Imported Upstream version 1.4upstream/1.4

1 files changed, 72 insertions, 0 deletions

diff --git a/src/crypto/cipher/cfb.go b/src/crypto/cipher/cfb.go
new file mode 100644
index 000000000..9b4eebf5b
--- /dev/null
+++ b/src/crypto/cipher/cfb.go
@@ -0,0 +1,72 @@
+// Copyright 2010 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.
+
+// CFB (Cipher Feedback) Mode.
+
+package cipher
+
+type cfb struct {
+	b       Block
+	next    []byte
+	out     []byte
+	outUsed int
+
+	decrypt bool
+}
+
+func (x *cfb) XORKeyStream(dst, src []byte) {
+	for len(src) > 0 {
+		if x.outUsed == len(x.out) {
+			x.b.Encrypt(x.out, x.next)
+			x.outUsed = 0
+		}
+
+		if x.decrypt {
+			// We can precompute a larger segment of the
+			// keystream on decryption. This will allow
+			// larger batches for xor, and we should be
+			// able to match CTR/OFB performance.
+			copy(x.next[x.outUsed:], src)
+		}
+		n := xorBytes(dst, src, x.out[x.outUsed:])
+		if !x.decrypt {
+			copy(x.next[x.outUsed:], dst)
+		}
+		dst = dst[n:]
+		src = src[n:]
+		x.outUsed += n
+	}
+}
+
+// NewCFBEncrypter returns a Stream which encrypts with cipher feedback mode,
+// using the given Block. The iv must be the same length as the Block's block
+// size.
+func NewCFBEncrypter(block Block, iv []byte) Stream {
+	return newCFB(block, iv, false)
+}
+
+// NewCFBDecrypter returns a Stream which decrypts with cipher feedback mode,
+// using the given Block. The iv must be the same length as the Block's block
+// size.
+func NewCFBDecrypter(block Block, iv []byte) Stream {
+	return newCFB(block, iv, true)
+}
+
+func newCFB(block Block, iv []byte, decrypt bool) Stream {
+	blockSize := block.BlockSize()
+	if len(iv) != blockSize {
+		// stack trace will indicate whether it was de or encryption
+		panic("cipher.newCFB: IV length must equal block size")
+	}
+	x := &cfb{
+		b:       block,
+		out:     make([]byte, blockSize),
+		next:    make([]byte, blockSize),
+		outUsed: blockSize,
+		decrypt: decrypt,
+	}
+	copy(x.next, iv)
+
+	return x
+}