* Latency for the CA waiting for a notary server to respond
* Business models for notary servers
As a rule open source software works really well as the marginal cost of production is zero. Open source services tend to sux because even though the marginal cost of a service is negligible, large numbers times negligible adds up to big numbers. Running a DNS server for a university department costs very little, running it for the whole university starts to cost real money and running a registry like .com with 99.9999% reliability ends up with $100 million hardware costs.
So the idea that I plug my business into a network of notary servers being run by amateurs or as a community service is a non-starter for me. We have to align the responsibility for running any server that the CA has a critical dependency on with a business model.
Looking at the CT proposal, it seems to me that we could fix the business model issue and remove a lot of the CA operational issues as follows:
1) Each browser provider that is interested in enforcing a CT requirement stands up a meta-notary server.
2) Each CA runs their own notary server and this is the only resource that needs to have a check in at certificate issue.
3) Each CA notary server checkpoints to one or more meta-notary servers every 60 minutes. As part of the check in process it uploads the whole information for all the certificates issued in that time interval.
4) Meta-Notaries deliver tokens that assert that the CA notaries are current every 60 minutes. Note here that 'current' is according to the criteria set by the meta notary. This is an intentional piece of 'slop' in the system.
5) The OCSP tokens delivered by the CA contain the information necessary to checkpoint the certificate to the Meta-Notaries.
6) A browser enforcing CT disclosure pulls a list of anchor points from its chosen meta-notary every 60 minutes and uses them to validate the CT assertions delivered in certs.
The 'slop' introduced at the meta-notary can of course be removed if we want to ensure that the system is robust even if there is a collusion between the CA and the meta-notary. But since the whole point of the scheme is transparency, the meta-notary operation can be audited by third parties in any event.