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

Peter Gutmann <> Tue, 06 January 2015 11:37 UTC

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From: Peter Gutmann <>
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Thread-Topic: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis
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Date: Tue, 06 Jan 2015 11:37:13 +0000
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Subject: Re: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis
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"Paterson, Kenny" <> writes:

>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.

Ooh, good to know, thanks!  The truncated-MAC extension is actually kind of
silly, there's only one size allowed, 80 bits, which isn't really useful (you
save all of 48 bits for a 128-bit MAC) while for ridiculous-sized MACs like
HMAC-SHA256 and HMAC-SHA512 where you would want to truncate, say to 128 bits,
you can't do it.

>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.

Actually it does affect AES, since you can only truncate to 80 bits.