ITPEC FE Subject A October 2024 Question 29
Which of the following description is an appropriate use of asymmetric encryption to ensure the confidentiality of a message that a sender is going to send to a receiver?
Explanation
Encrypt with the receiver's public key — only the matching private key, held by the receiver alone, can then decrypt the message.
In public-key cryptography each party has a key pair. For confidentiality the sender encrypts with the receiver's public key, which anyone may obtain; decryption needs the receiver's private key, so nobody else, not even the sender, can read the ciphertext.
Answer (c)
Why not others:
- •(a) — a pre-shared secret key used by both sides is symmetric (common-key) encryption, not asymmetric
- •(b) — anything encrypted with the sender's private key can be decrypted by anyone holding the sender's public key; this proves who sent it (a digital signature) but hides nothing
- •(d) — a message encrypted with the sender's public key needs the sender's private key to decrypt, so the receiver could not read it
Key rule: Confidentiality: encrypt with the receiver's public key, decrypt with the receiver's private key. Signature: sign with the sender's private key, verify with the sender's public key.
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