Re: [TLS] padding bug

Peter Gutmann <> Tue, 24 September 2013 12:31 UTC

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From: Peter Gutmann <>
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Date: Tue, 24 Sep 2013 12:31:12 +0000
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Subject: Re: [TLS] padding bug
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Nikos Mavrogiannopoulos <> writes:

>Since you are the one claiming that MAC truncation isn't necessary when all
>existing protocols do it (even TLS does it on the Finished message MAC), it
>may be better for you to present evidence that this isn't necessary.

Before that, you'd have to present evidence that "all existing protocols do
it".  As I've already pointed out, S/MIME doesn't, PGP doesn't, TLS doesn't,
SSH doesn't, ... .  In fact of the major IETF application-level security
protocols that I can think of, only IPsec does, and AFAIK (meaning based on
long-ago discussions with one of the IPsec folks) that was done in order to
fit the MAC into the fixed-size AH header (although there seems to be some
disagreement on the details).

As I've also already pointed out, TLS has for some years now provided for MAC
truncation to a range of sizes.  If anyone's really worried about this, they
can request truncation to whatever size they prefer.