IPA IP 2024 Question 57

A: Public key cryptography, B: Common key cryptography, C: Slow, D: Fast — the key characteristics identify the method, and the method decides the speed.

• •Row A: different keys for encryption and decryption, and easy safe distribution. That is public key cryptography: the encryption key is published, and only the decryption (private) key is kept secret, so nothing secret has to be delivered. Its mathematical operations are heavy, so it is slow.
• •Row B: the same key for both, and difficult safe distribution. That is common key cryptography: the one shared key must reach the partner without anyone else seeing it. Its operations are simple, so it is fast.

Answer (c)

Why not others:

• •(a) — swaps the two methods: the row with two different keys and easy distribution is public key cryptography, not common key cryptography
• •(b) — also swaps the methods; its speeds fit the names it gives (common fast, public slow), but the names do not fit the key characteristics of the rows
• •(d) — names the methods correctly but reverses their speeds: public key cryptography is the slow one

Key rule: Common key cryptography is fast but the shared key is hard to distribute; public key cryptography solves distribution but is slow, so hybrid schemes exchange a common key with public key cryptography and encrypt the data with it.

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