crypto/cipher: fix CTR infinite loop with large block sizes

Additionally, add a test for CTR mode to cover a range of block sizes.

Fixes #12975

Change-Id: I458aac1616228747e62f92f823768d55e874877a
Reviewed-on: https://go-review.googlesource.com/16050
Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/cipher/ctr.go

@@ -41,13 +41,10 @@ func NewCTR(block Block, iv []byte) Stream {
 
 func (x *ctr) refill() {
 	remain := len(x.out) - x.outUsed
-	if remain > x.outUsed {
-		return
-	}
 	copy(x.out, x.out[x.outUsed:])
 	x.out = x.out[:cap(x.out)]
 	bs := x.b.BlockSize()
-	for remain < len(x.out)-bs {
+	for remain <= len(x.out)-bs {
 		x.b.Encrypt(x.out[remain:], x.ctr)
 		remain += bs
 

src/crypto/cipher/ctr_test.go

@@ -0,0 +1,55 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cipher_test
+
+import (
+	"bytes"
+	"crypto/cipher"
+	"testing"
+)
+
+type noopBlock int
+
+func (b noopBlock) BlockSize() int        { return int(b) }
+func (noopBlock) Encrypt(dst, src []byte) { copy(dst, src) }
+func (noopBlock) Decrypt(dst, src []byte) { copy(dst, src) }
+
+func inc(b []byte) {
+	for i := len(b) - 1; i >= 0; i++ {
+		b[i]++
+		if b[i] != 0 {
+			break
+		}
+	}
+}
+
+func xor(a, b []byte) {
+	for i := range a {
+		a[i] ^= b[i]
+	}
+}
+
+func TestCTR(t *testing.T) {
+	for size := 64; size <= 1024; size *= 2 {
+		iv := make([]byte, size)
+		ctr := cipher.NewCTR(noopBlock(size), iv)
+		src := make([]byte, 1024)
+		for i := range src {
+			src[i] = 0xff
+		}
+		want := make([]byte, 1024)
+		copy(want, src)
+		counter := make([]byte, size)
+		for i := 1; i < len(want)/size; i++ {
+			inc(counter)
+			xor(want[i*size:(i+1)*size], counter)
+		}
+		dst := make([]byte, 1024)
+		ctr.XORKeyStream(dst, src)
+		if !bytes.Equal(dst, want) {
+			t.Errorf("for size %d\nhave %x\nwant %x", size, dst, want)
+		}
+	}
+}