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// Copyright 2011 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.

// OFB (Output Feedback) Mode.

package cipher

type ofb struct {

        b       Block

        out     []byte

        outUsed int

}

// NewOFB returns a Stream that encrypts or decrypts using the block cipher b

// in output feedback mode. The initialization vector iv's length must be equal

// to b's block size.

func NewOFB(b Block, iv []byte) Stream {

        blockSize := b.BlockSize()

        if len(iv) != blockSize {

                return nil

        }

        x := &ofb{

                b:       b,

                out:     make([]byte, blockSize),

                outUsed: 0,

        }

        b.Encrypt(x.out, iv)

        return x

}

func (x *ofb) XORKeyStream(dst, src []byte) {

        for i, s := range src {

                if x.outUsed == len(x.out) {

                        x.b.Encrypt(x.out, x.out)

                        x.outUsed = 0

                }

                dst[i] = s ^ x.out[x.outUsed]

                x.outUsed++

        }

}