ITPEC FE Morning April 2023 Question 38
Which of the following is an appropriate description of “asymmetric encryption,” used for encrypting and decrypting messages for secure communication?
Explanation
Encrypt with the receiver's public key, decrypt with the receiver's private key — each party owns a key pair, and only the private key can undo what its public key encrypts.
The receiver publishes the public key and keeps the private key secret. Anyone can encrypt a message for the receiver with the public key, but only the holder of the matching private key can decrypt it, so no secret has to be shared in advance. The reverse use, signing with the sender's private key and verifying with the sender's public key, gives a digital signature, which proves origin rather than hiding content.
Answer (b)
Why not others:
- •(a) — encryption does not shorten a message; public-key ciphertext is as long as or longer than the plaintext block. Shortening is compression.
- •(c) — a private key is never open to everyone; this swaps the roles of the two keys, and a message anyone could decrypt would not be secret.
- •(d) — one secret key shared among everyone who must decrypt is symmetric (common key) encryption, such as AES.
Key rule: For secrecy use the receiver's key pair, public to encrypt and private to decrypt; for a signature use the sender's, private to sign and public to verify.
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