56 lines
1.2 KiB
Go
56 lines
1.2 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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// See NIST SP 800-38A, pp 13-15
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package cipher
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type ctr struct {
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b 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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// NewCTR returns a Stream which encrypts/decrypts using the given Block in
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// counter mode. The length of iv must be the same as the Block's block size.
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func NewCTR(block Block, iv []byte) Stream {
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if len(iv) != block.BlockSize() {
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panic("cipher.NewCTR: iv length must equal block size")
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}
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return &ctr{
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b: block,
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ctr: dup(iv),
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out: make([]byte, len(iv)),
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outUsed: len(iv),
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}
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}
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func (x *ctr) XORKeyStream(dst, src []byte) {
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for i := 0; i < len(src); i++ {
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if x.outUsed == len(x.ctr) {
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x.b.Encrypt(x.out, x.ctr)
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x.outUsed = 0
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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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}
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dst[i] = src[i] ^ x.out[x.outUsed]
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x.outUsed++
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}
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}
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