#!/usr/bin/env python3

def vigDecrypt(ciphertext, key):
    decrypted = ''
    for i, ch in enumerate(ciphertext):
        decrypted += unshiftLetter(ch, key[i % len(key)])
    return decrypted

def unshiftLetter(letter, keyLetter):
    letter = ord(letter) - ord("A")
    keyLetter = ord(keyLetter) - ord("A")
    new = (letter - keyLetter) % 26
    return chr(new + ord("A"))