def feistel_round(half_block, subkey):
assert len(half_block) == 32
assert len(subkey) == 48
expansion_output = destools.permute(half_block, EXPANSION)
xor_output = destools.xor(expansion_output, subkey)
sbox_output = substituion_round(xor_output)
permute_output = destools.permute(sbox_output, PERMUTATION)
log(" Feistel(Right Block, Subkey):");
log(" Expand(Right Block)=", convert_bits_to_string(expansion_output))
log(" Expanded(...) XOR Subkey=", convert_bits_to_string(xor_output, 6))
log(" S-Box(...)=", convert_bits_to_string(sbox_output))
log(" Permutation(...) (output) =", convert_bits_to_string(permute_output))
return permute_output
def encrypt(block, key, decrypt=False):
nrounds = 16
assert len(block) == 64
assert len(key) == 64
if decrypt:
log("Decrypting text to plaintext:", convert_bits_to_string(block))
else:
log("Encrypting plaintext to ciphertext:", convert_bits_to_string(block))
# Generate substitution keys
subkeys = []
key_left = destools.permute(key, PERMUTED_CHOICE_1_LEFT)
key_right = destools.permute(key, PERMUTED_CHOICE_1_RIGHT)
assert len(key_left) == 28
assert len(key_right) == 28
log("Generating Subkeys:")
log(" Initial Key =", convert_bits_to_string(key))
log(" Permuting into Left and Right keys")
log(" Left Half =", convert_bits_to_string(key_left))
log(" Right Half =", convert_bits_to_string(key_right))
for i in xrange(nrounds):
shift_amount = KEY_SHIFT_AMOUNTS[i]
destools.left_shift(key_left, shift_amount)
destools.left_shift(key_right, shift_amount)
subkey = destools.permute(key_left + key_right, PERMUTED_CHOICE_2)
subkeys.append(subkey)
log("")
log("Subkey %s:" % i)
log(" Shifting key halves to the left by %s bits" % shift_amount)
log(" Left Half =", convert_bits_to_string(key_left))
log(" Right Half =", convert_bits_to_string(key_right))
log(" Permuting Left and Right key into subkey")
log(" Subkey =", convert_bits_to_string(subkey))
# Apply subkeys in reverse order if decrypting
log("")
if decrypt:
log("Reversing order of subkeys")
subkeys = subkeys[::-1]
# Initial Permutation
block = destools.permute(block, INITIAL_PERMUTATION)
log("Initial Permutation:", convert_bits_to_string(block))
log("")
# Rounds
left_block = block[0:32]
right_block = block[32:]
for i in xrange(nrounds):
log("Round %s:" % i)
log(" Input:")
log(" Subkey =", convert_bits_to_string(subkeys[i]))
log(" Left Block =", convert_bits_to_string(left_block))
log(" Right Block =", convert_bits_to_string(right_block))
tmp = right_block
fiestel_out = feistel_round(right_block, subkeys[i])
right_block = destools.xor(left_block, fiestel_out)
left_block = tmp
log(" Output:")
log(" Left Block = Left Block XOR Feistel(...)")
log(" =", convert_bits_to_string(right_block))
log(" Right Block (Unchanged)")
if i == 15:
log(" DO NOT SWITCH right and left block after the last round")
else:
log(" Left and Right blocks are switched and input into next round.")
log("")
# Final Permutation
# right and left are switched here because the final round does not switch
# them. Here we just switch them back.
encrypted = destools.permute(right_block + left_block, FINAL_PERMUTATION)
log("Result after all rounds = Left Block + Right Block")
log(" =", convert_bits_to_string(right_block+left_block))
log("After Final Permutation =", convert_bits_to_string(encrypted))
log("")
return encrypted
