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133 lines
3.4 KiB
V
133 lines
3.4 KiB
V
// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
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// Use of this source code is governed by an MIT license
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// 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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// NOTE this will be moved to crypto.cipher interface (joe-c)
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module aes
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import (
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crypto.cipher
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crypto.internal.subtle
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)
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struct AesCbc {
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mut:
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b AesCipher
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block_size int
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iv []byte
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tmp []byte
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}
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// internal
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fn new_aes_cbc(b AesCipher, iv []byte) AesCbc {
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return AesCbc{
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b: b,
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block_size: b.block_size(),
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iv: iv.clone(),
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tmp: [byte(0)].repeat(b.block_size()),
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}
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}
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// new_cbc_encrypter 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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pub fn new_cbc(b AesCipher, iv []byte) AesCbc {
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if iv.len != b.block_size() {
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panic('crypto.cipher.new_cbc_encrypter: IV length must equal block size')
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}
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return new_aes_cbc(b, iv)
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}
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pub fn (x &AesCbc) block_size() int { return x.block_size }
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pub fn (x &AesCbc) encrypt_blocks(dst mut []byte, src_ []byte) {
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mut src := src_
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if src.len%x.block_size != 0 {
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panic('crypto.cipher: input not full blocks')
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}
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if dst.len < src.len {
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panic('crypto.cipher: output smaller than input')
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}
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if subtle.inexact_overlap(dst[..src.len], src) {
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panic('crypto.cipher: invalid buffer overlap')
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}
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mut iv := x.iv
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for src.len > 0 {
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// Write the xor to dst, then encrypt in place.
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cipher.xor_bytes(mut dst[..x.block_size], src[..x.block_size], iv)
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x.b.encrypt(dst[..x.block_size], dst[..x.block_size])
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// Move to the next block with this block as the next iv.
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iv = dst[..x.block_size]
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if x.block_size >= src.len {
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src = []
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} else {
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src = src[x.block_size..]
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}
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*dst = dst[x.block_size..]
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}
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// Save the iv for the next crypt_blocks call.
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copy(x.iv, iv)
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}
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pub fn (x mut AesCbc) decrypt_blocks(dst mut []byte, src []byte) {
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if src.len%x.block_size != 0 {
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panic('crypto.cipher: input not full blocks')
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}
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if dst.len < src.len {
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panic('crypto.cipher: output smaller than input')
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}
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if subtle.inexact_overlap(dst[..src.len], src) {
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panic('crypto.cipher: invalid buffer overlap')
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}
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if src.len == 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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mut end := src.len
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mut start := end - x.block_size
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mut prev := start - x.block_size
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// Copy the last block of ciphertext in preparation as the new iv.
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copy(x.tmp, src.slice(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.slice(start, end), src.slice(start, end))
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cipher.xor_bytes(mut dst.slice(start, end), dst.slice(start, end), src.slice(prev, start))
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end = start
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start = prev
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prev -= x.block_size
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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.slice(start, end), src.slice(start, end))
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cipher.xor_bytes(mut dst.slice(start, end), dst.slice(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
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x.tmp = x.iv
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}
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fn (x &AesCbc) set_iv(iv []byte) {
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if iv.len != x.iv.len {
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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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