Which of the following is the MOST likely cause of users being unable to verify a single user’s email signature and that user being unable to decrypt sent
messages?

A.
Unmatched key pairs

B.
Corrupt key escrow

C.
Weak public key

D.
Weak private key

Explanation:
In a PKI the sender encrypts the data using the receiver’s public key. The receiver decrypts the data using his own private key. The sender and receiver must have
a matching key in order for the receiver to decrypt the data.
Incorrect Answers:
B: Key escrow is not used for verifying signatures or for decrypting data.
C: Public keys are public and known to all parties. They are weak by nature.
D: A weak private(secret) key could allow third parties to compromise the security, but would not cause problems verifying signatures or decrypting data.

Dulaney, Emmett and Chuck Eastton, CompTIA Security+ Study Guide, Sixth Edition, Sybex, Indianapolis, 2014, pp 262, 279-285