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/*
Implementation of the Lilliput-AE tweakable block cipher.
Authors, hereby denoted as "the implementer":
Kévin Le Gouguec,
2019.
For more information, feedback or questions, refer to our website:
https://paclido.fr/lilliput-ae
To the extent possible under law, the implementer has waived all copyright
and related or neighboring rights to the source code in this file.
http://creativecommons.org/publicdomain/zero/1.0/
---
This file implements Lilliput-AE's nonce-respecting mode based on ΘCB3.
*/
#include <stdbool.h>
#include <stdint.h>
#include <string.h>
#include "cipher.h"
#include "lilliput-ae.h"
#include "lilliput-ae-utils.h"
static const uint8_t _0n[BLOCK_BYTES] = {
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
};
static void _fill_msg_tweak(
uint8_t prefix,
const uint8_t N[NONCE_BYTES],
size_t block_index,
uint8_t tweak[TWEAK_BYTES]
)
{
/* With an s-bit block index, the t-bit tweak is filled as follows:
*
* - bits [ 1, t-|N|-4]: block index
* [ 1, s]: actual block index
* [ s+1, t-|N|-4]: 0-padding
* - bits [t-|N|-3, t-4]: nonce
* - bits [ t-3, t]: 4-bit prefix
*/
copy_block_index(block_index, tweak);
size_t N_start = TWEAK_BYTES - NONCE_BYTES - 1;
tweak[N_start] = lower_nibble(N[0]) << 4;
for (size_t i=1; i<NONCE_BYTES; i++)
{
tweak[N_start+i] = lower_nibble(N[i]) << 4 ^ upper_nibble(N[i-1]);
}
tweak[TWEAK_BYTES-1] = prefix << 4 ^ upper_nibble(N[NONCE_BYTES-1]);
}
static void _encrypt_message(
const uint8_t key[KEY_BYTES],
size_t M_len,
const uint8_t M[M_len],
const uint8_t N[NONCE_BYTES],
uint8_t C[M_len+BLOCK_BYTES],
uint8_t Final[BLOCK_BYTES]
)
{
size_t l = M_len / BLOCK_BYTES;
size_t rest = M_len % BLOCK_BYTES;
uint8_t tweak[TWEAK_BYTES];
uint8_t checksum[BLOCK_BYTES];
memset(tweak, 0, TWEAK_BYTES);
memset(checksum, 0, BLOCK_BYTES);
for (size_t j=0; j<l; j++)
{
xor_into(checksum, &M[j*BLOCK_BYTES]);
_fill_msg_tweak(0x0, N, j, tweak);
encrypt(key, tweak, &M[j*BLOCK_BYTES], &C[j*BLOCK_BYTES]);
}
if (rest == 0)
{
_fill_msg_tweak(0x1, N, l, tweak);
encrypt(key, tweak, checksum, Final);
}
else
{
uint8_t M_rest[BLOCK_BYTES];
uint8_t Pad[BLOCK_BYTES];
pad10(rest, &M[l*BLOCK_BYTES], M_rest);
xor_into(checksum, M_rest);
_fill_msg_tweak(0x4, N, l, tweak);
encrypt(key, tweak, _0n, Pad);
xor_arrays(rest, &C[l*BLOCK_BYTES], &M[l*BLOCK_BYTES], Pad);
_fill_msg_tweak(0x5, N, l+1, tweak);
encrypt(key, tweak, checksum, Final);
}
}
static void _decrypt_message(
const uint8_t key[KEY_BYTES],
size_t C_len,
const uint8_t C[C_len],
const uint8_t N[NONCE_BYTES],
uint8_t M[C_len],
uint8_t Final[BLOCK_BYTES]
)
{
size_t l = C_len / BLOCK_BYTES;
size_t rest = C_len % BLOCK_BYTES;
uint8_t tweak[TWEAK_BYTES];
uint8_t checksum[BLOCK_BYTES];
memset(tweak, 0, TWEAK_BYTES);
memset(checksum, 0, BLOCK_BYTES);
for (size_t j=0; j<l; j++)
{
_fill_msg_tweak(0x0, N, j, tweak);
decrypt(key, tweak, &C[j*BLOCK_BYTES], &M[j*BLOCK_BYTES]);
xor_into(checksum, &M[j*BLOCK_BYTES]);
}
if (rest == 0)
{
_fill_msg_tweak(0x1, N, l, tweak);
encrypt(key, tweak, checksum, Final);
}
else
{
uint8_t M_rest[BLOCK_BYTES];
uint8_t Pad[BLOCK_BYTES];
_fill_msg_tweak(0x4, N, l, tweak);
encrypt(key, tweak, _0n, Pad);
xor_arrays(rest, &M[l*BLOCK_BYTES], &C[l*BLOCK_BYTES], Pad);
pad10(rest, &M[l*BLOCK_BYTES], M_rest);
xor_into(checksum, M_rest);
_fill_msg_tweak(0x5, N, l+1, tweak);
encrypt(key, tweak, checksum, Final);
}
}
static void _generate_tag(
const uint8_t Final[BLOCK_BYTES],
const uint8_t Auth[BLOCK_BYTES],
uint8_t tag[TAG_BYTES]
)
{
xor_arrays(TAG_BYTES, tag, Final, Auth);
}
void lilliput_ae_encrypt(
size_t message_len,
const uint8_t message[message_len],
size_t auth_data_len,
const uint8_t auth_data[auth_data_len],
const uint8_t key[KEY_BYTES],
const uint8_t nonce[NONCE_BYTES],
uint8_t ciphertext[message_len],
uint8_t tag[TAG_BYTES]
)
{
uint8_t auth[BLOCK_BYTES];
process_associated_data(key, auth_data_len, auth_data, auth);
uint8_t final[BLOCK_BYTES];
_encrypt_message(key, message_len, message, nonce, ciphertext, final);
_generate_tag(final, auth, tag);
}
bool lilliput_ae_decrypt(
size_t ciphertext_len,
const uint8_t ciphertext[ciphertext_len],
size_t auth_data_len,
const uint8_t auth_data[auth_data_len],
const uint8_t key[KEY_BYTES],
const uint8_t nonce[NONCE_BYTES],
const uint8_t tag[TAG_BYTES],
uint8_t message[ciphertext_len]
)
{
uint8_t auth[BLOCK_BYTES];
process_associated_data(key, auth_data_len, auth_data, auth);
uint8_t final[BLOCK_BYTES];
_decrypt_message(key, ciphertext_len, ciphertext, nonce, message, final);
uint8_t effective_tag[TAG_BYTES];
_generate_tag(final, auth, effective_tag);
return memcmp(tag, effective_tag, TAG_BYTES) == 0;
}
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