ITPEC FE Morning October 2023 Question 36
Mr. A encrypts a message to be sent to Mr. B using an asymmetric key encryption method so that only Mr. B can decrypt the message. Which of the following is(are) the private key(s) used to decrypt the message?
Explanation
Only Mr. B's — in public key encryption the sender uses the recipient's public key, and only the recipient's private key can decrypt.
Each party has a key pair. To make a message only Mr. B can read, Mr. A encrypts it with Mr. B's public key, which anyone may have. The matching private key, which only Mr. B holds, is the one key that decrypts it. Mr. A's own keys are not involved.
Answer (d)
Why not others:
- •(a) A third party's — nobody else's key matches Mr. B's public key, and involving a third party would defeat the confidentiality
- •(b) Both — Mr. A's private key plays no part; decryption needs only the key matching the one used for encryption
- •(c) Only Mr. A's — Mr. A's private key is for signing what he sends; it cannot open a message encrypted with Mr. B's public key
Key rule: For confidentiality, encrypt with the recipient's public key; for a digital signature, sign with the sender's private key.
Security · AI-written, may contain errors
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