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// Copyright (c) 2019-2021 Alexander Medvednikov. All rights reserved.
// Use of this source code is governed by an MIT 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
// NOTE this will be moved to crypto.cipher interface (joe-c)
module aes
import crypto.cipher
import crypto.internal.subtle
struct AesCbc {
mut:
b AesCipher
block_size int
iv []byte
tmp []byte
}
// internal
fn new_aes_cbc(b AesCipher, iv []byte) AesCbc {
return AesCbc{
b: b
block_size: b.block_size()
iv: iv.clone()
tmp: []byte{len: (b.block_size())}
}
}
// new_cbc returns a `AesCbc` 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.
pub fn new_cbc(b AesCipher, iv []byte) AesCbc {
if iv.len != b.block_size() {
panic('crypto.cipher.new_cbc_encrypter: IV length must equal block size')
}
return new_aes_cbc(b, iv)
}
// block_size returns the block size of the checksum in bytes.
pub fn (x &AesCbc) block_size() int {
return x.block_size
}
// encrypt_blocks encrypts the blocks in `src_` to `dst_`.
// Please note: `dst_` is mutable for performance reasons.
pub fn (x &AesCbc) encrypt_blocks(mut dst_ []byte, src_ []byte) {
unsafe {
mut dst := *dst_
mut src := src_
if src.len % x.block_size != 0 {
panic('crypto.cipher: input not full blocks')
}
if dst.len < src.len {
panic('crypto.cipher: output smaller than input')
}
if subtle.inexact_overlap(dst[..src.len], src_) {
panic('crypto.cipher: invalid buffer overlap')
}
mut iv := x.iv
for src.len > 0 {
// Write the xor to dst, then encrypt in place.
cipher.xor_bytes(mut dst[..x.block_size], src[..x.block_size], iv)
x.b.encrypt(mut dst[..x.block_size], mut dst[..x.block_size])
// Move to the next block with this block as the next iv.
iv = dst[..x.block_size]
if x.block_size >= src.len {
src = []
} else {
src = src[x.block_size..]
}
dst = dst[x.block_size..]
}
// Save the iv for the next crypt_blocks call.
copy(x.iv, iv)
}
}
// decrypt_blocks decrypts the blocks in `src` to `dst`.
// Please note: `dst` is mutable for performance reasons.
pub fn (mut x AesCbc) decrypt_blocks(mut dst []byte, src []byte) {
if src.len % x.block_size != 0 {
panic('crypto.cipher: input not full blocks')
}
if dst.len < src.len {
panic('crypto.cipher: output smaller than input')
}
if subtle.inexact_overlap((*dst)[..src.len], src) {
panic('crypto.cipher: invalid buffer overlap')
}
if src.len == 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.
mut end := src.len
mut start := end - x.block_size
mut prev := start - x.block_size
// 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 {
mut src_chunk := src[start..end]
x.b.decrypt(mut (*dst)[start..end], mut src_chunk)
cipher.xor_bytes(mut (*dst)[start..end], (*dst)[start..end], src[prev..start])
end = start
start = prev
prev -= x.block_size
}
// The first block is special because it uses the saved iv.
mut src_chunk := src[start..end]
x.b.decrypt(mut (*dst)[start..end], mut src_chunk)
cipher.xor_bytes(mut (*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
}
fn (x &AesCbc) set_iv(iv []byte) {
if iv.len != x.iv.len {
panic('cipher: incorrect length IV')
}
copy(x.iv, iv)
}
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