Webauthn in Linux with a TPM via the HID gadget



Account security on the modern web is a bit of a nightmare. Everyone understands the need for strong passwords which are different for each account, but managing them is problematic because the human mind just can’t remember hundreds of complete gibberish words so everyone uses a password manager (which, lets admit it, for a lot of people is to write it down). A solution to this problem has long been something called two factor authentication (2FA) which authenticates you by something you know (like a password) and something you posses (like a TPM or a USB token). The problem has always been that you ideally need a different 2FA for each website, so that a compromise of one website doesn’t lead to the compromise of all your accounts.
Enter webauthn. This is designed as a 2FA protocol that uses public key cryptography instead of shared secrets and also uses a different public/private key pair for each website. Thus aspiring to be a passwordless secure scalable 2FA system for the web. However, the webauthn standard only specifies how the protocol works when the browser communicates with the remote website, there’s a different standard called FIDO or U2F that specifies how the browser communicates with the second factor (called an authenticator in FIDO speak) and how that second factor works.
It turns out that the FIDO standards do specify a TPM as one possible backend, so what, you might ask does this have to do with the Linux Gadget subsystem? The answer, it turns out, is that although the standards do recommend a TPM as the second factor, they don’t specify how to connect to one. The only connection protocols in the Client To Authenticator Protocol (CTAP) specifications are USB, BLE and NFC. And, in fact, the only one that’s really widely implemented in browsers is USB, so if you want to connect your laptop’s TPM to a browser it’s going to have to go over USB meaning you need a Linux USB gadget. Conspiracy theorists will obviously notice that if the main current connector is USB and FIDO requires new USB tokens because it’s a new standard then webauthn is a boon to token manufacturers.
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