OpenTitan Secure Boot process consists of several stages. On power-on reset, or wake-up from a deep sleep the execution enters the Mask ROM, which is “baked” into the OpenTitan metal masks. This means that the Mask ROM, once manufactured, cannot be updated. To add a degree of flexibility, in particular to allow for a manufacturer specific configuration and facilities to deliver security updates - OpenTitan design provisions the Extension ROM (ROM_EXT) that lives in the flash memory.

ROM_EXT consists of a manifest and the image itself. When the image is generated - the manifest data is “blank”. The responsibility of ROM_EXT signer is to update the manifest, sign the image and add the signature to it.


ROM_EXT signer is a host side tool that is written in Rust.

Why Rust?

Rust has been chosen for the reach language and standard library features, as well as incredible and versatile infrastructure. Rust’s Cargo (build/packaging system) and mature community driven database (, makes it easy and cheap (in terms of effort) to find, integrate and use relevant functionality.

** This makes rust great for host side tooling **


This tool consist of four major parts:

  • Configuration file and parsing/deserealisation

  • Image manipulation (updating the manifest)

  • Image signing

  • “Receipt” production

Configuration file and parsing/deserialisation

The configuration file is in .hjson format, which is deserialised (turned into Rust structures) using hjson_serde and serde crates.

Configuration file contains data for all of the fields that need updating in the manifest before signing.

Please note that signature key modulus and signature key public exponent are extracted separately. The configuration file has a link to the private key, and public key is extracted from it via ring library.

Generic (u32 or u64) integers are defines as an array. This is done to make updating the manifest easier. Complex fields such as “Peripheral Lockdown Info” have a separate data structure, and are spliced into the image separately.

Image manipulation (updating the manifest)

Reads the binary image of the disk. It uses the parsed configuration to update the manifest before signing, and is responsible for generating the signed file.

Image signing


“Receipt” production