Q$4$: One$-$way security in a public key encryption scheme means that, given a challenge ciphertext corresponding to a random message, no efficient adversary can successfully recover the entire plaintext. Explain, using a standard reduction $($proof by contradiction$),$ why any public key encryption scheme that is secure under the IND$-$CPA model must also be one$-$way secure$.$

Q$5$: Suppose Alice and Bob reach an agreement and wish to formalize it$.$ Alice writes down the agreement in a file, $m,$ and encrypts $m$ using Bob's public key, $pk,$ to obtain $c.$ Alice then sends $c$ to Bob in an attempt to convince Bob that $c$ represents her commitment to the agreement. Would this method be effective? Please explain your reasoning.