Subversion Repositories openrisc

// 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++

        }

}