1 files changed, 76 insertions, 0 deletions
diff --git a/src/crypto/cipher/ctr.go b/src/crypto/cipher/ctr.go
new file mode 100644
index 000000000..70ac40f6a
--- /dev/null
+++ b/src/crypto/cipher/ctr.go
@@ -0,0 +1,76 @@
+// Copyright 2009 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.
+
+// Counter (CTR) mode.
+
+// CTR converts a block cipher into a stream cipher by
+// repeatedly encrypting an incrementing counter and
+// xoring the resulting stream of data with the input.
+
+// See NIST SP 800-38A, pp 13-15
+
+package cipher
+
+type ctr struct {
+	b       Block
+	ctr     []byte
+	out     []byte
+	outUsed int
+}
+
+const streamBufferSize = 512
+
+// NewCTR returns a Stream which encrypts/decrypts using the given Block in
+// counter mode. The length of iv must be the same as the Block's block size.
+func NewCTR(block Block, iv []byte) Stream {
+	if len(iv) != block.BlockSize() {
+		panic("cipher.NewCTR: IV length must equal block size")
+	}
+	bufSize := streamBufferSize
+	if bufSize < block.BlockSize() {
+		bufSize = block.BlockSize()
+	}
+	return &ctr{
+		b:       block,
+		ctr:     dup(iv),
+		out:     make([]byte, 0, bufSize),
+		outUsed: 0,
+	}
+}
+
+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 {
+		x.b.Encrypt(x.out[remain:], x.ctr)
+		remain += bs
+
+		// Increment counter
+		for i := len(x.ctr) - 1; i >= 0; i-- {
+			x.ctr[i]++
+			if x.ctr[i] != 0 {
+				break
+			}
+		}
+	}
+	x.out = x.out[:remain]
+	x.outUsed = 0
+}
+
+func (x *ctr) XORKeyStream(dst, src []byte) {
+	for len(src) > 0 {
+		if x.outUsed >= len(x.out)-x.b.BlockSize() {
+			x.refill()
+		}
+		n := xorBytes(dst, src, x.out[x.outUsed:])
+		dst = dst[n:]
+		src = src[n:]
+		x.outUsed += n
+	}
+}