crypto/cipher: add CFB and OCFB mode.

        (Files which I left out of the initial commit to keep it small.)

R=rsc
CC=golang-dev
https://golang.org/cl/3183043

src/pkg/crypto/cipher/Makefile

@@ -9,6 +9,8 @@ GOFILES=\
 	cbc.go\
 	cipher.go\
 	ctr.go\
-	io.go
+	io.go\
+	ocfb.go\
+	cfb.go
 
 include ../../../Make.pkg

src/pkg/crypto/cipher/cfb.go

+// 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
+	out     []byte
+	outUsed int
+	decrypt bool
+}
+
+// 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 {
+		return nil
+	}
+
+	x := &cfb{
+		b:       block,
+		out:     make([]byte, blockSize),
+		outUsed: 0,
+		decrypt: decrypt,
+	}
+	block.Encrypt(x.out, iv)
+
+	return x
+}
+
+func (x *cfb) XORKeyStream(dst, src []byte) {
+	for i := 0; i < len(src); i++ {
+		if x.outUsed == len(x.out) {
+			x.b.Encrypt(x.out, x.out)
+			x.outUsed = 0
+		}
+
+		if x.decrypt {
+			t := src[i]
+			dst[i] = src[i] ^ x.out[x.outUsed]
+			x.out[x.outUsed] = t
+		} else {
+			x.out[x.outUsed] ^= src[i]
+			dst[i] = x.out[x.outUsed]
+		}
+		x.outUsed++
+	}
+}

src/pkg/crypto/cipher/cfb_test.go

+// 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.
+
+package cipher
+
+import (
+	"bytes"
+	"crypto/aes"
+	"crypto/rand"
+	"testing"
+)
+
+func TestCFB(t *testing.T) {
+	block, err := aes.NewCipher(commonKey128)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	plaintext := []byte("this is the plaintext")
+	iv := make([]byte, block.BlockSize())
+	rand.Reader.Read(iv)
+	cfb := NewCFBEncrypter(block, iv)
+	ciphertext := make([]byte, len(plaintext))
+	cfb.XORKeyStream(ciphertext, plaintext)
+
+	cfbdec := NewCFBDecrypter(block, iv)
+	plaintextCopy := make([]byte, len(plaintext))
+	cfbdec.XORKeyStream(plaintextCopy, ciphertext)
+
+	if !bytes.Equal(plaintextCopy, plaintext) {
+		t.Errorf("got: %x, want: %x", plaintextCopy, plaintext)
+	}
+}

src/pkg/crypto/cipher/ocfb.go

+// 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.
+
+// OpenPGP CFB Mode. http://tools.ietf.org/html/rfc4880#section-13.9
+
+package cipher
+
+type ocfb struct {
+	b       Block
+	fre     []byte
+	outUsed int
+}
+
+// NewOCFBEncrypter returns a Stream which encrypts data with OpenPGP's cipher
+// feedback mode using the given Block, and an initial amount of ciphertext.
+// randData must be random bytes and be the same length as the Block's block
+// size.
+func NewOCFBEncrypter(block Block, randData []byte) (Stream, []byte) {
+	blockSize := block.BlockSize()
+	if len(randData) != blockSize {
+		return nil, nil
+	}
+
+	x := &ocfb{
+		b:       block,
+		fre:     make([]byte, blockSize),
+		outUsed: 0,
+	}
+	prefix := make([]byte, blockSize+2)
+
+	block.Encrypt(x.fre, x.fre)
+	for i := 0; i < blockSize; i++ {
+		prefix[i] = randData[i] ^ x.fre[i]
+	}
+
+	block.Encrypt(x.fre, prefix[:blockSize])
+	prefix[blockSize] = x.fre[0] ^ randData[blockSize-2]
+	prefix[blockSize+1] = x.fre[1] ^ randData[blockSize-1]
+
+	block.Encrypt(x.fre, prefix[2:])
+	return x, prefix
+}
+
+// NewOCFBDecrypter returns a Stream which decrypts data with OpenPGP's cipher
+// feedback mode using the given Block. Prefix must be the first blockSize + 2
+// bytes of the ciphertext, where blockSize is the Block's block size. If an
+// incorrect key is detected then nil is returned.
+func NewOCFBDecrypter(block Block, prefix []byte) Stream {
+	blockSize := block.BlockSize()
+	if len(prefix) != blockSize+2 {
+		return nil
+	}
+
+	x := &ocfb{
+		b:       block,
+		fre:     make([]byte, blockSize),
+		outUsed: 0,
+	}
+	prefixCopy := make([]byte, len(prefix))
+	copy(prefixCopy, prefix)
+
+	block.Encrypt(x.fre, x.fre)
+	for i := 0; i < blockSize; i++ {
+		prefixCopy[i] ^= x.fre[i]
+	}
+
+	block.Encrypt(x.fre, prefix[:blockSize])
+	prefixCopy[blockSize] ^= x.fre[0]
+	prefixCopy[blockSize+1] ^= x.fre[1]
+
+	if prefixCopy[blockSize-2] != prefixCopy[blockSize] ||
+		prefixCopy[blockSize-1] != prefixCopy[blockSize+1] {
+		return nil
+	}
+
+	block.Encrypt(x.fre, prefix[2:])
+	return x
+}
+
+func (x *ocfb) XORKeyStream(dst, src []byte) {
+	for i := 0; i < len(src); i++ {
+		if x.outUsed == len(x.fre) {
+			x.b.Encrypt(x.fre, x.fre)
+			x.outUsed = 0
+		}
+
+		dst[i] = x.fre[x.outUsed] ^ src[i]
+		x.outUsed++
+	}
+}

src/pkg/crypto/cipher/ocfb_test.go

+// 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.
+
+package cipher
+
+import (
+	"bytes"
+	"crypto/aes"
+	"crypto/rand"
+	"testing"
+)
+
+func TestOCFB(t *testing.T) {
+	block, err := aes.NewCipher(commonKey128)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	plaintext := []byte("this is the plaintext")
+	randData := make([]byte, block.BlockSize())
+	rand.Reader.Read(randData)
+	ocfb, prefix := NewOCFBEncrypter(block, randData)
+	ciphertext := make([]byte, len(plaintext))
+	ocfb.XORKeyStream(ciphertext, plaintext)
+
+	ocfbdec := NewOCFBDecrypter(block, prefix)
+	if ocfbdec == nil {
+		t.Error("NewOCFBDecrypter failed")
+		return
+	}
+	plaintextCopy := make([]byte, len(plaintext))
+	ocfbdec.XORKeyStream(plaintextCopy, ciphertext)
+
+	if !bytes.Equal(plaintextCopy, plaintext) {
+		t.Errorf("got: %x, want: %x", plaintextCopy, plaintext)
+	}
+}