Which of the following is essential for authenticating the source of a digital message?

Authenticating the source of a digital message primarily involves the use of a public key. In asymmetric encryption, the sender uses their private key to sign the message, which generates a unique digital signature that confirms the message's authenticity. The recipient can then use the associated public key to verify that the signature was created using the corresponding private key, thus confirming that the message truly originated from the claimed sender.

Using a public key in this process allows for secure communication without sharing the private key, enabling trust in the identity of the sender. The verification process ensures that the message has not been altered in transit and is indeed from the correct source. Thus, the essential nature of the public key in the context of digital signatures reinforces its crucial role in message authentication.