openpgp: support SHA384

SHA384 is a natural hashing choice for P-384 ECDSA. The only thing
needed to make it usable, is adding it to the list of candidates.

Change-Id: I61f66f371774f95dfc1de30d10fab66f92c21b6b
Reviewed-on: https://go-review.googlesource.com/c/137956
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

openpgp/write.go

@@ -271,6 +271,7 @@ func Encrypt(ciphertext io.Writer, to []*Entity, signed *Entity, hints *FileHint
 	// These are the possible hash functions that we'll use for the signature.
 	candidateHashes := []uint8{
 		hashToHashId(crypto.SHA256),
+		hashToHashId(crypto.SHA384),
 		hashToHashId(crypto.SHA512),
 		hashToHashId(crypto.SHA1),
 		hashToHashId(crypto.RIPEMD160),
@@ -349,6 +350,7 @@ func Sign(output io.Writer, signed *Entity, hints *FileHints, config *packet.Con
 	// These are the possible hash functions that we'll use for the signature.
 	candidateHashes := []uint8{
 		hashToHashId(crypto.SHA256),
+		hashToHashId(crypto.SHA384),
 		hashToHashId(crypto.SHA512),
 		hashToHashId(crypto.SHA1),
 		hashToHashId(crypto.RIPEMD160),