one-time pad

from random import randrange

def otp(a, b, mode):
    """one-time pad
    mode=0: encrypt; mode=1: decrypt"""
    a, b = otpformat(a), otpformat(b)
    output = ''
    i = 0
    if mode == 0: sign = 1
    elif mode == 1: sign = -1
    for letter in a:
        output += chr((((ord(letter)-65)
               + (ord(b[i%len(b)])-65)*sign)%26) + 65)
        i += 1
    return output

def otpformat(a):
    a = a.replace(' ','')
    a = a.upper()
    return a

def otpgen(length):
    """one-time pad key generator"""
    output = ''
    for i in range(length):
        output += chr(randrange(65,91))
    return output

def xor(a, b):
    """exclusive or"""
    output = ''
    i = 0
    for bit in a:
        if bit == b[i % len(b)]:
            output += '0'
        elif bit != b[i % len(b)]:
            output += '1'
        i += 1
    return output