One time pad - pure python implementation

otp.py

import os
import sys
import argparse

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('infile')
    parser.add_argument('padfile')
    parser.add_argument('outfile')
    chunksize = 8192
    args = parser.parse_args()
    infile = args.infile
    padfile = args.padfile
    outfile = args.outfile
    if os.path.getsize(infile) > os.path.getsize(padfile):
        print('Error: padding is smaller than input file')
        sys.exit(1)
    with open(infile, 'rb') as inf, open(padfile, 'rb') as pf, open(outfile, 'wb') as of:
        while True:
            inchunk  = inf.read(chunksize)
            padchunk = pf.read(chunksize)
            if inchunk:
                outchunk = ''.join([chr(ord(i) ^ ord(p)) for i, p in zip(inchunk, padchunk)])
                of.write(outchunk)
            else:
                break