diff --git a/chacha20_test.go b/chacha20_test.go
index <HASH>..<HASH> 100644
--- a/chacha20_test.go
+++ b/chacha20_test.go
@@ -5,8 +5,12 @@ import (
 	"encoding/hex"
 	"fmt"
 	"testing"
+	"crypto/cipher"
 )
 
+// assert that a pointer to Cipher actually meets the cipher.Stream interface
+var _ cipher.Stream = &Cipher{}
+
 // stolen from http://tools.ietf.org/html/draft-agl-tls-chacha20poly1305-00#section-7
 var testVectors = [][]string{
 	[]string{
Lock down the commitment to cipher.Stream.