It's already build in e.g. Mail.app.

We need no software. We need public awareness, tutorials and probably some easy to use CA.

I bet the adoption rate of S/MIME is way beyond gpg if you check corporations and large enterprises.

for example: Germany

https://gist.github.com/rmoriz/5945400

I've thought some more about S/MIME, and right now my biggest concern is the CAs. I don't like having that central point of trust/failure.

Do you know if S/MIME can work on a distributed model?

Also, what are the advantages of S/MIME over PGP? I hear what you're saying about enterprise adoption, but I'm more concerned with the thoroughness of peer review than usage rates.

CAs are not a single point of failure because your private key should never leave your computer.

S/MIME is based on X.509, same thing that powers TLS/SSL.

CAs are a single point of failure if a government can get a key that your counterpart's UI will claim belongs to you.

then don't trust government controlled CA's, just as you wouldn't trust government controlled keyservers

The argument in favor of S/MIME is that it's already there and just works.

Not necessarily true. There are certainly CAs that are able to keep their private keys out of the hands of most governments. But there is definite uncertainty about who to trust. For the truly cautious, wouldn't it make sense to explore setting up your own CA? Something like OpenCA or TinyCA should do the trick.

You're missing the point. If the UI says "yes, that's 23david" if I can get any CA to certify that, the security of the system is no better than that of the weakest CA. Sure, your CA may be perfect, but why would the attacker go for the strongest point?

So perhaps that's an issue with the UI not clearly showing which CA is verifying the identity, and alerting you clearly if an encrypted email is using a different CA than prior ones.

Depending on the client you're using, it shouldnt be too hard to prune the trusted CA list to only include providers you choose to trust. If you want, only include your CA and remove all others.

So instead of PGP - which is already quite daunting, mind - the user now get to assess the security of 200+ CAs, most of which they've never heard of?

You're right, this wouldn't make a lot of sense for most users.

But this would be useful in a corporation where it's possible to centrally manage CA lists for approved applications.

Congrats, you've just reinvented PGP's web-of-trust.

Probably makes sense to start deciding which CAs we should or shouldn't trust? Has anyone reliable done any work on rating or evaluating the trustworthiness/security of different CAs?