To: Peter Gutmann <pgut001@cs.auckland.ac.nz>, "<tls@ietf.org>" <tls@ietf.org>
Thread-Topic: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis
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Date: Tue, 6 Jan 2015 10:35:16 +0000
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Subject: Re: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis
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Hi

On 06/01/2015 10:27, "Peter Gutmann" <pgut001@cs.auckland.ac.nz>; wrote:

>Michael Clark <michael@metaparadigm.com>; writes:
>
>>Before I've read through can we do this with it?
>>
>>AES-256-GCM + hmac_null   = 128 bits authentication
>>AES-256-CBC + hmac_sha128 = 128 bits authentication
>>
>>AES-256-GCM + hmac_sha128 = 256 bits authentication
>>AES-256-CBC + hmac_sha256 = 256 bits authentication
>>
>>AES-256-GCM + hmac_sha256 = 384 bits authentication
>>AES-256-CBC + hmac_sha384 = 384 bits authentication
>
>The MAC is whatever is negotiated for the session, and if you want a
>shorter
>one you can send the truncated-MAC extension (which, however, nothing
>supports
>AFAIK).  

It's good that nothing supports the truncated-MAC extension, because, in
combination with TLS's support for variable length padding, it introduces
a security vulnerability. See:

http://www.isg.rhul.ac.uk/~kp/mee-comp.pdf


which shows that if the tag size (MAC length) is shorter than the CBC-mode
block size, then there's a distinguishing attack that can tell the
difference between the encryption of a short message and a longer message
(even though both are padded to the same size before encryption).

This is not an issue for any of the cihersuites proposed above, but it's
something to be kept in mind when considering "exotic" ciphersuites.

Regards

Kenny 


>Having said that, see my previous message about not needing a
>thousand options, we only need one that everyone uses.
>
>Peter.
>
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