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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++
        }
}