input: Write a python function to decrypt a given ciphertext using a provided cipher alphabet.
  assert decrypt_ciphertext("sft")=="abc"
  assert decrypt_ciphertext("tqhp zt")=="defg hd"
  assert decrypt_ciphertext("pqc")=="gij"
output: def decrypt_ciphertext(ciphertext):
      cipher_alphabet = "zyxwvutsrqponmlkjihgfedcba"
      normal_alphabet = "abcdefghijklmnopqrstuvwxyz"
      decryption_dict = {cipher_alphabet[i]: normal_alphabet[i] for i in range(len(cipher_alphabet))}
      decrypted_message = ''.join(decryption_dict[char] if char in decryption_dict else char for char in ciphertext)
      return decrypted_message