On Monday 21 December 2015 14:54:23 Brian Smith wrote: > Eric Rescorla <e...@rtfm.com> wrote: > > Sorry, I'm still confused TLS 1.2 uses a specific PRF. TLS 1.3 uses > > HKDF. Are you suggesting TLS 1.2 use the TLS 1.2 PRF with SHA-512 > > and that TLS 1.2 use SHA-512 with HKDF, or something different? > > I mean that TLS 1.2 should use SHA-512 with the TLS 1.2 PRF and that > TLS 1.3 should use SHA-512 with HKDF. > > > Nobody should pay attention to what the MTI cipher suite for TLS 1.2 > > is,> > >> because it's obsolete; in fact, one would be making a huge mistake > >> to > >> deploy it now if one's application didn't have legacy backward > >> compatibility concerns. And, we should change the MTI cipher suite > >> for TLS 1.3 to the ChaCha20-Poly1305 ones, because they solve a > >> lot of problems. For example, they remove any question of any need > >> to implement rekeying, they avoid the weird IV construction hacks > >> that are necessary for 128-bit cipher suites like AES-GCM, and > >> they can be implemented efficiently in a safe way, unlike AES-GCM. > > > > This seems like a separate question. > > You are the one that brought the MTI stuff into this, not me. > > > SHA-256-using cipher suites are widely deployed and not going away > > any time soon, so what resource are you trying to conserve here? > I'm trying to minimize the number of algorithms (amount of code) > necessary to implement ChaCha20-Poly1305 using x25519 for key > agreement and Ed25519 for signatures. The different between needing > or not needing SHA-256 matters most for very small computers (AVR and > Cortex-M0), but doesn't really matter much for larger computers where > SHA-256 has an advantage. > > In particular, since there seems to be a notable amount of hardware > that is or will soon be released that optimized for > ChaCha20-Poly1305+x25519+Ed25519, because of Apple HomeKit, it would > be nice to take advantage of that for TLS. > > Besides that, the inconsistency regarding why these new > 256-bit-encryption-key cipher suites are currently defined to use > SHA-256 in the PRF whereas all the existing 256-bit-encryption-key > cipher suites use SHA-384 seems strange. Even if an application wants > to use AES-GCM cipher suites, it would be able to avoid needing > SHA-256 if it implemented the AES256-GCM cipher suites instead of > AES128-GCM.
I'm not convinced about SHA-512, but yes, they probably should use SHA-384 at the very least. And given that the algorithm for SHA-384 and SHA-512 is essentially the same, using just different IVs, that should be usable for highly restricted hardware, wouldn't it? I would be against SHA-512 as that would be the very first cipher that uses SHA-512 PRF in TLS1.2, making its addition/implementation much more invasive to the underlying library, OTOH, we have multiple ciphers which use SHA-384 PRF. I think I just need to remind the delay after which NSS added support for SHA-384 compared to introduction to AES-128-GCM TLS ciphers... -- Regards, Hubert Kario Senior Quality Engineer, QE BaseOS Security team Web: www.cz.redhat.com Red Hat Czech s.r.o., Purkyňova 99/71, 612 45, Brno, Czech Republic
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