I wrote an extensive piece on this as part of a presentation in 2004, called Authentication Without Encryption for Ham Radio - https://rietta.com/blog/2009/08/17/authentication-without-en....

Gave examples of using PGP cleartext signatures and Cram-MD5 (shudder) challenge-response for authentication without obscuring the meaning of the communication. At the time I thought that IPSEC with Authentication, but Encryption may be the best thing, but that has been slow to take off altogether.