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>
diff --git a/openpgp/write.go b/openpgp/write.go
index d6dede7..4ee7178 100644
--- a/openpgp/write.go
+++ b/openpgp/write.go
@@ -271,6 +271,7 @@
 	// 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 @@
 	// 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),