Which of the following is NOT a property of a public key cryptosystem?
(Let P represent the private key, Q represent the public key and M the
plaintext message.)

A.
P and Q are difficult to generate from a particular key value.

B.
P[Q(M)] = M

C.
Q[P(M)] = M

D.
It is computationally infeasible to derive P from Q.

Explanation:
The answer refers to the initial computation wherein the private
and public keys are computed. The computation in this direction is
relatively straightforward. Answers “Q[P(M)] = M” and “P[Q(M)] = M” state the true property of
public key cryptography which is that a plaintext message
encrypted with the private key can be decrypted by the public key and vice versa. Answer “It is
computationally infeasible to derive P from Q” states that it is computationally infeasible to derive
the private key from the public key. Obviously, this is a critical property of public key cryptography.