crypto/cipher: add OFB mode

R=rsc, r
CC=golang-dev
https://golang.org/cl/3994042

src/pkg/crypto/cipher/ofb.go

+// 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
+
+type ofb struct {
+	b       Block
+	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 {
+		return nil
+	}
+
+	x := &ofb{
+		b:       b,
+		out:     make([]byte, blockSize),
+		outUsed: 0,
+	}
+	b.Encrypt(x.out, iv)
+
+	return x
+}
+
+func (x *ofb) XORKeyStream(dst, src []byte) {
+	for i, s := range src {
+		if x.outUsed == len(x.out) {
+			x.b.Encrypt(x.out, x.out)
+			x.outUsed = 0
+		}
+
+		dst[i] = s ^ x.out[x.outUsed]
+		x.outUsed++
+	}
+}

src/pkg/crypto/cipher/ofb_test.go

+// 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.
+
+// OFB AES test vectors.
+
+// See U.S. National Institute of Standards and Technology (NIST)
+// Special Publication 800-38A, ``Recommendation for Block Cipher
+// Modes of Operation,'' 2001 Edition, pp. 52-55.
+
+package cipher
+
+import (
+	"bytes"
+	"crypto/aes"
+	"testing"
+)
+
+type ofbTest struct {
+	name string
+	key  []byte
+	iv   []byte
+	in   []byte
+	out  []byte
+}
+
+var ofbTests = []ofbTest{
+	// NIST SP 800-38A pp 52-55
+	{
+		"OFB-AES128",
+		commonKey128,
+		commonIV,
+		commonInput,
+		[]byte{
+			0x3b, 0x3f, 0xd9, 0x2e, 0xb7, 0x2d, 0xad, 0x20, 0x33, 0x34, 0x49, 0xf8, 0xe8, 0x3c, 0xfb, 0x4a,
+			0x77, 0x89, 0x50, 0x8d, 0x16, 0x91, 0x8f, 0x03, 0xf5, 0x3c, 0x52, 0xda, 0xc5, 0x4e, 0xd8, 0x25,
+			0x97, 0x40, 0x05, 0x1e, 0x9c, 0x5f, 0xec, 0xf6, 0x43, 0x44, 0xf7, 0xa8, 0x22, 0x60, 0xed, 0xcc,
+			0x30, 0x4c, 0x65, 0x28, 0xf6, 0x59, 0xc7, 0x78, 0x66, 0xa5, 0x10, 0xd9, 0xc1, 0xd6, 0xae, 0x5e,
+		},
+	},
+	{
+		"OFB-AES192",
+		commonKey192,
+		commonIV,
+		commonInput,
+		[]byte{
+			0xcd, 0xc8, 0x0d, 0x6f, 0xdd, 0xf1, 0x8c, 0xab, 0x34, 0xc2, 0x59, 0x09, 0xc9, 0x9a, 0x41, 0x74,
+			0xfc, 0xc2, 0x8b, 0x8d, 0x4c, 0x63, 0x83, 0x7c, 0x09, 0xe8, 0x17, 0x00, 0xc1, 0x10, 0x04, 0x01,
+			0x8d, 0x9a, 0x9a, 0xea, 0xc0, 0xf6, 0x59, 0x6f, 0x55, 0x9c, 0x6d, 0x4d, 0xaf, 0x59, 0xa5, 0xf2,
+			0x6d, 0x9f, 0x20, 0x08, 0x57, 0xca, 0x6c, 0x3e, 0x9c, 0xac, 0x52, 0x4b, 0xd9, 0xac, 0xc9, 0x2a,
+		},
+	},
+	{
+		"OFB-AES256",
+		commonKey256,
+		commonIV,
+		commonInput,
+		[]byte{
+			0xdc, 0x7e, 0x84, 0xbf, 0xda, 0x79, 0x16, 0x4b, 0x7e, 0xcd, 0x84, 0x86, 0x98, 0x5d, 0x38, 0x60,
+			0x4f, 0xeb, 0xdc, 0x67, 0x40, 0xd2, 0x0b, 0x3a, 0xc8, 0x8f, 0x6a, 0xd8, 0x2a, 0x4f, 0xb0, 0x8d,
+			0x71, 0xab, 0x47, 0xa0, 0x86, 0xe8, 0x6e, 0xed, 0xf3, 0x9d, 0x1c, 0x5b, 0xba, 0x97, 0xc4, 0x08,
+			0x01, 0x26, 0x14, 0x1d, 0x67, 0xf3, 0x7b, 0xe8, 0x53, 0x8f, 0x5a, 0x8b, 0xe7, 0x40, 0xe4, 0x84,
+		},
+	},
+}
+
+func TestOFB(t *testing.T) {
+	for _, tt := range ofbTests {
+		test := tt.name
+
+		c, err := aes.NewCipher(tt.key)
+		if err != nil {
+			t.Errorf("%s: NewCipher(%d bytes) = %s", test, len(tt.key), err)
+			continue
+		}
+
+		for j := 0; j <= 5; j += 5 {
+			plaintext := tt.in[0 : len(tt.in)-j]
+			ofb := NewOFB(c, tt.iv)
+			ciphertext := make([]byte, len(plaintext))
+			ofb.XORKeyStream(ciphertext, plaintext)
+			if !bytes.Equal(ciphertext, tt.out[:len(plaintext)]) {
+				t.Errorf("%s/%d: encrypting\ninput % x\nhave % x\nwant % x", test, len(plaintext), plaintext, ciphertext, tt.out)
+			}
+		}
+
+		for j := 0; j <= 5; j += 5 {
+			ciphertext := tt.out[0 : len(tt.in)-j]
+			ofb := NewOFB(c, tt.iv)
+			plaintext := make([]byte, len(ciphertext))
+			ofb.XORKeyStream(plaintext, ciphertext)
+			if !bytes.Equal(plaintext, tt.in[:len(ciphertext)]) {
+				t.Errorf("%s/%d: decrypting\nhave % x\nwant % x", test, len(ciphertext), plaintext, tt.in)
+			}
+		}
+
+		if t.Failed() {
+			break
+		}
+	}
+}