[TLS] Why is resumption_context hashed?

David Benjamin <davidben@chromium.org> Fri, 15 July 2016 10:39 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Fri, 15 Jul 2016 10:39:43 +0000
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Subject: [TLS] Why is resumption_context hashed?
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Every time resumption_context is used, it's fed into the PRF hash.
Handshake Context gets hashed since that actually expands to the full
concatenation and we want to be able to maintain a rolling hash.
But resumption_context is always a short value and is already the size of
the PRF hash. (If not resuming, it is the zero key, which is sized
appropriately. If resuming, it is the size of the PRF hash of the original
connection. But we require that resumptions use the same PRF, so that too
will be the right size.)

Was there some other reason we needed to hash it, or is a guarantee of
constant size sufficient to use it directly? If it still needs to be
hashed, it seems we ought to redefine resumption_context to be
Hash(HKDF-Expand-Label(...)) instead, mostly as a hint to implementors that
one may as well store the final value in the ticket.

David