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.

How does Webauthn Work?

The protocol comes in two flavours, version 1 and version 2. Version 1 is fixed cryptography and version 2 is agile cryptography. However, version1 is simpler so that’s the one I’ll explain.

Webauthn essentially consists of two phases: a registration phase where the authenticator is tied to the account, which often happens when the remote account is created, and authentication where the authenticator is used to log in again to the website after registration. Obviously accounts often outlive the second factor, especially if it’s tied to a machine like the TPM, so the standard contemplates a single account having multiple registered authenticators.

The registration request consists of a random challenge supplied by the remote website to prevent replay and an application id which is constructed by the browser from the website supplied ID and the web origin of the site. The design is that the application ID should be unique for each remote account and not subject to being faked by the remote site to trick you into giving up some other application’s credentials.

The authenticator’s response consists of a unique public key, an opaque key handle, an attestation X.509 certificate containing a public key and a signature over the challenge, the application ID, the public key and the key handle using the private key of the certificate. The remote website can verify this signature against the certificate to verify registration. Additionally, Google recommends that the website also verifies the attestation certificate against a list of know device master certificates to prove it is talking to a genuine U2F authenticator. Since no-one is currently maintaining a database of “genuine” second factor master certificates, this last step mostly isn’t done today.

In version 1, the only key scheme allowed is Elliptic Curve over the NIST P-256 curve. This means that the public key is always 65 bytes long and an encrypted (or wrapped) form of the private key can be stashed inside the opaque key handle, which may be a maximum of 255 bytes. Since the key handle must be presented for each authentication attempt, it relieves the second factor from having to remember an ever increasing list of public/private key pairs because all it needs to do is unwrap the private key from the opaque handle and perform the signature and then forget the unwrapped private key. Note that this means per user account authenticator, the remote website must store the public key and the key handle, meaning about 300 bytes extra, but that’s peanuts compared to the amount of information remote websites usually store per registered account.

To perform an authentication the remote website presents a unique challenge, the raw ID from which the browser should construct the same application ID and the key handle. Ideally the authenticator should verify that the application ID matches the one used for registration (so it should be part of the wrapped key handle) and then perform a signature over the application ID, the challenge and a unique monotonically increasing counter number which is sent back in the response. To validly authenticate, the remote website verifies the signature is genuine and that the count has increased from the last time authentication has done (so it has to store the per authenticator 4 byte count as well). Any increase is fine, so each second factor only needs to maintain a single monotonically increasing counter to use for every registered site.

Problems with Webauthn and the TPM

The primary problem is the attestation certificate, which is actually an issue for the whole protocol. TPMs are actually designed to do attestation correctly, which means providing proof of being a genuine TPM without compromising the user’s privacy. The way they do this is via a somewhat complex attestation protocol involving a privacy CA. The problem they’re seeking to avoid is that if you present the same certificate every time you use the device for registration you can be tracked via that certificate and your privacy is compromised. The way the TPM gets around this is that you can use a privacy CA to produce an arbitrary number of different certificates for the same TPM and you could present a new one every time, thus leaving nothing to be tracked by.

The ability to track users by certificate has been a criticism levelled at FIDO and the best the alliance can come up with is the idea that perhaps you batch the attestation certificates, so the same certificate is used in hundreds of new keys.

The problem for TPMs though is that until FIDO devices use proper privacy CA based attestation, the best you can do is generate a separate self signed attestation certificate. The reason is that the TPM does contain its own certificate, but it’s encryption only, not signing because of the way the TPM privacy CA based attestation works. Thus, even if you were willing to give up your privacy you can’t use the TPM EK certificate as the FIDO attestation certificate. Plus, if Google carries out its threat to verify attestation certificates, this scheme is no longer going to work.

Aside about Browsers and CTAP

The crypto aware among you will recognise that there is already a library based standard that can be used to talk to a variety of USB tokens and even the TPM called PKCS#11. Mozilla Firefox, for instance, already supports using this as I demonstrated in a previous blog post. One might think, based on what I said about the one token per key problem in the introduction, that PKCS#11 can’t support the new key wrapping based aspect of FIDO but, in fact, it can via the C_WrapKey/C_UnwrapKey API. The only thing PKCS#11 can’t do is the new monotonic counter.

