ssh: allow up to 255 bytes of padding in AES-GCM

The writing side would generate a maximum of 19 bytes of padding, so
the reading side erroneously checked this. However, RFC 5647 specifies
255 as the maximum amount of padding for AES-GCM.

Fixes golang/go#18953.

Change-Id: I416b0023c6e4cbd91a6a1b4214a03f1663b77248
Reviewed-on: https://go-review.googlesource.com/47590
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/cipher.go

@@ -392,7 +392,9 @@ func (c *gcmCipher) readPacket(seqNum uint32, r io.Reader) ([]byte, error) {
 	c.incIV()
 
 	padding := plain[0]
-	if padding < 4 || padding >= 20 {
+	if padding < 4 {
+		// padding is a byte, so it automatically satisfies
+		// the maximum size, which is 255.
 		return nil, fmt.Errorf("ssh: illegal padding %d", padding)
 	}