The SSH protocol allows any client and server programs built to the protocol's specifications to communicate securely and to be used interchangeably.
Two varieties of SSH (version 1 and version 2) currently exist. SSH version 1 makes use of several patented encryption algorithms (however, some of these patents have expired) and is vulnerable to a well known security exploit that allows an attacker to insert data into the communication stream. The OpenSSH suite under Red Hat Enterprise Linux uses SSH version 2 which has an enhanced key exchange algorithm not vulnerable to the exploit in version 1. However, the OpenSSH suite does support version 1 connections.
It is recommended that only SSH version 2-compatible servers and clients are used whenever possible.