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// Copyright 2010 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.

// CFB (Cipher Feedback) Mode.

package cipher

type cfb struct {

        b       Block

        out     []byte

        outUsed int

        decrypt bool

}

// NewCFBEncrypter returns a Stream which encrypts with cipher feedback mode,

// using the given Block. The iv must be the same length as the Block's block

// size.

func NewCFBEncrypter(block Block, iv []byte) Stream {

        return newCFB(block, iv, false)

}

// NewCFBDecrypter returns a Stream which decrypts with cipher feedback mode,

// using the given Block. The iv must be the same length as the Block's block

// size.

func NewCFBDecrypter(block Block, iv []byte) Stream {

        return newCFB(block, iv, true)

}

func newCFB(block Block, iv []byte, decrypt bool) Stream {

        blockSize := block.BlockSize()

        if len(iv) != blockSize {

                return nil

        }

        x := &cfb{

                b:       block,

                out:     make([]byte, blockSize),

                outUsed: 0,

                decrypt: decrypt,

        }

        block.Encrypt(x.out, iv)

        return x

}

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

        for i := 0; i < len(src); i++ {

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

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

                        x.outUsed = 0

                }

                if x.decrypt {

                        t := src[i]

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

                        x.out[x.outUsed] = t

                } else {

                        x.out[x.outUsed] ^= src[i]

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

                }

                x.outUsed++

        }

}