def crypt(s,offset):
    A = ord("A")
    crypted = []
    for i in s:
        crypted.append(chr((ord(i)-A+offset)%26+A))
    return "".join(crypted)

def encrypt(plaintext):
    return crypt(plaintext,1)

def decrypt(ciphertext):
    return crypt(ciphertext,-1)

encrypt("APPLE")

'BQQMF'

decrypt("BQQMF")

'APPLE'