Very simple Python code for breaking substitution ciphers.

substitution.py

from string import maketrans

def decrypt(in, out, test):
  """
  Given a plaintext input, encrypted output, and an encrypted test string, return a
  decrypted test string. This will only work for a substitution-style cipher.
  """
  
  cipher = maketrans(out, in)
  return test.translate(cipher)