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by trash_panda 2847 days ago
You don't need to have private keys to exploit this scenario. Let's say you own example.com, and you add a certificate by Let's Encrypt. If Let's Encrypt is a malicious actor, they could MiTM a connection to your site, and present a VALID certificate to the target user, as they hold the private keys used to sign the public certificate.

The value of a CA is that it's a trusted 3rd party that holds a private key used to sign public keys (certificates). Never a CA should get hold of your private keys.
2 comments
schoen 2847 days ago
> If Let's Encrypt is a malicious actor, they could MiTM a connection to your site, and present a VALID certificate to the target user, as they hold the private keys used to sign the public certificate.

I'm not sure if you're referring to CAs' ability to issue fake certificates, or if you're suggesting that the certificate issuer can directly MITM connections.

CAs' ability to issue fake certificates is a very serious concern which has led to the Certificate Transparency system where all issued certificates must be publicly disclosed (in a system outside of the issuer's control) in order to be publicly trusted. A site doesn't have to use a certificate from a particular CA in order to be vulnerable to misissuance by that CA, as in the Iranian Comodo and DigiNotar attacks, where Gmail was briefly vulnerable to MITM attacked involving misissued certificates from these CAs even though it didn't normally use certificates from either of them at all.

CAs don't have the ability to use their signing keys directly to MITM connections involving certificates that they issued, because the signing key isn't used for any cryptographic purpose other than validating that the certificate (which refers to the site's public key) was validly issued.

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trash_panda 2847 days ago
Of course, you're right. My phrasing was not the best. The rogue CA would need to perform a classical MiTM as all the other mortals do, having access to the signing keys does not give you special MiTM powers, other than when you actually are able to conduct a MiTM through other means, you'll have valid certs to intercept the connection.

Totally agree with your point about trust being a very hard problem to solve, that's why CAs first came in to place, and now we have CT (which is not widely adopted yet). It is a problem that has no clear and definite solution yet.

Edit: Also, CT is no magical solution. It's just another "node" in the graph of trust we're establishing. As many other things have in the past, the CT system itself could also fail.

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roblabla 2847 days ago
Chrome requires certs to be published in CT to trust it[0], since chrome 68[1]. Because of this, I would believe CT is widely adopted.

[0]: http://www.certificate-transparency.org/certificate-transpar... , Certificate Inclusion Check

[1]: https://groups.google.com/a/chromium.org/forum/#!msg/ct-poli...

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jancsika 2847 days ago
> The rogue CA would need to perform a classical MiTM as all the other mortals do, having access to the signing keys does not give you special MiTM powers, other than when you actually are able to conduct a MiTM through other means, you'll have valid certs to intercept the connection.

But this thread is operating under the thought experiment that the NSA already owns LetsEncrypt. And in reality-- at least according to the Snowden leaks-- NSA currently has classical MiTM capabilities. (Can't remember which program it was that was using some node between the user and the desired server to send back a forged response that would almost always beat the server to the punch.)

So in this thought experiment there are only two pieces of Triforce and NSA has them both.

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schoen 2847 days ago
> (Can't remember which program it was that was using some node between the user and the desired server to send back a forged response that would almost always beat the server to the punch.)

These were called QUANTUM (with various sub-projects related to specific applications of that capability).

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Ajedi32 2847 days ago
The point is that the NSA doesn't need to own Let's Encrypt to do that; they could use literally _any_ certificate authority.

Also there _is_ a third piece of the triforce; certificate transparency logs; and those would be very difficult to compromise without the certificate transparency monitors noticing.

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kodablah 2847 days ago
> If Let's Encrypt is a malicious actor, they could MiTM a connection to your site, and present a VALID certificate to the target user, as they hold the private keys used to sign the public certificate.

And if they were able to turn off certificate transparency logs or targeted users only without it (or a hacked browser or whatever) to keep site owners from knowing about invalid certs being given.

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