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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
}
const streamBufferSize = 512
// 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")
}
bufSize := streamBufferSize
if bufSize < block.BlockSize() {
bufSize = block.BlockSize()
}
return &ctr{
b: block,
ctr: dup(iv),
out: make([]byte, 0, bufSize),
outUsed: 0,
}
}
func (x *ctr) refill() {
remain := len(x.out) - x.outUsed
if remain > x.outUsed {
return
}
copy(x.out, x.out[x.outUsed:])
x.out = x.out[:cap(x.out)]
bs := x.b.BlockSize()
for remain < len(x.out)-bs {
x.b.Encrypt(x.out[remain:], x.ctr)
remain += bs
// Increment counter
for i := len(x.ctr) - 1; i >= 0; i-- {
x.ctr[i]++
if x.ctr[i] != 0 {
break
}
}
}
x.out = x.out[:remain]
x.outUsed = 0
}
func (x *ctr) XORKeyStream(dst, src []byte) {
for len(src) > 0 {
if x.outUsed >= len(x.out)-x.b.BlockSize() {
x.refill()
}
n := xorBytes(dst, src, x.out[x.outUsed:])
dst = dst[n:]
src = src[n:]
x.outUsed += n
}
}
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