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Noise says[1] that identity keys should be hashed after the prologue,
but I missed this during the switch to KN handshakes. Thanks to Dirk
Balfanz for picking up on that.

Mobile counterparts will need to be updated to match this.

BUG=1002262

[1] http://www.noiseprotocol.org/noise.html#:~:text=once%20for%20each%20public%20key%20listed%20in%20the%20pre-messages%20from

Change-Id: I6b4fef90e640371100fa1b1be64113141099c71a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2224987
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Auto-Submit: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#774165}