[AmericanChopper]

I'm not sure if you intended to or not, but you've perfectly proven my point. DigiNotar and StartComm were both forced out of business because people couldn't trust them.

I can generate a set of magic bits on my own computer without any input from a CA, and I'll be able to use them to encrypt a TLS session. What I can't generate on my computer is some trust, which is what I get when I buy a cert from a CA. Also, as a customer of CAs, I have a selection of different companies to choose from, and I base that selection mostly on trustworthiness. It's not so difficult for me to determine which CAs have had the least number of trust-undermining incidents.

To your point of likelihood around CAs being de-trusted, something that happens even less often (as fas as I'm aware), is relying parties claiming on the relying part warranty, so I don't think appealing to likelihood is an especially valid point here.

[thatcks]

My apologies for not being clear about my DigiNotar and StartComm examples. What I intended to point out here is that in neither case were these failures something which one could have discovered in advance by reasonable amounts of checking how trustworthy the CA was. DigiNotar's compromise was (as far as I know) completely unpredictable and appears to have come from a state-level attack; StartComm actively hid itself and took significant work to uncover. Both CAs would have passed most people's checks done beforehand.

If advance warning is nonexistent, there's no point in even trying to check. If advance warning is hidden, you have a cost/benefit tradeoff to make, and I believe that in most cases (ie with low amounts of money involved), it's not good use of scarce and expensive engineer time to try to assess the state of a CA.

(This is especially the case if your company doesn't already have an engineer who is relatively expert in the CA ecosystem and knows where to even start looking. Without that expertise, would something like StartComm's quiet sale to WoSign have raised any alarms even if you discovered it?)

As far as magic bits go, the one thing that self-generated magic bits can't do is insure that a complete first time visitor can't be MITM'd by an active attacker. Whether this matters (or matters more than the scary browser warnings) depends on circumstances.

[thatcks]

I think I can boil down an example about why I prefer not to talk about trust by itself as it relates to CAs. Imagine a hypothetical version of Let's Encrypt that has all of its operational excellence and security, but that uses root certificates that are not cross-signed and not included in any browsers. I would argue that this CA is exactly as trust-able as LE is (by hypothesis its procedures and technologies are the same, and we trust LE's), but clearly it is not as useful as a CA as LE is because it is not included in the root set of any browser (which we call 'trusted' and which generally implies that the people behind the browser believe that the CA will not issue certificates improperly).

If we say that this CA is not 'trustworthy' here, what we really mean is 'this CA is not in browser root sets and so the TLS certificates it issues provoke browser warnings'. This is useful in one sense (it is what most people care about), but I prefer to be explicit about what we mean (partly because 'trust' is a loaded term with tangled implications).