forked from mirror/zip
105 lines
2.2 KiB
Go
105 lines
2.2 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Counter (CTR) mode.
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// CTR converts a block cipher into a stream cipher by
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// repeatedly encrypting an incrementing counter and
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// xoring the resulting stream of data with the input.
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// This is a reimplementation of Go's CTR mode to allow
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// for little-endian, left-aligned uint32 counter. Go's
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// NewCTR follows the NIST Standard SP 800-38A, pp 13-15
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// which has a big-endian, right-aligned counter. WinZip
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// AES requires the CTR mode to have a little-endian,
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// left-aligned counter.
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package zip
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import "crypto/cipher"
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type ctr struct {
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b cipher.Block
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ctr []byte
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out []byte
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outUsed int
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}
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const streamBufferSize = 512
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// NewWinZipCTR returns a Stream which encrypts/decrypts using the given Block in
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// counter mode. The counter is initially set to 1.
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func NewWinZipCTR(block cipher.Block) cipher.Stream {
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bufSize := streamBufferSize
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if bufSize < block.BlockSize() {
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bufSize = block.BlockSize()
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}
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// Set the IV (counter) to 1
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iv := make([]byte, block.BlockSize())
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iv[0] = 1
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return &ctr{
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b: block,
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ctr: iv,
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out: make([]byte, 0, bufSize),
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outUsed: 0,
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}
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}
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func (x *ctr) refill() {
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remain := len(x.out) - x.outUsed
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if remain > x.outUsed {
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return
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}
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copy(x.out, x.out[x.outUsed:])
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x.out = x.out[:cap(x.out)]
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bs := x.b.BlockSize()
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for remain < len(x.out)-bs {
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x.b.Encrypt(x.out[remain:], x.ctr)
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remain += bs
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// Increment counter
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// for i := len(x.ctr) - 1; i >= 0; i-- {
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// x.ctr[i]++
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// if x.ctr[i] != 0 {
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// break
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// }
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// }
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// Change to allow for little-endian,
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// left-aligned counter
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for i := 0; i < len(x.ctr); i++ {
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x.ctr[i]++
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if x.ctr[i] != 0 {
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break
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}
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}
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}
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x.out = x.out[:remain]
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x.outUsed = 0
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}
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func (x *ctr) XORKeyStream(dst, src []byte) {
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for len(src) > 0 {
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if x.outUsed >= len(x.out)-x.b.BlockSize() {
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x.refill()
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}
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n := xorBytes(dst, src, x.out[x.outUsed:])
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dst = dst[n:]
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src = src[n:]
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x.outUsed += n
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}
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}
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func xorBytes(dst, a, b []byte) int {
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n := len(a)
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if len(b) < n {
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n = len(b)
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}
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for i := 0; i < n; i++ {
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dst[i] = a[i] ^ b[i]
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}
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return n
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}
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