$7\mathrm{.}2.$ Let $$ be an encryption scheme with CPA security. Let ${}^{\text{'}}$ be the encryption scheme

defined by:

${}^{\text{'}}.$Enc$(k,m)=00||*Enc(k,m)$

The decryption algorithm in ${}^{\text{'}}$ simply throws away the first two bits of the ciphertext and

then calls $.$Dec.

$($a$)$ Does ${}^{\text{'}}$ have CPA$ security? Prove or disprove $($if disproving, show a distinguisher

and calculate its advantage$).$

$($b$)$ Does ${}^{\text{'}}$ have CPA security? Prove or disprove $($if disproving, show a distinguisher and

calculate its advantage$).$