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import random

import sys

import cryptomath_module as cryptomath

SYMBOLS = (

r""" !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`"""

r"""abcdefghijklmnopqrstuvwxyz{|}~"""

)

def main():

"""

>>> key = get_random_key()

>>> msg = "This is a test!"

>>> decrypt_message(key, encrypt_message(key, msg)) == msg

True

"""

message = input("Enter message: ").strip()

key = int(input("Enter key [2000 - 9000]: ").strip())

mode = input("Encrypt/Decrypt [E/D]: ").strip().lower()

if mode.startswith("e"):

mode = "encrypt"

translated = encrypt_message(key, message)

elif mode.startswith("d"):

mode = "decrypt"

translated = decrypt_message(key, message)

print(f"\n{mode.title()}ed text: \n{translated}")

def check_keys(keyA, keyB, mode):

if mode == "encrypt":

if keyA == 1:

sys.exit(

"The affine cipher becomes weak when key "

"A is set to 1. Choose different key"

)

if keyB == 0:

sys.exit(

"The affine cipher becomes weak when key "

"B is set to 0. 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 "

f"be between 0 and {len(SYMBOLS) - 1}."

)

if cryptomath.gcd(keyA, len(SYMBOLS)) != 1:

sys.exit(

f"Key A {keyA} and the symbol set size {len(SYMBOLS)} "

"are not relatively prime. Choose a different key."

)

def encrypt_message(key: int, message: str) -> str:

"""

>>> encrypt_message(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 = divmod(key, len(SYMBOLS))

check_keys(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 decrypt_message(key: int, message: str) -> str:

"""

>>> decrypt_message(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 = divmod(key, len(SYMBOLS))

check_keys(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 get_random_key():

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()

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