TLS clients need a set of root CA certificates to establish the chain of trust when connecting to a service on the Internet. On Chromium OS systems, three sets of certificates exist for different purposes:
This document explains in detail the goal of each store, and how various clients use them.
Google distributes a Chrome OS root store for Chrome OS systems. It is maintained by Google explicitly for the purpose of making safe and secure TLS connections. Clients like Chrome and programs that link libchrome source this store for roots.
The certificates in this store are installed as a shared library (libnss) by the package chromiumos-overlay/dev-libs/nss. It is a fork of a Gentoo upstream package with the same name, with added/removed certificates based on Google's policy.
For Chromium OS systems not distributed by Google, the Mozilla NSS store is provided. It includes a broad selection of CAs, across a variety of use cases (such as trusting both TLS and S/MIME CAs). Users of this store should be aware that because it does not express trusted purposes, it may contain CAs that are not suitable for their use case. Applications and distributors are encouraged to manage and maintain their own root store, and to ensure its fitness for purpose and policy.
The certificates in this store are installed by the portage-stable/app-misc/ca-certificates package at /etc/ssl/certs, both individually and as a concatenated PEM file at /etc/ssl/certs/ca-certificates.crt. This is done so because some clients like OpenSSL expect individual certificates (with hashed names, man X509_LOOKUP_HASH_DIR), while others (Golang) look for a single file at /etc/ssl/certs/ca-certificates.crt.
The portage-stable/app-misc/ca-certificates package unpacks the certificate list directly from NSS release tarballs, but uses the update-ca-certificate tool from the Debian ca-certificates package to install them to the locations mentioned above. Hence the package is versioned with both NSS and ca-certificate. For example, app-misc/ca-certificates-20170717.3.36.1 uses the scripts from the 20170717 release of the ca-certificate Debian package and installs certificates from the 3.36.1 release of the Mozilla NSS module.
While the NSS root store can serve a variety of applications, and the Chrome OS root store is suitable for Chrome on Chrome OS, clients that will/should only ever connect to Google services use a more restrictive set. This set, synchronized with pki.goog/roots.pem, contains those CAs that Google does or may use for various services, and is a significantly smaller set than a more general root store intended for interacting with the Web at large. You can find more details at this FAQ page.
The certificates in this store are installed by the chromiumos-overlay/chromeos-base/chromeos-ca-certificates package at /usr/share/chromeos-ca-certificates. Clients that should only connect to Google, e.g., the update engine and crash sender, are explicitly configured to source these certificates as roots.
While both stores trace their roots to the Mozilla NSS module (through dev-libs/nss and app-misc/ca-certificates respectively) and they are likely to have a large overlap, Google may and have added/removed certificates to ensure that only CAs trusted for TLS (via policy) are trusted for TLS (via code). The biggest difference of the two is that the NSS store includes CAs trusted for S/MIME CAs while the Chrome OS store doesn't.
To ensure that any changes to both stores are reviewed, and any differences expected, the security_RootCA test checks to ensure there are no accidental additions or removals, and that all differences are intentional.
This was an attempt to replace portage-stable/app-misc/ca-certificates with a Chromium OS fork, chromeos-base/root-certificates. It existed from 2013 to May 2018 and was predicated on the risk due to the lack of Gentoo policies - that they would pick any CA. That has changed now and as explained above app-misc/ca-certificates now takes the certificates directly from NSS. Therefore chromeos-base/root-certificates was deleted.
