$10.$ If an encryption scheme is IND$-$CPA secure$,$ show that the set Dec$1$

$($m$)($the set of all

ciphertexts that would decrypt to m$)$ must not be of polynomial size in the security

parameter for any efficiently computable message m$.$ Conclude in particular that a

public$-$key, deterministic encryption scheme $($like vanilla RSA$)$ can never be IND$-$CPA

secure$.$