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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 block
import (
"io"
)
type cbcCipher struct {
c Cipher
blockSize int
iv []byte
tmp []byte
}
func newCBC(c Cipher, iv []byte) *cbcCipher {
n := c.BlockSize()
x := new(cbcCipher)
x.c = c
x.blockSize = n
x.iv = dup(iv)
x.tmp = make([]byte, n)
return x
}
func (x *cbcCipher) BlockSize() int { return x.blockSize }
func (x *cbcCipher) Encrypt(dst, src []byte) {
for i := 0; i < x.blockSize; i++ {
x.iv[i] ^= src[i]
}
x.c.Encrypt(x.iv, x.iv)
for i := 0; i < x.blockSize; i++ {
dst[i] = x.iv[i]
}
}
func (x *cbcCipher) Decrypt(dst, src []byte) {
x.c.Decrypt(x.tmp, src)
for i := 0; i < x.blockSize; i++ {
x.tmp[i] ^= x.iv[i]
x.iv[i] = src[i]
dst[i] = x.tmp[i]
}
}
// NewCBCDecrypter returns a reader that reads data from r and decrypts it using c
// in cipher block chaining (CBC) mode with the initialization vector iv.
// The returned Reader does not buffer or read ahead except
// as required by the cipher's block size.
func NewCBCDecrypter(c Cipher, iv []byte, r io.Reader) io.Reader {
return NewECBDecrypter(newCBC(c, iv), r)
}
// NewCBCEncrypter returns a writer that encrypts data using c
// in cipher block chaining (CBC) mode with the initialization vector iv
// and writes the encrypted data to w.
// The returned Writer does no buffering except as required
// by the cipher's block size, so there is no need for a Flush method.
func NewCBCEncrypter(c Cipher, iv []byte, w io.Writer) io.Writer {
return NewECBEncrypter(newCBC(c, iv), w)
}
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