Sender of the message wants to ensure that the message should be digitally signed. To ensure this, he performs following steps: Step 1: create the hash of the message. Step 2: encrypts the hash of the message by using an encryption key (i.e. creating the digital signature) Step 3: attach the encrypted hash (as arrived in step 2) along with the message. This encrypted hash will serve as a digital signature. On receipt of the message, the receiver will perform following steps: Step 4: decrypt the digital signature (encrypted hash) using a key and check the hash. If decryption is successful then it confirms the digital signature. Which of the following keys is used by the receiver of the message to confirm and validate a digital signature in step 4 mentioned above?