Doing some archeaology, since the initial OpenSSL commit, key_arg has been
omitted from the serialization if key_arg_length was 0. Since this is an
SSLv2-only field and resuming an SSLv2 session with SSLv3+ is not possible,
there is no need to support parsing those sessions.
Interestingly, it is actually not the case that key_arg_length was only ever
set in SSLv2, historically. In the initial commit of OpenSSL, SSLeay 0.8.1b,
key_arg was used to store what appears to be the IV. That was then removed in
the next commit, an import of SSLeay 0.9.0b, at which point key_arg was only
ever set in SSLv3. That is old enough that there is certainly no need to
parse pre-SSLeay-0.9.0b sessions...
Change-Id: Ia768a2d97ddbe60309be20e2efe488640c4776d9
Reviewed-on: https://boringssl-review.googlesource.com/2050
Reviewed-by: Adam Langley <agl@google.com>
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