ITPEC IP October 2019 Question 44

The same key encrypts and decrypts — that is what defines symmetric (common key) cryptography.

In symmetric key cryptography the sender and the receiver share one secret key, used both to encrypt and to decrypt, so the key must be kept secret and delivered safely. In public key cryptography each party has a key pair: data encrypted with the public key can be decrypted only with the matching private key, so the encryption key may be published.

Answer (d)

Why not others:

• •(a) — describes public key cryptography: the encryption key is the public key, and only the private key decrypts; with a symmetric key, whoever learns the key can decrypt
• •(b) — describes public key cryptography: a receiver needs only its own key pair, since every sender uses the same public key; with symmetric keys each pair of parties needs a separate secret key
• •(c) — a digital certificate issued by a CA binds a public key to its owner; it belongs to public key cryptography (PKI)

Key rule: Symmetric keys: one shared secret, fast, but one key per pair of parties, n(n − 1) ÷ 2 in all; public key: a key pair per party, 2n in all, with certificates proving whose public key it is.

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