Caesar Cipher in Python

caesar.py

#!/usr/bin/python
import sys
import optparse

dic = {
  "a": ord("a") - 97,
  "b": ord("b") - 97,
  "c": ord("c") - 97,
  "d": ord("d") - 97,
  "e": ord("e") - 97,
  "f": ord("f") - 97,
  "g": ord("g") - 97,
  "h": ord("h") - 97,
  "i": ord("i") - 97,
  "j": ord("j") - 97,
  "k": ord("k") - 97,
  "l": ord("l") - 97,
  "m": ord("m") - 97,
  "n": ord("n") - 97,
  "o": ord("o") - 97,
  "p": ord("p") - 97,
  "q": ord("q") - 97,
  "r": ord("r") - 97,
  "s": ord("s") - 97,
  "t": ord("t") - 97,
  "u": ord("u") - 97,
  "v": ord("v") - 97,
  "w": ord("w") - 97,
  "x": ord("x") - 97,
  "y": ord("y") - 97,
  "z": ord("z") - 97
}

def getIdx(string: str, k: int) -> list:
  idx = []
  for ch in string:
    if ch in dic:
      idx.append((dic[ch] + k) % 26)
  return idx

def encrypt(plain: str, k: int) -> str:
  cipher = ""
  cIdx = getIdx(plain, k)
  for i in cIdx:
    cipher += chr(i + 97)
  return cipher

def decrypt(cipher: str, k: int) -> str:
  cIdx = getIdx(cipher, -k)
  plain = ""
  for i in cIdx:
    plain += chr(i + 97)
  return plain

if __name__ == "__main__":
  parser = optparse.OptionParser("\n\t./caesar.py -e \"HELLO WORLD\" -k 3\n\t./caesar.py -d \"KHOORZRUOG\" -k 3")
  parser.add_option("-e", "--encrypt", dest="plain", type="string", help="encrypt mode")
  parser.add_option("-d", "--decrypt", dest="cipher", type="string", help="decrypt mode")
  parser.add_option("-k", dest="k", type="int", help="shift value")
  options, args = parser.parse_args()

  if(options.k == None or (options.cipher == None and options.plain == None)):
    print(parser.usage)
    sys.exit(1)

  k = options.k
  plain = options.plain
  cipher = options.cipher

  if(plain != None):
    cipher = encrypt(plain.lower(), k)
    print(cipher.upper())
  else:
    plain = decrypt(cipher.lower(), k)
    print(plain.upper())