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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++
        }
}