How does a digital signature ensure authenticity?

A digital signature ensures authenticity primarily by using the sender’s private key for encryption. When a sender creates a digital signature, they generate a hash of the message and then encrypt that hash using their private key. This process demonstrates that the signature was created by the sender and that the message has not been altered since it was signed.

Since only the sender possesses their private key, the digital signature can be verified by anyone who has access to the sender's public key. This means that the recipient can be confident that the message genuinely came from the sender, ensuring both the sender's identity is authentic and the integrity of the message is intact.

The other options do not directly contribute to establishing authenticity. For instance, encrypting a message with the receiver's public key is a method for securing confidentiality rather than confirming the identity of the sender. Masking the sender's identity is contrary to the purpose of digital signatures, which seek to verify identities. Generating a unique key for each message may enhance security but does not specifically address the verification of the sender's identity and the integrity of the message like the proper use of a private key in digital signatures does.