Plain PSK omits the ServerKeyExchange when there is no hint and includes
it otherwise (it should have always sent it), while other PSK ciphers
like ECDHE_PSK cannot omit the hint. Having different capabilities here
is odd and RFC 4279 5.2 suggests that all PSK ciphers are capable of
"[not] provid[ing] an identity hint".
Interpret this to mean no identity hint and empty identity hint are the
same state. Annoyingly, this gives a plain PSK implementation two
options for spelling an empty hint. The spec isn't clear and this is not
really a battle worth fighting, so I've left both acceptable and added a
test for this case.
See also https://android-review.googlesource.com/c/275217/. This is also
consistent with Android's PskKeyManager API, our only consumer anyway.
https://developer.android.com/reference/android/net/PskKeyManager.html
Change-Id: I8a8e6cc1f7dd1b8b202cdaf3d4f151bebfb4a25b
Reviewed-on: https://boringssl-review.googlesource.com/11087
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
786793411a