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.