x/crypto/ssh: add padding oracle countermeasures for AES-CBC.

This deprives an attacker of feedback for guesses against the packet
length given by the connection dropping.

Change-Id: I14939a82e5243a86d192bb18be93d45589227147
Reviewed-on: https://go-review.googlesource.com/9908
Reviewed-by: Adam Langley <agl@golang.org>
2 files changed