[torbid]

I understand that. But..

If 12 CAs send to the same log and all have to save up to their latest entry not to be declared incompetent to be CAs, how would all 12 possibly do a worse job of providing that log on demand than a random 3rd party who has no particular investment at risk?

(Every other CA in a log is a 3rd party with respect to any other, but they are one who can actually be told to keep something indefinitely because they would also need to return it for legitimizing their own issuance.)

[michaelt]

As far as I know, CAs don't have to "save up to their latest entry"

The info they get back from the CT log may be a Merkle Hash that partly depends on the other entries in the log - but they don't have to store the entire log, just a short checksum.

[torbid]

Right and this is what I am saying is backwards with the protocol. It is not in anyone's best interest that some random 3rd party takes responsibility to preserve data for CAs indefinitely to prove things. The CA should identify where it has its copy in the extension and looking at one CAs copy one would find every other CAs copy of the same CT log.