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// 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.

// Cipher block chaining (CBC) mode.

// CBC provides confidentiality by xoring (chaining) each plaintext block
// with the previous ciphertext block before applying the block cipher.

// See NIST SP 800-38A, pp 10-11

package cipher

type cbc struct {
        b         Block
        blockSize int
        iv        []byte
        tmp       []byte
}

func newCBC(b Block, iv []byte) *cbc {
        return &cbc{
                b:         b,
                blockSize: b.BlockSize(),
                iv:        dup(iv),
                tmp:       make([]byte, b.BlockSize()),
        }
}

type cbcEncrypter cbc

// NewCBCEncrypter returns a BlockMode which encrypts in cipher block chaining
// mode, using the given Block. The length of iv must be the same as the
// Block's block size.
func NewCBCEncrypter(b Block, iv []byte) BlockMode {
        return (*cbcEncrypter)(newCBC(b, iv))
}

func (x *cbcEncrypter) BlockSize() int { return x.blockSize }

func (x *cbcEncrypter) CryptBlocks(dst, src []byte) {
        for len(src) > 0 {
                for i := 0; i < x.blockSize; i++ {
                        x.iv[i] ^= src[i]
                }
                x.b.Encrypt(x.iv, x.iv)
                for i := 0; i < x.blockSize; i++ {
                        dst[i] = x.iv[i]
                }
                src = src[x.blockSize:]
                dst = dst[x.blockSize:]
        }
}

type cbcDecrypter cbc

// NewCBCDecrypter returns a BlockMode which decrypts in cipher block chaining
// mode, using the given Block. The length of iv must be the same as the
// Block's block size and must match the iv used to encrypt the data.
func NewCBCDecrypter(b Block, iv []byte) BlockMode {
        return (*cbcDecrypter)(newCBC(b, iv))
}

func (x *cbcDecrypter) BlockSize() int { return x.blockSize }

func (x *cbcDecrypter) CryptBlocks(dst, src []byte) {
        for len(src) > 0 {
                x.b.Decrypt(x.tmp, src[:x.blockSize])
                for i := 0; i < x.blockSize; i++ {
                        x.tmp[i] ^= x.iv[i]
                        x.iv[i] = src[i]
                        dst[i] = x.tmp[i]
                }

                src = src[x.blockSize:]
                dst = dst[x.blockSize:]
        }
}