The premise of end-to-end encryption (E2EE) is that the client is secure and trustworthy, your end devices is secure and trustworthy, but the network and server need not be trusted. You’ve read all the code in the client, or someone you trust has done so for you. But after loading the code onto your phone — installing the Keybase app — and starting a chat with your friend, you still need to verify that the server sent you the right encryption key. Since you can’t host your own server, it has to be the Keybase, Inc’s server that sends you the encryption key of your friend.
It’s pretty standard for E2EE software that the (client) code is open, the server sends you the encryption key, and you can check the encryption keys out of band. In Signal you check the safety number, in Wire you check the device fingerprints, and in Telegram you check the encryption key. If they match, you know (based on the open client code and the cryptography it uses) that there is no (wo)man in the middle.
How does this work with Keybase? Their documentation explains parts of it:
Alice and Bob share a symmetric encryption key, which they pass through the server by encrypting it to each of their devices’ public encryption keys.
Okay, so either we verify the symmetric key (identical on both phones), or we verify the public key (I should be able to display mine on my phone, and my friend will call up what their phone thinks my public key is, and that should match).
The weird thing is that there is no button to do either of this. The documentation doesn’t mention any and I can’t find it when looking around.
Further on, it explains the public keys are put into your signature chain:
All Keybase devices publish a
crypto_box public key and a
crypto_sign public key when they’re first provisioned. Those keys live in the user’s signature chain […] A chat symmetric encryption key is 32 random bytes. It’s shared by everyone in the chat, […] When a new device needs this shared key, another device that has it will encrypt it with the new device’s public
crypto_box key and then upload it to the Keybase server.
So if I’m reading this right, when my friend opens a new chat with me, their client generates the shared secret, it will take the public key from the
crypto_box inside my signature chain, and upload that to Keybase’s server so that I can download and decrypt it, and thereby establish the shared secret and start the chat.
… but where does that signature chain come from? This has to be fetched from Keybase servers, so since there is no way to display it, I have to trust the server to send me the right key. How does that make sense when they claim “all messages are secure, end-to-end encrypted”?
There is some documentation that goes through some verification step by step, but aside from that it’s broken (there is no “sigs” field in “root_desc”, the result of
root.json?hash=X), this is not something I can do on my phone. A malicious server could return the correct answers to my command line client while telling the mobile app that the signature chain contains an entirely different key (namely the one the malicious actor uses to perform the (W)MitM).
Somehow, when this comes up in conversation, this blockchain thing is given as the reason why it’s secure without needing to verify anything yourself, but I can’t figure out how it works.
- Do Keybase chats require manual verification of any kind of key, or does the blockchain magic (which the app could verify under the hood) somehow prevent us from having to compare fingerprints/safety-numbers/keys/sigchains?
- If we do need to compare fingerprints/safety-numbers/keys/sigchains, how can that be done in the Keybase app in order to verify that no MitM is going on?