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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.
// Counter (CTR) mode.
// CTR converts a block cipher into a stream cipher by
// repeatedly encrypting an incrementing counter and
// xoring the resulting stream of data with the input.
// See NIST SP 800-38A, pp 13-15
package cipher
type ctr struct {
b Block
ctr []byte
out []byte
outUsed int
}
// NewCTR returns a Stream which encrypts/decrypts using the given Block in
// counter mode. The length of iv must be the same as the Block's block size.
func NewCTR(block Block, iv []byte) Stream {
if len(iv) != block.BlockSize() {
panic("cipher.NewCTR: IV length must equal block size")
}
return &ctr{
b: block,
ctr: dup(iv),
out: make([]byte, len(iv)),
outUsed: len(iv),
}
}
func (x *ctr) XORKeyStream(dst, src []byte) {
for i := 0; i < len(src); i++ {
if x.outUsed == len(x.ctr) {
x.b.Encrypt(x.out, x.ctr)
x.outUsed = 0
// Increment counter
for i := len(x.ctr) - 1; i >= 0; i-- {
x.ctr[i]++
if x.ctr[i] != 0 {
break
}
}
}
dst[i] = src[i] ^ x.out[x.outUsed]
x.outUsed++
}
}
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