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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
import (
"bytes"
"crypto/internal/alias"
"crypto/subtle"
)
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: bytes.Clone(iv),
tmp: make([]byte, b.BlockSize()),
}
}
type cbcEncrypter cbc
// cbcEncAble is an interface implemented by ciphers that have a specific
// optimized implementation of CBC encryption, like crypto/aes.
// NewCBCEncrypter will check for this interface and return the specific
// BlockMode if found.
type cbcEncAble interface {
NewCBCEncrypter(iv []byte) BlockMode
}
// 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 {
if len(iv) != b.BlockSize() {
panic("cipher.NewCBCEncrypter: IV length must equal block size")
}
if cbc, ok := b.(cbcEncAble); ok {
return cbc.NewCBCEncrypter(iv)
}
return (*cbcEncrypter)(newCBC(b, iv))
}
// newCBCGenericEncrypter 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. This always returns the generic non-asm encrypter for use
// in fuzz testing.
func newCBCGenericEncrypter(b Block, iv []byte) BlockMode {
if len(iv) != b.BlockSize() {
panic("cipher.NewCBCEncrypter: IV length must equal block size")
}
return (*cbcEncrypter)(newCBC(b, iv))
}
func (x *cbcEncrypter) BlockSize() int { return x.blockSize }
func (x *cbcEncrypter) CryptBlocks(dst, src []byte) {
if len(src)%x.blockSize != 0 {
panic("crypto/cipher: input not full blocks")
}
if len(dst) < len(src) {
panic("crypto/cipher: output smaller than input")
}
if alias.InexactOverlap(dst[:len(src)], src) {
panic("crypto/cipher: invalid buffer overlap")
}
iv := x.iv
for len(src) > 0 {
// Write the xor to dst, then encrypt in place.
subtle.XORBytes(dst[:x.blockSize], src[:x.blockSize], iv)
x.b.Encrypt(dst[:x.blockSize], dst[:x.blockSize])
// Move to the next block with this block as the next iv.
iv = dst[:x.blockSize]
src = src[x.blockSize:]
dst = dst[x.blockSize:]
}
// Save the iv for the next CryptBlocks call.
copy(x.iv, iv)
}
func (x *cbcEncrypter) SetIV(iv []byte) {
if len(iv) != len(x.iv) {
panic("cipher: incorrect length IV")
}
copy(x.iv, iv)
}
type cbcDecrypter cbc
// cbcDecAble is an interface implemented by ciphers that have a specific
// optimized implementation of CBC decryption, like crypto/aes.
// NewCBCDecrypter will check for this interface and return the specific
// BlockMode if found.
type cbcDecAble interface {
NewCBCDecrypter(iv []byte) BlockMode
}
// 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 {
if len(iv) != b.BlockSize() {
panic("cipher.NewCBCDecrypter: IV length must equal block size")
}
if cbc, ok := b.(cbcDecAble); ok {
return cbc.NewCBCDecrypter(iv)
}
return (*cbcDecrypter)(newCBC(b, iv))
}
// newCBCGenericDecrypter 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. This always returns the generic non-asm decrypter for use in
// fuzz testing.
func newCBCGenericDecrypter(b Block, iv []byte) BlockMode {
if len(iv) != b.BlockSize() {
panic("cipher.NewCBCDecrypter: IV length must equal block size")
}
return (*cbcDecrypter)(newCBC(b, iv))
}
func (x *cbcDecrypter) BlockSize() int { return x.blockSize }
func (x *cbcDecrypter) CryptBlocks(dst, src []byte) {
if len(src)%x.blockSize != 0 {
panic("crypto/cipher: input not full blocks")
}
if len(dst) < len(src) {
panic("crypto/cipher: output smaller than input")
}
if alias.InexactOverlap(dst[:len(src)], src) {
panic("crypto/cipher: invalid buffer overlap")
}
if len(src) == 0 {
return
}
// For each block, we need to xor the decrypted data with the previous block's ciphertext (the iv).
// To avoid making a copy each time, we loop over the blocks BACKWARDS.
end := len(src)
start := end - x.blockSize
prev := start - x.blockSize
// Copy the last block of ciphertext in preparation as the new iv.
copy(x.tmp, src[start:end])
// Loop over all but the first block.
for start > 0 {
x.b.Decrypt(dst[start:end], src[start:end])
subtle.XORBytes(dst[start:end], dst[start:end], src[prev:start])
end = start
start = prev
prev -= x.blockSize
}
// The first block is special because it uses the saved iv.
x.b.Decrypt(dst[start:end], src[start:end])
subtle.XORBytes(dst[start:end], dst[start:end], x.iv)
// Set the new iv to the first block we copied earlier.
x.iv, x.tmp = x.tmp, x.iv
}
func (x *cbcDecrypter) SetIV(iv []byte) {
if len(iv) != len(x.iv) {
panic("cipher: incorrect length IV")
}
copy(x.iv, iv)
}
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