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 \(K_{x}^{-}\) and \(K_{x}^{+}\) for \(x = A, B\), respectively. Let \(K_{x}(m)\) represent the operation of encrypting \(m\) with a key \(K_{x}\) 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?
GATE CS - 2016,
Answer: B
The correct way is \(\{m, K_{B}^{-}(H(m))\}\).
A digital signature is made with the sender's private key: B hashes the message and encrypts the digest with \(K_{B}^{-}\), then sends it with \(m\). A decrypts the signature with B's public key \(K_{B}^{+}\), hashes the received \(m\) herself and compares. A match proves the message came from B (only B has \(K_{B}^{-}\)) and was not altered.
Encrypting with B's public key proves nothing, since anyone has it. A's private key is unknown to B. \(K_{A}^{+}(m)\) gives confidentiality, not a signature, and has no digest.
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