Hey! I'm David, a security engineer at the Blockchain team of Facebook, previously a security consultant for the Cryptography Services of NCC Group. This is my blog about cryptography and security and other related topics that I find interesting.

# Key Compromise Impersonation attacks (KCI) posted September 2016

You might have heard of KCI attacks on TLS: an attacker gets to install a client certificate on your device and can then impersonate websites to you. I had thought the attack was a highly impractical one, but looking at the video of the attack (done against facebook.com at the time) it seemed to contradict my first instinct.

I skimmed the paper and it answered some of my questions and doubts. So here's a tl;dr of it.

The issue is pretty easy to comprehend once you understand how a Diffie-Hellman key exchange works.

Imagine that you navigate to myCompany.com and you do a key exchange with both of your public keys.

• your public key: $g^a \pmod{n}$

• your company's public key: $g^b \pmod{n}$

where $g$ and $n$ are public parameters that you agreed on. Let's ignore $n$ from now on: here, both ends can create the shared secret by doing either $(g^b)^a$ or $(g^a)^b$.

In other words, shared secret = (other dude's public key)(my private key)

If you know either one of them's private key, you can observe the other public key (it's public!) and compute the shared secret. Then you have enough to replace one of the end in the TLS communication.

That's the theory.

If you replace the client's certificate with your own keypair, or know the private key of the certificate, you can break whatever key exchange they try to do with that certificate. What I mean by "break": from a key exchange you can compute the session keys and replace any party during the following communications.

My doubts had originally came from the fact that most implementations rarely use plain Diffie-Hellman, instead they usually offer ephemeral DH or RSA-based key exchanges (which are not vulnerable to this attack). The paper brought me back to reality:

Support for fixed DH client authentication has been very recently added to the OpenSSL 1.0.2 branch.

But then how would you make the client do such a un-used key exchange? The paper washed me from doubts once again:

the attacker, under the hood, interferes with the connection initialization to facebook, and forces the client to use an insecure handshake with client authentication, requesting the previously installed certificate from the system.

Now a couple of questions come to my mind.

How does facebook have these non-ephemeral DH ciphersuites?

→ from the paper, the server doesn't even need to use a static DH ciphersuite. If it has an ECDSA certificate, and didn't specify that it's only to be used to sign then you can use it for the attack (the keyUsage field in the certificate is apparently never used correctly, the few values listed in this RFC tell you how to correctly limit the authorized usages of your public key)

How can you trigger the client to use this ciphersuite?

→ just reply with a serverHello only displaying static ECDH in its ciphersuite list (contrarily to ECDHE, notice the last E for ephemeral). Then show the real ECDSA certificate (the client will interpret that as a ECDH cert because of the fake ciphersuites) and then ask the client for the specific cert you know the private key of.

In addition to the ECDSA → ECDH trumpery, this is all possible because none of these messages are signed at that moment. They are later authenticated via the shared secret, but it is too late =) I don't know if TLS 1.3 is doing a better job in this regard.

It also seems to me that in the attack, instead of installing a client cert you could just install a CA cert and MITM ALL TLS connections (except the pinned ones). But then, this attack is more sneaky, and it's another way of doing exactly this. So I appreciate that.

Well done! You've reached the end of my post. Now you can leave me a comment :)

#### Clemens Hlauschek

Thanks for the summary. I couldn't have explained it more succinctly.
-clemens