Refactor encryption.Cipher to be an Encrypt/Decrypt Interface

All Encrypt/Decrypt Cipher implementations will now take
and return []byte to set up usage in future binary compatible
encoding schemes to fix issues with bloat encrypting to strings
(which requires base64ing adding 33% size)

pkg/apis/sessions/session_state_test.go

@@ -145,7 +145,7 @@ func TestExpired(t *testing.T) {
 type testCase struct {
 	sessions.SessionState
 	Encoded string
-	Cipher  *encryption.Cipher
+	Cipher  encryption.Cipher
 	Error   bool
 }