Whether the communications partner really has the desired system isn't checked, and there is no encryption of the data flow of the actual session. The SRA telnet and ftp methods only encode the user's id and password in standard cases. The necessary key is dynamically generated for each session by the Diffie Hellman method. This was first introduced in the classic 'New Directions in Cryptography' article in 1976 and shows the oldest public key method for encryption. It's not suitable for encryption or generation a digital signatures, but it does make possible the trusted agreement of a shared key between two communication partners, without this having to have shared knowledge of some sort before.
Modified versions offer the additional encryption of the data flow.
For authentication of the client rather than the server a normal, but sent encrypted, Unix password is used. The server can't prove its authenticity, but is de facto trustworthy. This method offer protection against passive, but not against active, attacks. These are less widely distributed and less well tested though. They are considerable extensions of the already existing sources, whereas the SSH safety concept is completely newly developed and produced solution.