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// 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.
// The block package implements standard block cipher modes
// that can be wrapped around low-level block cipher implementations.
// See
// and NIST Special Publication 800-38A.
package block
import "io";
// A Cipher represents an implementation of block cipher
// using a given key. It provides the capability to encrypt
// or decrypt individual blocks. The mode implementations
// extend that capability to streams of blocks.
type Cipher interface {
// BlockSize returns the cipher's block size.
BlockSize() int;
// Encrypt encrypts the first block in src into dst.
// Src and dst may point at the same memory.
Encrypt(src, dst []byte);
// Decrypt decrypts the first block in src into dst.
// Src and dst may point at the same memory.
Decrypt(src, dst []byte);
// TODO(rsc): Digest belongs elsewhere.
// A Digest is an implementation of a message digest algorithm.
// Write data to it and then call Sum to retreive the digest.
// Calling Reset resets the internal state, as though no data has
// been written.
type Digest interface {
Sum() []byte;
// Utility routines
func shift1(src, dst []byte) byte {
var b byte;
for i := len(src)-1; i >= 0; i-- {
bb := src[i]>>7;
dst[i] = src[i]<<1 | b;
b = bb;
return b;
func same(p, q []byte) bool {
if len(p) != len(q) {
return false;
for i := 0; i < len(p); i++ {
if p[i] != q[i] {
return false;
return true;
func copy(p []byte) []byte {
q := make([]byte, len(p));
for i, b := range p {
q[i] = b;
return q;