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ciphers.py
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ciphers.py
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#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
# ciphers.py
#
# Copyright 2017 Jesse Rominske
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
# MA 02110-1301, USA.
#
# Program to quickly encipher a plaintext
import math
import sys
import factorize
# converts the letter input to numbers (using ASCII)
def textConvert(text):
nums = []
for t in list(text):
nums.append(ord(t) - 65) #A should be 0, for example
return nums
# converts simple numbers into 4-long blocks
def numToBlock(nums):
blocks = []
prev = 26 # set to an invalid value to be an indicator
for n in nums:
if prev == 26: # this is invalid, so we know this means it can be replaced
prev = n # store the number for next time
else:
blocks.append(n + (100 * prev)) # make 4-digit block from numbers
prev = 26 #reset the value of prev to 26 for next time
return blocks
# converts 4-long blocks into simple numbers
def blockToNum(blocks):
nums = []
for b in blocks:
nums.append((b - (b % 100)) / 100) # append its first two digits
nums.append(b % 100) # append its last two digits
return nums
# converts number input to letters (using ASCII)
def numConvert(nums):
text = ""
for n in nums:
text += (str(chr(n + 65)))
return text
# finds an inverse of a given mod m
def modInverse(a, m):
i = 1
for i in range(1, m):
if (a * i) % m == 1:
print("Found inverse of " + str(a) + " mod " + str(m) + ": " + str(i))
return i
# the lines below will only execute if the loop finished and never returned i
print("No inverse of " + str(a) + " mod " + str(m))
return 1
# uses given affine cipher to encode a message
def affineEncode(plainText, a, b):
plainNums = textConvert(plainText)
cipherNums = []
for p in plainNums:
cipherNums.append(((p * a) + b) % 26)
return numConvert(cipherNums)
# uses given affine cipher to decode a message
def affineDecode(cipherText, a, b):
cipherNums = textConvert(cipherText)
plainNums = []
aBar = modInverse(a, 26)
for c in cipherNums:
plainNums.append(((c - b) * aBar) % 26)
return numConvert(plainNums)
# uses given Vigenère cipher to decode a message
def vigenDecode(cipherText, key):
cipherNums = textConvert(cipherText)
keyNums = textConvert(key)
plainNums = []
i = 0
for c in cipherNums:
plainNums.append((c - keyNums[i % len(keyNums)]) % 26)
i += 1
return numConvert(plainNums)
# uses given modular exponentation cipher to encode message
def modExpEncode(plainText, p, e):
plainBlocks = numToBlock(textConvert(plainText))
cipherBlocks = []
for b in plainBlocks:
c = 1
for i in range(0, e): # up to but not including the power
c = (c * b) % p
cipherBlocks.append(c)
return cipherBlocks
# uses given modular exponentation cipher to decode message
def modExpDecode(cipherBlocks, p, e):
d = modInverse(e, p-1)
plainBlocks = []
for c in cipherBlocks:
pb = 1
for i in range(0, d): # up to but not including the power
pb = (pb * c) % p
plainBlocks.append(pb)
return numConvert(blockToNum(plainBlocks))
# uses given RSA cipher to decode message
def rsaDecode(cipherBlocks, e, n):
factors = factorize.factorize(n) # will be a 2-tuple for RSA
phi = (factors[0] - 1) * (factors[1] - 1)
d = modInverse(e, phi)
plainBlocks = []
for c in cipherBlocks:
pb = 1
for i in range(0, d): # up to but not including the power
pb = (pb * c) % n
plainBlocks.append(pb)
return numConvert(blockToNum(plainBlocks))
# main program structure
def main(args):
#problems from Rosen, Elementary Number Theory, 5th ed.
#8.1: 4
print(affineEncode("THERIGHTCHOICE", 15, 14))
#8.1: 6
print(affineDecode("RTOLKTOIK", 3, 24))
#8.2: 2
print(vigenDecode("WBRCSLAZGJMGKMFV", "SECRET"))
#8.3: 2
print(modExpEncode("SWEETDREAMS", 2621, 7))
#8.3: 4
print(modExpDecode([1213, 902, 539, 1208, 1234, 1103, 1374], 2591, 13))
#8.4: 8
print(rsaDecode([504, 1874, 347, 515, 2088, 2356, 736, 468], 5, 2881))
if __name__ == '__main__':
sys.exit(main(sys.argv))