The guidelines from ASHRAE establish 64.4-80.6degF (18-27degC) as the optimal temperature for a data center.

Explanation
Virtual private networks (VPNs) are used to provide administrative personnel with secure communication channels through security systems and into trust zones. They allow staff who perform system administration tasks to have access to ports and systems that are not allowed from the public Internet. IPSec is an encryption protocol for point-to-point communications at the network level, and may be used within a trust zone but not to give access into a trust zone. TLS enables encryption of communications between systems and services and would likely be used to secure the VPN communications, but it does not represent the overall concept being asked for in the question. SSH allows for secure shell access to systems, but not for general access into trust zones.