def edit(self, ciphertext, offset, new_text):
     plaintext = util.ctr_decrypt(ciphertext, self.key)
     new_plaintext = plaintext[:offset] + new_text + plaintext[offset+len(new_text):]
     return self.encrypt(new_plaintext)
 def decrypt(self, text):
     return util.ctr_decrypt(text, self.key)