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Consider that B wants to send a message m that is digitally signed to A. Let the pair of private and public keys for A and B be denoted by K_A⁻ and K_A⁺ for x = A,B, respectively. Let Kₓ(m) represent the operation of encrypting m with a key Kₓ and H(m) represent the message digest. Which one of the following indicates the CORRECT way of sending the message m along with the digital signature to A?

1. {m, K_B^+(H(m))}
2. {m, K_B^-(H(m))}
3. {m, K_A^-(H(m))}
4. {m, K_A^+(m)}

Correct answer: {m, K_B^-(H(m))}

Solution

The correct option is right because B needs to sign the message digest H(m) using their private key K_B⁻, which allows A to verify the signature using B's public key K_B⁺. This ensures the authenticity and integrity of the message.

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