1 files changed, 51 insertions, 0 deletions
diff --git a/src/pkg/crypto/cipher/ctr.go b/src/pkg/crypto/cipher/ctr.go
new file mode 100644
index 000000000..04436ec23
--- /dev/null
+++ b/src/pkg/crypto/cipher/ctr.go
@@ -0,0 +1,51 @@
+// 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
+}
+
+// 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 {
+	return &ctr{
+		b:       block,
+		ctr:     dup(iv),
+		out:     make([]byte, len(iv)),
+		outUsed: len(iv),
+	}
+}
+
+func (x *ctr) XORKeyStream(dst, src []byte) {
+	for i := 0; i < len(src); i++ {
+		if x.outUsed == len(x.ctr) {
+			x.b.Encrypt(x.out, x.ctr)
+			x.outUsed = 0
+
+			// Increment counter
+			for i := len(x.ctr) - 1; i >= 0; i-- {
+				x.ctr[i]++
+				if x.ctr[i] != 0 {
+					break
+				}
+			}
+		}
+
+		dst[i] = src[i] ^ x.out[x.outUsed]
+		x.outUsed++
+	}
+}