Whoever thought about this riddle, has no clue about cryptography. He has set both private key and public key to value 255. Public key is actually calculated by (g ^ private) mod p Which is in this case 289. That is the point of the bloody algorithm it is difficult to reverse the operation. Alice would not be able to decrypt the message. The bloody web calculator, did allow setting invalid key. I am really annoyed BTW: 10 is not a generator over 307