Trust assertions are bits of trust information used by applications to make trust decisions about certificates. For example, trust assertions can represent certificate authority anchors, pinned certificate exceptions, or revocation lists. Trust assertions do not represent the trust decision itself, but they're used in a trust decision.
By using trust assertions applications (and libraries) can make consistent trust decisions and not confuse the poor user with different security in each app when making TLS connections.
For example all the applications on the user's desktop would use the same set of certificate authorities when making TLS connections. And the user can then easily manage that set of certificates. It's also easy to store per-host pinned certificate exceptions for self-signed certificates, and have all applications use them consistently.
I've put together a spec for storing and looking up trust assertions via PKCS#11 which allows a loose coupling between applications and the storage of these trust assertions. I've also implemented support for storing trust assertions in Gnome Keyring, and client side support in libgcr.
To make it all very easy to use, I've added a GcrCertificateChain class which builds up a certificate chain, based on trust assertions and gets it ready for verification by your favorite crypto library.
All this goodness is available in the trust-store branch of gnome-keyring, and it looks like empathy will be the first app to make use of it. I'm gonna try and see how we can fit this into the nice new GTlsConnection support in glib.
I'm looking forward to the security devroom at FOSDEM and hope to talk about some of this stuff.