ITPEC FE Morning April 2020 Question 40
Which of the following is the result when person A sends an e-mail that is encrypted by person B’s public key to person B and person C? Here, these three persons have the public keys of all three persons, and all of them have their own private keys.
Explanation
Only B, with B's own private key — data encrypted with a public key can be decrypted only with the matching private key.
A used B's public key. Its partner is B's private key, which only B holds. C receives the same encrypted mail and has everyone's public keys, but those only encrypt, and C's own private key matches C's public key, not B's. To reach both, A would have to encrypt a copy with each recipient's public key.
Answer (d)
Why not others:
- •(a) — B's public key is the encrypting key and is known to everyone; if it also decrypted, the encryption would protect nothing
- •(b) — C's private key pairs with C's public key, which was not used
- •(c) — A's public key played no part in the encryption; A's key pair is used when A signs, not when A encrypts for someone else
Key rule: For confidentiality, encrypt with the recipient's public key; for a digital signature, sign with the sender's private key.
Security · AI-written, may contain errors
This question comes from an official ITPEC past paper. ITPEC Practice is an independent study tool and is not affiliated with ITPEC. See the official FE past-paper collection or Report an issue.