x86 Archives - Michael Altfield's Tech Blog

Trusted Boot (Anti-Evil-Maid, Heads, and PureBoot)

By Michael Altfield, on February 16th, 2023

Verifying Boot Integrity with Heads, PureBoot

This post will help to provide historical context and demystify what's under the hood of Heads, PureBoot, and other tools to provide Trusted Boot.

I will not be presenting anything new in this article; I merely hope to provide a historical timeline and a curated list of resources.

Intro

The Librem Key cryptographically verifies the system's integrity and flashes red if it's detected tampering

I've always felt bad about two things:

Because I run QubesOS, I usually disable "Secure Boot" on my laptop I travel a lot, and I don't have a good way to verify the integrity of my laptop (eg from an Evil Maid that gains physical access to my computer)

To address this, I have turned to Heads and PureBoot -- a collection of technologies including an open-source firmware/BIOS, TPM, and a USB security key that can cryptographically verify the integrity of the lowest firmware (and up the chain to the OS).

While Purism has written many articles about PureBoot and has some (minimal) documentation, I found they did a lot of hand waving without explaining how the technology works (what the hell is a "BIOS measurement"?). So I spent a great deal of
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gcc Optimizations for Arithmetic Operations using Bit Shifts

By Michael Altfield, on December 2nd, 2009

I've got a hellacious project due and finals all next week, but this was just too much fun to pass up. In any case, compiler optimization increases compile time, and anything that gives me more time to sword fight on $1000 office chairs is worth a little R&D.

I'm working on writing this cut-down MIPS processor simulator for my Computer Organization class at UCF. I googled "word alignments" to help me better understand the most efficient calculations for converting the Byte Aligned Program Counter address to the Word Aligned Memory array when I ran across an interesting article showing that the mere *order* of variable declarations in a C program can affect the amount of memory used by that program.

The article explained the situation very well, and it makes sense why this issue would happen, but I was surprised that the compiler wouldn't try to optimize situations like this by re-ordering a set of concurrent variable declarations of alternating data types.

In any case, I continued to hack away at my project when I began to think about whether or not gcc translates multiplication and division operations where one of the operands is a power of 2 into simple
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