Everyone by now heard of security breach of DigiNotar. The Internet is full of stories about it! I won't go into details what happened. Instead, I'll try to pinpoint what actual problem is, and, based on that, I'll try to outline possible solution.
Let us start with the problem. The problem is that every single CA is actually single point of failure of the whole distributed system. Do you need fraudulent Google certificate? No problem, attack the weakest CA you can find, or try to attack more of them, and there you go. Now, I can here you say: Remove weakest CA! Well, it's not so easy. Applying this rule recursively you'll end up with one, or no CAs at all. This is not a solution either. And this also adds another dimension to the problem, the less CAs the more fragile the Internet becomes because each CA is anyway highly likely target. And you know the main premise of security: You are never ever absolutely secure!
So, what is the solution? I believe that the solution is to keep the system as it is, but to introduce signatures from multiple CAs in a single certificate. This won't resolve the problem, but it will make life harder to hackers. Besides, absolute security doesn't exist, as I already mentioned.
From the implementation standpoint, it is possible to do this either by changing certificate structure, or to change implementations so that they can check multiple certificates. In case multiple certificates are used it's obviously necessary to have some common information that will allow all those certificates to be related.
Validity of certificate (or certificates) could be calculated probabilisticaly. Additionally, some independent measure of correlation between CAs could be defined so that the validity of a single site that uses this system can be evaluated based on this correlation measure (meaning, the less correlated CAs signed the more valid it is).
Note that if a single CA goes into bankruptcy, or is removed from trusted CA list, doesn't mean that everyone has to issue a new certificate imediatelly.
I would say that CAs implemented this way would be somewhere between current CA system and PGP Web of Trust.
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