On Wed 2016-06-15 04:44:59 -0400, Yoav Nir wrote: > I disagree that this is a low level crypto decision, or at least that this is > mainly so. > > There is the question of whether using the same key for application data and > handshake is harmful. That question is mainly low level crypto and could be > asked of CFRG. > > There is the other question of whether exposing the fact that there are > handshake messages and when they occur is harmful. That is security-related, > but not at all related to crypto. > > Weighing these two potential harms against each other and coming to a > decision is entirely an engineering issue, and we should not offload that to > CFRG.
To be clear, we're being asked to trade these things off against each other here, but there are other options which were ruled out in the prior framing of the question which don't rule either of them out. In particular, if we're willing to pay the cost of a slightly more complex key schedule (and an increased TLS record size), we could have "packet header" keys which protect the content-type itself for all non-cleartext TLS records. If we do that, these keys might as well also be used to protect the TLS record size itself. This would result in an opaque data stream (though obviously record size would still leak in DTLS, and timing and framing is still likely to leak the record size in the lowest-latency TLS applications). The current framing of the question pits these things against each other as a tradeoff, but if we want to, we could protect them all. --dkg
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