crypto/rc4: test the portable version too

Prevent bitrot. (similar to the previous sha1 and md5 CLs)

Fixes #6642

LGTM=agl
R=agl, dave
CC=golang-codereviews
https://golang.org/cl/65690043

src/pkg/crypto/rc4/rc4.go

@@ -50,3 +50,20 @@ func (c *Cipher) Reset() {
 	}
 	c.i, c.j = 0, 0
 }
+
+// xorKeyStreamGeneric sets dst to the result of XORing src with the
+// key stream.  Dst and src may be the same slice but otherwise should
+// not overlap.
+//
+// This is the pure Go version. rc4_{amd64,386,arm}* contain assembly
+// implementations. This is here for tests and to prevent bitrot.
+func (c *Cipher) xorKeyStreamGeneric(dst, src []byte) {
+	i, j := c.i, c.j
+	for k, v := range src {
+		i += 1
+		j += uint8(c.s[i])
+		c.s[i], c.s[j] = c.s[j], c.s[i]
+		dst[k] = v ^ uint8(c.s[uint8(c.s[i]+c.s[j])])
+	}
+	c.i, c.j = i, j
+}

src/pkg/crypto/rc4/rc4_ref.go

@@ -9,12 +9,5 @@ package rc4
 // XORKeyStream sets dst to the result of XORing src with the key stream.
 // Dst and src may be the same slice but otherwise should not overlap.
 func (c *Cipher) XORKeyStream(dst, src []byte) {
-	i, j := c.i, c.j
-	for k, v := range src {
-		i += 1
-		j += uint8(c.s[i])
-		c.s[i], c.s[j] = c.s[j], c.s[i]
-		dst[k] = v ^ uint8(c.s[uint8(c.s[i]+c.s[j])])
-	}
-	c.i, c.j = i, j
+	c.xorKeyStreamGeneric(dst, src)
 }

src/pkg/crypto/rc4/rc4_test.go

@@ -117,19 +117,30 @@ func TestGolden(t *testing.T) {
 }
 
 func TestBlock(t *testing.T) {
+	testBlock(t, (*Cipher).XORKeyStream)
+}
+
+// Test the pure Go version.
+// Because we have assembly for amd64, 386, and arm, this prevents
+// bitrot of the reference implementations.
+func TestBlockGeneric(t *testing.T) {
+	testBlock(t, (*Cipher).xorKeyStreamGeneric)
+}
+
+func testBlock(t *testing.T, xor func(c *Cipher, dst, src []byte)) {
 	c1a, _ := NewCipher(golden[0].key)
 	c1b, _ := NewCipher(golden[1].key)
 	data1 := make([]byte, 1<<20)
 	for i := range data1 {
-		c1a.XORKeyStream(data1[i:i+1], data1[i:i+1])
-		c1b.XORKeyStream(data1[i:i+1], data1[i:i+1])
+		xor(c1a, data1[i:i+1], data1[i:i+1])
+		xor(c1b, data1[i:i+1], data1[i:i+1])
 	}
 
 	c2a, _ := NewCipher(golden[0].key)
 	c2b, _ := NewCipher(golden[1].key)
 	data2 := make([]byte, 1<<20)
-	c2a.XORKeyStream(data2, data2)
-	c2b.XORKeyStream(data2, data2)
+	xor(c2a, data2, data2)
+	xor(c2b, data2, data2)
 
 	if !bytes.Equal(data1, data2) {
 		t.Fatalf("bad block")