6736ef96ea
The next revision, 19185, renames several runtime files, and will be handled in a separate change. From-SVN: r211328
134 lines
3.5 KiB
Go
134 lines
3.5 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Cipher block chaining (CBC) mode.
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// CBC provides confidentiality by xoring (chaining) each plaintext block
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// with the previous ciphertext block before applying the block cipher.
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// See NIST SP 800-38A, pp 10-11
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package cipher
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type cbc struct {
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b Block
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blockSize int
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iv []byte
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tmp []byte
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}
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func newCBC(b Block, iv []byte) *cbc {
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return &cbc{
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b: b,
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blockSize: b.BlockSize(),
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iv: dup(iv),
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tmp: make([]byte, b.BlockSize()),
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}
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}
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type cbcEncrypter cbc
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// NewCBCEncrypter returns a BlockMode which encrypts in cipher block chaining
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// mode, using the given Block. The length of iv must be the same as the
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// Block's block size.
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func NewCBCEncrypter(b Block, iv []byte) BlockMode {
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if len(iv) != b.BlockSize() {
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panic("cipher.NewCBCEncrypter: IV length must equal block size")
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}
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return (*cbcEncrypter)(newCBC(b, iv))
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}
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func (x *cbcEncrypter) BlockSize() int { return x.blockSize }
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func (x *cbcEncrypter) CryptBlocks(dst, src []byte) {
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if len(src)%x.blockSize != 0 {
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panic("crypto/cipher: input not full blocks")
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}
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if len(dst) < len(src) {
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panic("crypto/cipher: output smaller than input")
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}
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iv := x.iv
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for len(src) > 0 {
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// Write the xor to dst, then encrypt in place.
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xorBytes(dst[:x.blockSize], src[:x.blockSize], iv)
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x.b.Encrypt(dst[:x.blockSize], dst[:x.blockSize])
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// Move to the next block with this block as the next iv.
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iv = dst[:x.blockSize]
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src = src[x.blockSize:]
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dst = dst[x.blockSize:]
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}
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// Save the iv for the next CryptBlocks call.
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copy(x.iv, iv)
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}
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func (x *cbcEncrypter) SetIV(iv []byte) {
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if len(iv) != len(x.iv) {
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panic("cipher: incorrect length IV")
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}
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copy(x.iv, iv)
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}
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type cbcDecrypter cbc
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// NewCBCDecrypter returns a BlockMode which decrypts in cipher block chaining
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// mode, using the given Block. The length of iv must be the same as the
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// Block's block size and must match the iv used to encrypt the data.
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func NewCBCDecrypter(b Block, iv []byte) BlockMode {
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if len(iv) != b.BlockSize() {
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panic("cipher.NewCBCDecrypter: IV length must equal block size")
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}
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return (*cbcDecrypter)(newCBC(b, iv))
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}
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func (x *cbcDecrypter) BlockSize() int { return x.blockSize }
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func (x *cbcDecrypter) CryptBlocks(dst, src []byte) {
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if len(src)%x.blockSize != 0 {
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panic("crypto/cipher: input not full blocks")
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}
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if len(dst) < len(src) {
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panic("crypto/cipher: output smaller than input")
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}
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if len(src) == 0 {
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return
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}
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// For each block, we need to xor the decrypted data with the previous block's ciphertext (the iv).
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// To avoid making a copy each time, we loop over the blocks BACKWARDS.
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end := len(src)
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start := end - x.blockSize
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prev := start - x.blockSize
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// Copy the last block of ciphertext in preparation as the new iv.
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copy(x.tmp, src[start:end])
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// Loop over all but the first block.
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for start > 0 {
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x.b.Decrypt(dst[start:end], src[start:end])
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xorBytes(dst[start:end], dst[start:end], src[prev:start])
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end = start
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start = prev
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prev -= x.blockSize
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}
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// The first block is special because it uses the saved iv.
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x.b.Decrypt(dst[start:end], src[start:end])
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xorBytes(dst[start:end], dst[start:end], x.iv)
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// Set the new iv to the first block we copied earlier.
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x.iv, x.tmp = x.tmp, x.iv
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}
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func (x *cbcDecrypter) SetIV(iv []byte) {
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if len(iv) != len(x.iv) {
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panic("cipher: incorrect length IV")
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}
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copy(x.iv, iv)
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}
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