1. A method comprising: receiving a first request from a first end-user system for a speaker-verification digital signature for a corresponding communication; authenticating, via a processor, a claimed identity associated with a sender of the corresponding communication, the authenticating comprising obtaining an audio signal from the first end-user system corresponding to a text-phrase provided at the first end-user system and a voice template that corresponds to the claimed identity, comparing the audio signal and voice template to generate a confidence score, and generating an authentication certificate when the confidence score exceeds a threshold; after authenticating the claimed identity, generating a session ID to be inserted in the corresponding communication, the session ID being unique with respect to other communications for a destination address of the corresponding communication, and the session ID being available for use as another session ID for a different destination address; and responsive to generating the session ID, separately transmitting the corresponding communication and authentication information having the session ID to the destination address indicated in the corresponding communication, wherein the authentication information is generated by packaging the session ID with the authentication certificate if the confidence score exceeded the threshold, else the authentication information is generated by packaging the session ID with the confidence score.