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