1 files changed, 77 insertions, 0 deletions
diff --git a/src/crypto/cipher/ofb.go b/src/crypto/cipher/ofb.go
new file mode 100644
index 0000000..1195fdd
--- /dev/null
+++ b/src/crypto/cipher/ofb.go
@@ -0,0 +1,77 @@
+// Copyright 2011 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.
+
+// OFB (Output Feedback) Mode.
+
+package cipher
+
+import (
+	"crypto/internal/alias"
+	"crypto/subtle"
+)
+
+type ofb struct {
+	b       Block
+	cipher  []byte
+	out     []byte
+	outUsed int
+}
+
+// NewOFB returns a Stream that encrypts or decrypts using the block cipher b
+// in output feedback mode. The initialization vector iv's length must be equal
+// to b's block size.
+func NewOFB(b Block, iv []byte) Stream {
+	blockSize := b.BlockSize()
+	if len(iv) != blockSize {
+		panic("cipher.NewOFB: IV length must equal block size")
+	}
+	bufSize := streamBufferSize
+	if bufSize < blockSize {
+		bufSize = blockSize
+	}
+	x := &ofb{
+		b:       b,
+		cipher:  make([]byte, blockSize),
+		out:     make([]byte, 0, bufSize),
+		outUsed: 0,
+	}
+
+	copy(x.cipher, iv)
+	return x
+}
+
+func (x *ofb) refill() {
+	bs := x.b.BlockSize()
+	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)]
+	for remain < len(x.out)-bs {
+		x.b.Encrypt(x.cipher, x.cipher)
+		copy(x.out[remain:], x.cipher)
+		remain += bs
+	}
+	x.out = x.out[:remain]
+	x.outUsed = 0
+}
+
+func (x *ofb) XORKeyStream(dst, src []byte) {
+	if len(dst) < len(src) {
+		panic("crypto/cipher: output smaller than input")
+	}
+	if alias.InexactOverlap(dst[:len(src)], src) {
+		panic("crypto/cipher: invalid buffer overlap")
+	}
+	for len(src) > 0 {
+		if x.outUsed >= len(x.out)-x.b.BlockSize() {
+			x.refill()
+		}
+		n := subtle.XORBytes(dst, src, x.out[x.outUsed:])
+		dst = dst[n:]
+		src = src[n:]
+		x.outUsed += n
+	}
+}