TLS 1.3 includes a server-random-based anti-downgrade signal, as a
workaround for TLS 1.2's ServerKeyExchange signature failing to cover
the entire handshake. However, because TLS 1.3 draft versions are each
doomed to die, we cannot deploy it until the final RFC. (Suppose a
draft-TLS-1.3 client checked the signal and spoke to a final-TLS-1.3
server. The server would correctly negotiate TLS 1.2 and send the
signal. But the client would then break. An anologous situation exists
with reversed roles.)
However, it appears that Cisco devices have non-compliant TLS 1.2
implementations[1] and copy over another server's server-random when
acting as a TLS terminator (client and server back-to-back).
Hopefully they are the only ones doing this. Implement a
measurement-only version with a different value. This sentinel must not
be enforced, but it will tell us whether enforcing it will cause
problems.
[1] https://www.ietf.org/mail-archive/web/tls/current/msg25168.html
Bug: 226
Change-Id: I976880bdb2ef26f51592b2f6b3b97664342679c8
Reviewed-on: https://boringssl-review.googlesource.com/24284
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
6df6540766