Trivial one-time pad implementation

otp.py

#!/usr/bin/python
import sys

if __name__ == "__main__":
  if len(sys.argv) != 3:
    print "Usage: otp.py <plaintext> <pad>"
    exit(1)

  plaintext = sys.argv[1].upper()
  pad = sys.argv[2].upper()

  if len(plaintext) != len(pad):
    print "Pad and plaintext must be the same length"
    exit(2)

  if not plaintext.isalpha() or not pad.isalpha():
    print "Pad and plaintext must be alphabetic"
    exit(3)

  ciphertext = ''
  for i in range(0, len(plaintext)):
    ciphertext += chr((ord(plaintext[i]) + ord(pad[i])) % 26 + ord('A'))

  print ciphertext