Byte-wise deciphering on base of known alphabet.

decipher.py

ab_name = "ab.txt"
cipher_name = "cipher.txt"
output_name = "output.txt"
 
import sys

if (len(sys.argv) < 2):
	print("Error: too few arguments")
	exit(0)

input_name = ""
inverse = False
if (len(sys.argv) == 3):
	if (sys.argv[1] == "--encode"):
		inverse = True
		input_name = sys.argv[2]
	elif (sys.argv[2] == "--encode"):
		inverse = True
		input_name = sys.argv[1]
	else:
		print("Error: unknown arguments")
		exit(0)
elif (len(sys.argv) == 2):
	input_name = sys.argv[1]

def try_conv(alphabet, code):
	if (alphabet[code] == -1):
		print("Warning: not found decoding for code %d (0x%02X)" % (code, code))
		return code
	return alphabet[code]
 
ab = open(ab_name, "rb")
src = [x for x in ab.read()]
ab.close()
 
cipher = open(cipher_name, "rb")
dst = [x for x in cipher.read()]
cipher.close()

if (inverse):
	src, dst = dst, src
 
if (len(src) != len(dst)):
	print("sizes of src and dst are different")
	exit(0)
 
alphabet = [-1] * 256
for i in range(0, len(src)):
	alphabet[dst[i]] = src[i]
	
# for i in range(0, len(alphabet)):
	# val = alphabet[i]
	# if (val != -1):
		# print("%02X \t=>\t %02X" % (i, val))
 
BSIZE = 1024
 
input = open(input_name, "rb")
output = open(output_name, "wb")
 
while True:
	data = input.read(BSIZE)
	if not data:
		break
	output.write(bytes([try_conv(alphabet, x) for x in data]))
 
input.close()
output.close()