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
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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
- [TLS] Last Call: <draft-ietf-tls-encrypt-then-mac… The IESG
- Re: [TLS] Last Call: <draft-ietf-tls-encrypt-then… Yoav Nir
- Re: [TLS] Last Call: <draft-ietf-tls-encrypt-then… Yoav Nir
- Re: [TLS] Last Call: <draft-ietf-tls-encrypt-then… Hugo Krawczyk
- Re: [TLS] Last Call: <draft-ietf-tls-encrypt-then… Yoav Nir
- Re: [TLS] Last Call: <draft-ietf-tls-encrypt-then… Martin Rex