There are two versions of the Secure Shell protocol. The current version, Secure Shell version 2 (SecSh v2, SSH2) provides several security improvements as compared to the original Secure Shell version 1 (SecSh v1, SSH1). SSH Tectia is based on SecSh v2, as SSH Communications Security considers SecSh v1 deprecated and does not recommend nor support its use anymore.
SSH G3, based on and compatible with the standard SSH2, considerably improves Secure Shell encryption and management performance, securing throughput-intensive file transfers and applications without causing processing bottlenecks. The SSH G3 architecture is incorporated in the SSH Tectia client/server solution version 5.0 and later.
More information on the protocol can be found in the Secure Shell RFCs (4250-4256) (at http://www.ietf.org/rfc.html).