EC0-350 Question #804: Real Exam Question with Answer & Explanation

In the context of using PKI, when Sven wishes to send a secret message to Bob, he looks up Bob's public key in a directory, uses it to encrypt the message before sending it off. Bob then uses his private key to decrypt the message and reads it. No one listening on can decrypt the message. Anyone can send an encrypted message to Bob but only Bob can read it. Thus, although many people may know Bob's public key and use it to verify Bob's signature, they cannot discover Bob's private key and use it to forge digital signatures. What does this principle refer to?