Question 33

In public-key cryptography, a digital signature provides message integrity, authentication, and non-repudiation.

The protocol operates as follows:

1. Signing (by Anarkali): Anarkali computes the hash of the message $h = H(M)$ and encrypts the hash using her private key $K_{\text{Anarkali, private}}$ to generate signature $\sigma$.
2. Verification (by Salim): Salim receives message $M$ and signature $\sigma$. He decrypts $\sigma$ using Anarkali's public key $K_{\text{Anarkali, public}}$ to recover the original hash $h$, and compares it with his locally computed hash $H(M)$.

Because only Anarkali's public key can decrypt something encrypted by Anarkali's private key, verification requires Anarkali's public key (option A).