ITPEC FE Morning October 2020 Question 39

The sender signs with the private key, and the recipient confirms the sender — a digital signature proves who sent a message and that it was not altered.

• •The sender computes a hash (message digest) of the message and encrypts it with the sender's own private key; the result is the signature, attached to the message
• •The recipient decrypts the signature with the sender's public key and compares the result with a fresh hash of the received message
• •A match proves the message came from the holder of that private key and was not changed on the way

Answer (b)

Why not others:

• •(a) — restoring an encrypted message with the recipient's private key is decryption for confidentiality, not a signature
• •(c) — a signature is built from a hash of the whole message, not an attached fixed string, and a mismatch only shows that the message was altered, not which sections were falsified
• •(d) — anyone with the sender's public key can decrypt what the private key encrypted, so this gives no confidentiality; secrecy needs encryption with the recipient's public key

Key rule: Sign with your own private key and verify with the sender's public key; encrypt with the recipient's public key and decrypt with the recipient's private key.

Security · AI-written, may contain errors