Encrypted Key Exchange (EKE)
The first password-authenticated key exchange (PAKE), where a weak password is enough because it can't be cracked offline.
Steve Bellovin and Michael Merritt at AT&T Bell Labs published EKE in 1992. The problem was well known: protocols that prove you know a password nearly always leak enough for an eavesdropper to run a dictionary attack at leisure, at home. EKE fixes this by encrypting an ephemeral Diffie-Hellman value with the password. The only way to test a guess is to talk to the server, one guess at a time.
The idea was patented, and the patent kept many implementers away until it expired in 2011. SRP was designed partly to route around it, and today's OPAQUE is a distant descendant.