def decryption_round(round_index, transformed, KHat):
if 0 <= round_index <= ROUNDS - 2:
to_s_box = inverse_linear_transformation(transformed)
elif round_index == ROUNDS - 1:
to_s_box = string_xor(transformed, KHat[ROUNDS])
else:
raise ValueError(f'Invalid round number {round_index}')
to_xor = ''.join([
sbox_inverse_func(round_index, to_s_box[ind:ind + 4])
for ind in range(0, len(to_s_box), 4)
])
output = string_xor(to_xor, KHat[round_index])
return output
def linear_transformation(inp):
assert len(inp) == 128, 'input to linear transf. is not 128 bits'
res = ''
for array in LTtable:
out = '0'
for e in array:
out = string_xor(out, inp[e])
res += out
return res
def encryption_round(round_index, round_input, KHat):
# First we xor the round input with the key
input_xor_key = string_xor(round_input, KHat[round_index])
# Then we apply the Sbox function on the result
sbox_res = ''.join([
sbox_func(round_index, input_xor_key[index:index + 4])
for index in range(0, len(input_xor_key), 4)
])
# If it is round 0 to 30 we apply the linear transformation
# Otherwise we do bitwise xor between the sbox_res and the last key
if 0 <= round_index <= ROUNDS - 2:
round_output = linear_transformation(sbox_res)
elif round_index == ROUNDS - 1:
round_output = string_xor(sbox_res, KHat[ROUNDS])
else:
raise ValueError(f'round number {round_index} out of range')
return round_output
def inverse_linear_transformation(inp):
assert len(inp) == 128, 'input to inverse linear transf. is not 128 bits'
res = ''
for array in LTInverseTable:
out = '0'
for array_element in array:
out = string_xor(out, inp[array_element])
res += out
return res
def xor_two_strings(s1, s2):
bit_str1 = string_to_bitstring(s1)
bit_str2 = string_to_bitstring(s2)
res = string_xor(bit_str1, bit_str2)
return bitstring_to_string(res)