Decryption of ASCII text

decrypt_affine.py

s = 'NTHELY'
a = 15
b = 3
n = 26

# Modular multiplicative inverse of a
a_inv = pow(a, -1, n)

for c in s:
    index = ord(c) - 65
    new_c = (a_inv * (index - b)) % n
    print(chr(new_c+65), end='')

decrypt_caesar.py

from sys import argv
import string

ALPHABET = string.ascii_uppercase

def caesar_decrypt(key, string):
    ret = ''
    for c in string.upper():
        if c in ALPHABET:
            index = ord(c) - 65
            new_index = (index + k) % len(ALPHABET)
            ret += chr(new_index + 65)
        else:
            ret += c
    return ret

if __name__ == '__main__':
    if len(argv) == 3:
        k = int(argv[1])
        print(caesar_decrypt(k, argv[2]))
    elif len(argv) == 2:
        for k in range(1, len(ALPHABET)):
            print(caesar_decrypt(k, argv[1]))
    else:
        print(f'Usage: {argv[0]} <key> <ciphertext>')
        exit()