btcec: add new IsCompressedPubKey function

This commit adds a new function to btcec: IsCompressedPubKey. This
function returns true iff the passed serialized public key is encoded
in compressed format.

btcec/pubkey.go

@@ -54,6 +54,15 @@ const (
 	pubkeyHybrid       byte = 0x6 // y_bit + x coord + y coord
 )
 
+// IsCompressedPubKey returns true the the passed serialized public key has
+// been encoded in compressed format, and false otherwise.
+func IsCompressedPubKey(pubKey []byte) bool {
+	// The public key is only compressed if it is the correct length and
+	// the format (first byte) is one of the compressed pubkey values.
+	return len(pubKey) == PubKeyBytesLenCompressed &&
+		(pubKey[0]&^byte(0x1) == pubkeyCompressed)
+}
+
 // ParsePubKey parses a public key for a koblitz curve from a bytestring into a
 // ecdsa.Publickey, verifying that it is valid. It supports compressed,
 // uncompressed and hybrid signature formats.

btcec/pubkey_test.go

@@ -282,3 +282,15 @@ func TestPublicKeyIsEqual(t *testing.T) {
 			"equal to %v", pubKey1, pubKey2)
 	}
 }
+
+func TestIsCompressed(t *testing.T) {
+	for _, test := range pubKeyTests {
+		isCompressed := IsCompressedPubKey(test.key)
+		wantCompressed := (test.format == pubkeyCompressed)
+		if isCompressed != wantCompressed {
+			t.Fatalf("%s (%x) pubkey: unexpected compressed result, "+
+				"got %v, want %v", test.name, test.key,
+				isCompressed, wantCompressed)
+		}
+	}
+}