proto_handshake: clear the cookie before freeing it - #441
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struct handshake_cookie holds x, the diffie-hellman private value, and the two diffie-hellman parameter MAC keys derived from the shared secret. All four sites which free the cookie did so without wiping it: handshakefail(), proto_handshake() at err1, handshakedone(), and proto_handshake_cancel(). handshakedone() runs once per successful connection, at exactly the point where x has served its purpose and should stop existing; forgetting it is what makes the PFS handshake forward-secret. Wipe on all four rather than only where x is known to be populated, so that "the cookie is cleared before it is freed" holds without tracing which paths populate what, as scryptenc.c does with dk at err1. The nonces and yh_* values go over the wire in the clear and are simply covered by wiping the whole structure.
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Fixes #440.
struct handshake_cookieholdsx— the diffie-hellman private value, whosebeing forgotten is what makes the PFS handshake forward-secret — and the two
diffie-hellman parameter MAC keys derived from the shared secret. All four
sites which free the cookie did so without wiping it:
handshakefail()proto_handshake()aterr1handshakedone(), which runs once per successful connection, at exactly thepoint where
xhas served its purposeproto_handshake_cancel()Four
insecure_memzero()calls and the include; no other change.Why all four rather than only where
xis populatederr1is reached before the diffie-hellman value is ever generated, sostrictly it has nothing to protect. I wiped it anyway so that "the cookie is
always cleared before it is freed" is a property that holds without the reader
having to trace which paths populate what — the same reasoning as
scryptenc.c, which wipesdkaterr1on paths where it may never havebeen filled in. Happy to drop that one if you would rather keep it exact.
The nonces and
yh_*values in the cookie go over the wire in the clear, sothey are not the point here; they are simply covered by wiping the whole
structure, which is cheaper and less fragile than picking members.