#### Affine Cipher

A
p
P
```import random
import sys

from maths.greatest_common_divisor import gcd_by_iterative

from . import cryptomath_module as cryptomath

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

def check_keys(key_a: int, key_b: int, mode: str) -> None:
if mode == "encrypt":
if key_a == 1:
sys.exit(
"The affine cipher becomes weak when key "
"A is set to 1. Choose different key"
)
if key_b == 0:
sys.exit(
"The affine cipher becomes weak when key "
"B is set to 0. Choose different key"
)
if key_a < 0 or key_b < 0 or key_b > 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 gcd_by_iterative(key_a, len(SYMBOLS)) != 1:
sys.exit(
f"Key A {key_a} 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'
"""
key_a, key_b = divmod(key, len(SYMBOLS))
check_keys(key_a, key_b, "encrypt")
cipher_text = ""
for symbol in message:
if symbol in SYMBOLS:
sym_index = SYMBOLS.find(symbol)
cipher_text += SYMBOLS[(sym_index * key_a + key_b) % len(SYMBOLS)]
else:
cipher_text += symbol
return cipher_text

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.'
"""
key_a, key_b = divmod(key, len(SYMBOLS))
check_keys(key_a, key_b, "decrypt")
plain_text = ""
mod_inverse_of_key_a = cryptomath.find_mod_inverse(key_a, len(SYMBOLS))
for symbol in message:
if symbol in SYMBOLS:
sym_index = SYMBOLS.find(symbol)
plain_text += SYMBOLS[
(sym_index - key_b) * mod_inverse_of_key_a % len(SYMBOLS)
]
else:
plain_text += symbol
return plain_text

def get_random_key() -> int:
while True:
key_b = random.randint(2, len(SYMBOLS))
key_b = random.randint(2, len(SYMBOLS))
if gcd_by_iterative(key_b, len(SYMBOLS)) == 1 and key_b % len(SYMBOLS) != 0:
return key_b * len(SYMBOLS) + key_b

def main() -> None:
"""
>>> 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}")

if __name__ == "__main__":
import doctest

doctest.testmod()
# main()
```