Recently we got several questions about automated installers for our certificates. While the new ca-cacert package in Debian Testing is a nice way for a verified installation it isn’t perfect. One issue is the initial download of the certificates when the source package is built by the maintainer; the second issue is that not everybody is using Debian.
As for a long time there was no way to automate the check of the trust anchor with tools you already have we used cryptography to make it work: DNSSEC. While you can’t directly download the certificates directly from DNS – the information would be to huge and hardly manageable – you still get enough information to bootstrap the verification from DNS. All you need is a way to query and validate TXT RRs from DNS, a way to download files via HTTP and a way to calculate some hashes.
The information about the fingerprints is stored in the DNS zone _fp.cacert.org – the underscore indicates non-host information. For each generation of root certificates a new sub-directory will be created. The current one is “g1”. To list all available certificates of a specific generation you can query the label _certs for that sub-directory given a DNS query for _certs.g1._fp.cacert.org yielding the two names “root class3” as the certificates. Each of those references in turn provides both an URL (“_url”) and a set of fingerprints (_md5, _sha1, _sha256) needed for the verified download of that certificate. To download the current (g1) root certificate you’d thus look for the download URL at _url.root.g1._fp.cacert.org and verify the SHA2-256 fingerprint given at _sha256.root.g1._fp.cacert.org. Fingerprints are always uppercase and without any delimiters.
For further technical details have a look into the Wiki