My (longer-than-planned) contribution to the conference aimed at introducing trust and security concepts, mainly in showing the prevalent role of hashes, and covered public-key cryptography uses, GPG, SSL CAs, trusting trust and reproducible builds.
CAcert is an SSL Certificate Authority based on the establishment of a web-of-trust à la PGP: rather than charging to issue certificates to anyone, it issues them only to members who have been vouched for by enough other trustworthy members (assurers).
For historical reasons, they were included in the Debian ca-certificates package. It was however recently removed, for justified reasons (CAcert is conducting an audit, and withdrew their demand for inclusion in the Mozilla chain until it’s done). Most other distributions mirror from this package to ship their root certificate, and have also dropped CAcert as a consequence.