A designated verifier signature is a signature

Digital signature

A digital signature or digital signature scheme is a mathematical scheme for demonstrating the authenticity of a digital message or document. A valid digital signature gives a recipient reason to believe that the message was created by a known sender, and that it was not altered in transit...

 scheme in which signatures can only be verified by a single designated verifier who is chosen by the signer. Designated verifier signatures were first proposed in 1996 by Jakobsson Sako, Kazue Sako, and Russell Impagliazzo. Proposed as a way to combine authentication and off-the-record messages, designated verifier signatures allow authenticated, private conversations to take place.