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# Implementation of the Lilliput-AE tweakable block cipher.
#
# Authors, hereby denoted as "the implementer":
# Kévin Le Gouguec,
# Léo Reynaud
# 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/
"""Lilliput-I Authenticated Encryption mode.
This module provides the functions for authenticated encryption and decryption
using Lilliput-AE's nonce-respecting mode based on ΘCB3.
"""
from enum import Enum
from .constants import BLOCK_BYTES, NONCE_BITS
from .ae_common import (
bytes_to_block_matrix,
block_matrix_to_bytes,
build_auth,
integer_to_byte_array,
pad10,
TagValidationError,
xor
)
from . import tbc
TWEAK_BITS = 192
TWEAK_BYTES = TWEAK_BITS//8
class _MessageTweak(Enum):
BLOCK = 0b0000
NO_PADDING = 0b0001
PAD = 0b0100
FINAL = 0b0101
def _upper_nibble(i):
return i >> 4
def _lower_nibble(i):
return i & 0b00001111
def _byte_from_nibbles(lower, upper):
return upper<<4 | lower
def _tweak_message(N, j, padding):
j = integer_to_byte_array(j, (TWEAK_BITS-NONCE_BITS-4)//8+1)
middle_byte = _byte_from_nibbles(
_lower_nibble(j[-1]), _lower_nibble(N[0])
)
shifted_N = [
_byte_from_nibbles(_upper_nibble(N[i-1]), _lower_nibble(N[i]))
for i in range(1, NONCE_BITS//8)
]
last_byte = _byte_from_nibbles(_upper_nibble(N[-1]), padding.value)
return j[:-1] + [middle_byte] + shifted_N + [last_byte]
def _treat_message_enc(M, N, key):
checksum = [0 for byte in range(0, BLOCK_BYTES)]
l = len(M)//BLOCK_BYTES
padding_bytes = len(M)%BLOCK_BYTES
M = bytes_to_block_matrix(M)
C = []
for j in range(0, l):
checksum = xor(checksum, M[j])
tweak = _tweak_message(N, j, _MessageTweak.BLOCK)
C.append(tbc.encrypt(tweak, key, M[j]))
if padding_bytes == 0:
tweak = _tweak_message(N, l, _MessageTweak.NO_PADDING)
Final = tbc.encrypt(tweak, key, checksum)
else:
m_padded = pad10(M[l])
checksum = xor(checksum, m_padded)
tweak = _tweak_message(N, l, _MessageTweak.PAD)
pad = tbc.encrypt(tweak, key, [0 for byte in range(0, BLOCK_BYTES)])
lower_part = pad[:padding_bytes]
C.append(xor(M[l], lower_part))
tweak_final = _tweak_message(N, l+1, _MessageTweak.FINAL)
Final = tbc.encrypt(tweak_final, key, checksum)
return Final, C
def _treat_message_dec(C, N, key):
checksum = [0 for byte in range(0, BLOCK_BYTES)]
l = len(C)//BLOCK_BYTES
padding_bytes = len(C)%BLOCK_BYTES
C = bytes_to_block_matrix(C)
M = []
for j in range(0, l):
tweak = _tweak_message(N, j, _MessageTweak.BLOCK)
M.append(tbc.decrypt(tweak, key, C[j]))
checksum = xor(checksum, M[j])
if padding_bytes == 0:
tweak = _tweak_message(N, l, _MessageTweak.NO_PADDING)
Final = tbc.encrypt(tweak, key, checksum)
else:
tweak = _tweak_message(N, l, _MessageTweak.PAD)
pad = tbc.encrypt(tweak, key, [0 for byte in range(0, BLOCK_BYTES)])
lower_part = pad[:padding_bytes]
M.append(xor(C[l], lower_part))
m_padded = pad10(M[l])
checksum = xor(checksum, m_padded)
tweak_final = _tweak_message(N, l+1, _MessageTweak.FINAL)
Final = tbc.encrypt(tweak_final, key, checksum)
return Final, M
def encrypt(A, M, N, key):
K = list(key)
N = list(N)
Auth = build_auth(TWEAK_BITS, A, K)
Final, C = _treat_message_enc(M, N, K)
tag = xor(Auth, Final)
return block_matrix_to_bytes(C), bytes(tag)
def decrypt(A, C, N, tag, key):
K = list(key)
N = list(N)
tag = list(tag)
Auth = build_auth(TWEAK_BITS, A, K)
Final, M = _treat_message_dec(C, N, K)
tag2 = xor(Auth, Final)
if tag != tag2:
raise TagValidationError(tag, tag2)
return block_matrix_to_bytes(M)
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