ITPEC IP April 2020 Question 39

Private key of Mr. B — what is encrypted with a public key can be decrypted only with the matching private key.

In public key cryptography each person has a key pair. The public key is published, so anyone can use it; the private key is kept secret by its owner. To send something only Mr. B may read, Mr. A encrypts it with Mr. B's public key. The only key that can decrypt it is Mr. B's private key, which only Mr. B holds, so the mail stays confidential even though the key used to encrypt it was public.

Answer (d)

Why not others:

• •(a) — Mr. A's public key belongs to a different key pair; it is used to send confidential data to Mr. A or to verify Mr. A's digital signature
• •(b) — Mr. A's private key creates Mr. A's signatures and decrypts mail sent to Mr. A; it cannot decrypt data locked with Mr. B's public key
• •(c) — Mr. B's public key did the encrypting and cannot undo it; if it could, anyone holding the published key could read the mail

Key rule: For confidentiality, encrypt with the receiver's public key and decrypt with the receiver's private key; for a digital signature it is the reverse, the sender signs with its own private key and anyone verifies with the sender's public key.

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