Much more detail than in the past, and a promise of open-source software release soon:

https://android-developers.googleblog.com/2018/10/building-titan-better-security-through.html

Titan M performs several security sensitive functions, including:

• Storing and enforcing the locks and rollback counters used by Android Verified Boot.
• Securely storing secrets and rate-limiting invalid attempts at retrieving them using the Weaver API.
• Providing backing for the Android Strongbox Keymaster module, including Trusted User Presence and Protected Confirmation. Titan M has direct electrical connections to the Pixel’s side buttons, so a remote attacker can’t fake button presses. These features are available to third-party apps, such as FIDO U2F Authentication.
• Enforcing factory-reset policies, so that lost or stolen phones can only be restored to operation by the authorized owner.
• Ensuring that even Google can’t unlock a phone or install firmware updates without the owner’s cooperation with Insider Attack Resistance.

A Primer on Trustworthy Secure Bootloading*
*exemplified on a RISC-V processor system

Hi, my name is Ilia and I work at MIT’s Computer Science and Artificial Intelligence Lab with Srini Devadas to imagine a world where users of computers worldwide can be safe from one other and from themselves. But who might you be? Here, I will assume you have some familiarity with computer system architecture for the deep dive into our case study of a secure RISC-V processor system. I will otherwise attempt to keep this article as accessible as possible. If you find this text confusing, misleading, or otherwise underwhelming, send me a note! I’d like to improve.

TL;DR: scroll down for an implementation example of a secure bootloader on a typical RISC-V system.[…]

Hmm, this WordPress blog does strange things with Medium.com-based URLs, I’ll include two versions below, one with a SPACE in it, so you can copy the text, the second will be processed by WordPress, may not be visible on some systems (like mine):

https://medium.com/ @ilia.lebedev/secure-boot-2d6e319b6978

View story at Medium.com

This is my latest article on a topic near and dear to my heart: making IDA Pro more modern and, well, better. Those familiar with IDA Pro probably know that feeling: there are glitches in the processor modules that you use, you don’t have the source code, and they are driving you crazy! Unfortunately, not all of the glitches discussed here qualify as bugs, meaning that the developers are unlikely to ever fix them—unless you fix them yourself.[…]

http://blog.ptsecurity.com/2018/10/modernizing-ida-pro-how-to-make.html