Dylan and Liam can easily achieve confidentiality and integrity by using the key to encrypt and decrypt messages, which is not what we are looking for here. They can also achieve authentication because they know that if a message decrypts with the key, it must have been encrypted by the only other person with knowledge of the key. They cannot, however, achieve non-repudiation because they have no way to prove to a third party that a message came from the other party and wasn’t forged by themselves.