Re: [TLS] Encrypt-then-MAC again (was Re: padding bug)

Juho Vähä-Herttua <juhovh@iki.fi> Fri, 29 November 2013 18:24 UTC

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From: =?utf-8?Q?Juho_V=C3=A4h=C3=A4-Herttua?= <juhovh@iki.fi>
Date: Fri, 29 Nov 2013 20:20:29 +0200
To: Robert Ransom <rransom.8774@gmail.com>
Cc: "<tls@ietf.org>" <tls@ietf.org>, Peter Gutmann <p.gutmann@auckland.ac.nz>
Subject: Re: [TLS] Encrypt-then-MAC again (was Re: padding bug)
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> On 29.11.2013, at 19.26, Robert Ransom <rransom.8774@gmail.com> wrote:
> 
>> On 11/29/13, Juho Vähä-Herttua <juhovh@iki.fi> wrote:
>> 
>> You have been quite clear about that, and I've got the impression that you
>> are the only but very vocal opponent of encrypt-then-mac on this list.
> 
> Nikos Mavrogiannopoulos has also been opposed to encrypt-then-HMAC.  I
> don't know of any others.

I understood his opposition was not strong, I think the last comment from him supports that.

>> I'm sure you have good reasons for this opposition, so could you please
>> explain them in one or two sentences. This excluding the "encrypting the
>> HMAC makes it safer" argument, which might be true but as I understand is
>> not well proven.
> 
> "not well proven" is the wrong phrase here.  Marsh Ray solidly debunked it.

I was probably being too careful with my words, not being a cryptographer myself. But let's change that to "based on current knowledge is debunked" :)

To correct myself a bit more, CBC+HMAC is possible in AEAD by modifying plaintext before encryption as in McGrew et al. draft, but that's not very relevant to the discussion.

> To put it even more bluntly: OS and hardware vendors will treat TLS
> 1.2 or AEAD support and an efficient-in-secure-software AEAD cipher as
> new features, and sit on them for a few *years*.  TLS, and every
> pre-1.2 TLS implementation in the world, needs a critical security
> bugfix *yesterday*, and the only bugfix I see that is ready to be
> deployed as such is draft-gutmann-tls-encrypt-then-mac.

I agree with this, sounds quite likely.


Juho