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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++
        }
}