affine cipher Algorithm
The Affine Cipher Algorithm is a type of monoalphabetic substitution cipher used in cryptography, where each letter in the plaintext is mapped to another letter in the ciphertext through a simple mathematical formula. This algorithm is based on modular arithmetic and uses two keys – a multiplication key (a) and an addition key (b) – for encryption and decryption. The relationship between the plaintext (P) and the ciphertext (C) can be expressed using the formula C ≡ aP + b (mod m), where m represents the size of the alphabet. For decryption, the inverse of the multiplication key is used along with the addition key, following the formula P ≡ a⁻¹(C - b) (mod m). The encryption and decryption processes are reversible, making it suitable for secure communication.
The security of the Affine Cipher Algorithm relies on the choice of the keys a and b, as well as the size of the alphabet m. To ensure that the cipher is invertible and unique, the multiplication key (a) must be chosen such that it is coprime to the size of the alphabet (m). This means that the greatest common divisor of a and m should be equal to 1. However, the Affine Cipher is vulnerable to frequency analysis and other cryptanalytic techniques, as the algorithm preserves the frequency distribution of the plaintext, making it easier for an attacker to analyze the ciphertext and recover the original message. Despite its simplicity, the Affine Cipher serves as a building block for more complex cryptographic systems and provides a foundation for understanding more advanced encryption techniques.
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()
