go.crypto/openpgp: test that hashes are compiled in before using them.

Previously, if Encrypt decided to use a hash function that wasn't
compiled in then it would panic.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12055043

openpgp/write.go

@@ -224,16 +224,32 @@ func Encrypt(ciphertext io.Writer, to []*Entity, signed *Entity, hints *FileHint
 		}
 	}
 
-	hashFunc := candidateHashes[0]
-	// If the hash specified by config is a candidate, we'll use that.
-	configuredHash := config.Hash()
-	for _, h := range candidateHashes {
-		if h == uint8(configuredHash) {
-			hashFunc = h
+	var hash crypto.Hash
+	for _, hashId := range candidateHashes {
+		if h, ok := s2k.HashIdToHash(hashId); ok && h.Available() {
+			hash = h
 			break
 		}
 	}
-	hash, _ := s2k.HashIdToHash(hashFunc)
+
+	// If the hash specified by config is a candidate, we'll use that.
+	if configuredHash := config.Hash(); configuredHash.Available() {
+		for _, hashId := range candidateHashes {
+			if h, ok := s2k.HashIdToHash(hashId); ok && h == configuredHash {
+				hash = h
+				break
+			}
+		}
+	}
+
+	if hash == 0 {
+		hashId := candidateHashes[0]
+		name, ok := s2k.HashIdToString(hashId)
+		if !ok {
+			name = "#" + strconv.Itoa(int(hashId))
+		}
+		return nil, errors.InvalidArgumentError("cannot encrypt because no candidate hash functions are compiled in. (Wanted " + name + " in this case.)")
+	}
 
 	symKey := make([]byte, cipher.KeySize())
 	if _, err := io.ReadFull(config.Random(), symKey); err != nil {