When you install a properly signed software update, maybe the update you got is different from what everyone else get. Maybe the attacker was able to sign a malicious update due to key compromise, or coercion of the legitimate key holder. How would you, or anyone else, notice?

One way to enable detection (but not prevention) of this kind of attack is transparency. When installing updates, verify the signature as usual. In addition, require that the signature is visible in a public append-only transparency log, where entries can be added but never removed.

Sigsum is a minimalistic transparency log that can accept signed checksum submissions for a wide variety of applications and entities that are neither known, nor trusted, by the log operator. The log itself does not become a trusted third party for applications, instead, applications depend on m-of-n trusted witnesses attesting that the log behaves correctly. One of the many use-cases of Sigsum logging is transparency for signed software packages and updates.

This talk explains the "detection, not prevention" benefits one can get from Sigsum, and describes the different roles that make up the Sigsum system.