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Re: [TLS] rfc7366: is encrypt-then-mac implemented?

Peter Gutmann <pgut001@cs.auckland.ac.nz> Thu, 30 October 2014 10:44 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: "<tls@ietf.org>" <tls@ietf.org>
Thread-Topic: [TLS] rfc7366: is encrypt-then-mac implemented?
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Date: Thu, 30 Oct 2014 10:44:19 +0000
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Subject: Re: [TLS] rfc7366: is encrypt-then-mac implemented?
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Manuel Pégourié-Gonnard <mpg@polarssl.org> writes:

>Sure, I'd never think of doing that. But I can certainly tell my code to
>compute
>
>  MAC_k( seq_num + type + version + length + encrypted_stuff )
>
>with length != length_of(encrypted_stuff). After all, for the MAC function,
>what we call "length" is just two meaningless bytes.

Oh, so the issue isn't that you're MAC'ing extra stuff that isn't there, but
that you're using a length value that includes stuff that isn't MAC'd?  In
other words in the above 'length' isn't 'length of encrypted_stuff that
follows the length value' but 'length of encrypted_stuff that follows the
length value + length of MAC'?

Peter.

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