Re: https://firmwaresecurity.com/2017/12/13/apple-secure-boot/

A New Level of Integration and Security
iMac Pro brings a new level of integration and security to the Mac with the T2 chip, Apple’s second generation custom Mac silicon. By designing and integrating several new controllers — such as the System Management Controller, image signal processor, audio controller and SSD controller — T2 delivers new capabilities to the Mac like enhanced imaging processing for the FaceTime HD camera. T2 also enables a new level of security by including a secure enclave coprocessor that provides the foundation for new encrypted storage and secure boot capabilities. Dedicated AES hardware encrypts data on the SSD without affecting performance, while secure boot ensures that only trusted software loads at startup.

https://www.apple.com/newsroom/2017/12/imac-pro-the-most-powerful-mac-ever-available-today/

I wish there was some firmware information in the tech specs:

https://www.apple.com/imac-pro/specs/