Brute Force

message = 'AVNYY'
LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'


for key in range(len(LETTERS)):


    translated = ''


    for symbol in message:
        if symbol in LETTERS:
            num = LETTERS.find(symbol) # get the number of the symbol
            num = num - key

            # handle the wrap-around if num is 26 or larger or less than 0
            if num < 0:
                num = num + len(LETTERS)

            # add number's symbol at the end of translated
            translated = translated + LETTERS[num]

        else:
            # just add the symbol without encrypting/decrypting
            translated = translated + symbol


    print('Key #%s: %s' % (key, translated))