Python/ciphers/affine_cipher.py
2018-10-20 14:45:08 -05:00

78 lines
2.8 KiB
Python

from __future__ import print_function
import sys, random, cryptomath_module as cryptoMath
SYMBOLS = """ !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~"""
def main():
message = input('Enter message: ')
key = int(input('Enter key [2000 - 9000]: '))
mode = input('Encrypt/Decrypt [E/D]: ')
if mode.lower().startswith('e'):
mode = 'encrypt'
translated = encryptMessage(key, message)
elif mode.lower().startswith('d'):
mode = 'decrypt'
translated = decryptMessage(key, message)
print('\n%sed text: \n%s' % (mode.title(), translated))
def getKeyParts(key):
keyA = key // len(SYMBOLS)
keyB = key % len(SYMBOLS)
return (keyA, keyB)
def checkKeys(keyA, keyB, mode):
if keyA == 1 and mode == 'encrypt':
sys.exit('The affine cipher becomes weak when key A is set to 1. Choose different key')
if keyB == 0 and mode == 'encrypt':
sys.exit('The affine cipher becomes weak when key A is set to 1. Choose different key')
if keyA < 0 or keyB < 0 or keyB > len(SYMBOLS) - 1:
sys.exit('Key A must be greater than 0 and key B must be between 0 and %s.' % (len(SYMBOLS) - 1))
if cryptoMath.gcd(keyA, len(SYMBOLS)) != 1:
sys.exit('Key A %s and the symbol set size %s are not relatively prime. Choose a different key.' % (keyA, len(SYMBOLS)))
def encryptMessage(key, message):
'''
>>> encryptMessage(4545, 'The affine cipher is a type of monoalphabetic substitution cipher.')
'VL}p MM{I}p~{HL}Gp{vp pFsH}pxMpyxIx JHL O}F{~pvuOvF{FuF{xIp~{HL}Gi'
'''
keyA, keyB = getKeyParts(key)
checkKeys(keyA, keyB, 'encrypt')
cipherText = ''
for symbol in message:
if symbol in SYMBOLS:
symIndex = SYMBOLS.find(symbol)
cipherText += SYMBOLS[(symIndex * keyA + keyB) % len(SYMBOLS)]
else:
cipherText += symbol
return cipherText
def decryptMessage(key, message):
'''
>>> decryptMessage(4545, 'VL}p MM{I}p~{HL}Gp{vp pFsH}pxMpyxIx JHL O}F{~pvuOvF{FuF{xIp~{HL}Gi')
'The affine cipher is a type of monoalphabetic substitution cipher.'
'''
keyA, keyB = getKeyParts(key)
checkKeys(keyA, keyB, 'decrypt')
plainText = ''
modInverseOfkeyA = cryptoMath.findModInverse(keyA, len(SYMBOLS))
for symbol in message:
if symbol in SYMBOLS:
symIndex = SYMBOLS.find(symbol)
plainText += SYMBOLS[(symIndex - keyB) * modInverseOfkeyA % len(SYMBOLS)]
else:
plainText += symbol
return plainText
def getRandomKey():
while True:
keyA = random.randint(2, len(SYMBOLS))
keyB = random.randint(2, len(SYMBOLS))
if cryptoMath.gcd(keyA, len(SYMBOLS)) == 1:
return keyA * len(SYMBOLS) + keyB
if __name__ == '__main__':
import doctest
doctest.testmod()
main()