Re: [TLS] Last Call: <draft-ietf-tls-encrypt-then-mac-02.txt> (Encrypt-then-MAC for TLS and DTLS) to Proposed Standard

mrex@sap.com (Martin Rex) Mon, 16 June 2014 15:35 UTC

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To: Hugo Krawczyk <hugo@ee.technion.ac.il>
Date: Mon, 16 Jun 2014 17:34:55 +0200 (CEST)
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From: mrex@sap.com (Martin Rex)
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Subject: Re: [TLS] Last Call: <draft-ietf-tls-encrypt-then-mac-02.txt> (Encrypt-then-MAC for TLS and DTLS) to Proposed Standard
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Hugo Krawczyk wrote:
>
> The technical results in my 2001 paper are correct but the conclusion
> regarding SSL/TLS is wrong. I assumed that TLS was using fresh IVs and that
> the MAC was computed on the encoded plaintext, i.e. Encode-Mac-Encrypt
> while TLS is doing Mac-Encode-Encrypt which is exactly what my theoretical
> example shows is insecure. The later padding attacks showed that the
> theoretical example of insecurity had a very practical instantiation in
> TLS.  While the paper shows correctly that MAC-then-Encrypt can be secure
> with both CBC and stream ciphers, it also shows that it requires a LOT of
> care about encoding - it turned out that TLS/SSL was not doing that. So if
> you want to keep Mac-then-Encrypt then you must change the encoding as well
> as how you apply the MAC. Changing to Encrypt-then-MAC is a much safer
> solution.

I agree with you that your paper demonstrates problem with
mac-extend-encrypt schemes.  And that it fails to notice that
TLS applies CBC padding after computing the MAC and before encryption,
and is therefore an mac-extend-encrypt scheme that can be susceptible to
decryption oracles.

But I strongly disagree to the assertion that "Encrypt-then-MAC" would
be a much safer scheme than *TRUE* MAC-then-Encrypt _withou_ any
extension inserted before the encryption such as CBC-padding.

In fact, the pad-mac-encrypt scheme for TLS-CBC-ciphersuites, as suggested
by Serge Vaudenay, is provably safer than the currently favoured
encrypt-then-MAC scheme.


-Martin