Chosen ciphertext insecurity in an ElGamal variant
There is no known way to compute $(g^a)^k \mod p = g^{ak} \mod p$,

given only 

$g^k \mod p$ and

$g^a \mod p$

as per the Decisional Diffie–Hellman assumption. This is roughly equivalent to the discrete log problem. This is described in more detail in this paper.