Even if PKCS#11 can’t perform all the necessary functions, what about a new or extended library based protocol? This is a good question to which I’ve been unable to get a satisfactory answer. Certainly doing CTAP correctly requires that your browser be able to speak directly to the USB, Bluetooth and NFC subsystems. Perhaps not too hard for a single platform browser like Internet Explorer, but fraught with platform complexity for generic browsers like FireFox where the only solution is to have a rust based accessor for every supported platform.

Certainly the lack of a library interface are where the TPM issues come from, because without that we have to plug the TPM based FIDO layer into a browser over an existing CTAP protocol it supports, i.e. USB. Fortunately Linux has the USB Gadget subsystem which fits the bill precisely.

Building Synthetic HID Devices with USB Gadget

Before you try this at home, I should point out that the Linux HID Gadget has a longstanding bug that will cause your machine to hang unless you have this patch applied. You have been warned!

The HID subsystem is for driving Human Interaction Devices meaning keyboard and mice. However, it has a simple packet (called report in USB speak) based protocol which is easy for most things to use. In order to facilitate this, Linux actually provides hidraw devices which allow you to send and receive these reports using read and write system calls (which, in fact, is how Firefox on Linux speaks CTAP). What the hid gadget does when set up is provide all the static emulation of HID device protocols (like discovery pages) while allowing you to send and receive the hidraw packets over the /dev/hidgX device tap, also via read and write (essentially operating like a tty/pty pair¹). To get the whole thing running, the final piece of the puzzle is that the browser (most likely running as you) needs to be able to speak to the hidraw device, so you need a udev rule to make it accessible because by default they’re 0600. Since the same goes for every other USB security token, you’ll find the template in the same rpm that installs the PKCS#11 library for the token.

The way CTAP works is that every transaction is split into 64 byte reports and sent over the hidraw interface. All you need to do to get this setup is initialise a report descriptor for this type of device. Since it’s somewhat cumbersome to do, I’ve created this script to do it (run it as root). Once you have this, the hidraw and hidg devices will appear (make them both user accessible with chmod 666) and then all you need is a programme to drive the hidg device and you’re done.

A TPM Based Hid Gadget Driver

Note: this section is written describing TPM 2.0.

The first thing we need out of the TPM is a monotonic counter, but all TPMs have NV counter indexes which can be created (all TPM counters are 8 byte, whereas the CTAP protocol requires 4 bytes, but we simply chop off the top 4 bytes). By convention I create the counter at NV index 01000101. Once created, this counter will be persistent and monotonic for the lifetime of the TPM.

The next thing you need is an attestation certificate and key. These must be NIST P-256 based, but it’s easy to get openssl to create them

openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:prime256v1 -pkeyopt ec_param_enc:named_curve -out reg_key.key

openssl req -new -x509 -subj '/CN=My Fido Token/' -key reg_key.key -out reg_key.der -outform DER

This creates a self signed certificate, but you could also create a certificate chain this way.

Finally, we need the TPM to generate one NIST P-256 key pair per registration. Here we use the TPM2_Create() call which gets the TPM to create a random asymmetric key pair and return the public and wrapped private pieces. We can simply bundle these up and return them as the key handle (fortunately, what the TPM spits back for a NIST P-256 key is about 190 bytes when properly marshalled). When the remote end requests an authentication, we extract the TPM key from the key handle and use a TPM2_Load to place it in the TPM and sign the hash and then unload it from the TPM. Putting this all together this project (which is highly experimental) provides the script to create the devices and a hidg driver that interfaces to the TPM. All you need to do is run it as

hidgd /dev/hidg0 reg_key.der reg_key.key

And you’re good to go. If you want to test it there are plenty of public domain webauthn test sites, webauthn.org and webauthn.io² are two I’ve tested as working.

TODO Items

The webauthn standard specifies the USB authenticator should ask for permission before performing either registration or authentication. Currently the TPM hid gadget doesn’t have any external verification, but in future I’ll add a configurable pinentry to add confirmation and possibly also a single password for verification.

The current code also does nothing to verify the application ID on a per authorization basis. This is a security problem because you are currently vulnerable to being spoofed by malicious websites who could hand you a snooped key handle and then use the signature to fake your login to a different site. To avoid this, I’m planning to use the policy area of the TPM key to hold the application ID. This should work because the generated keys have no authorization, either policy or password, so the policy area is effectively redundant. It is in the unwrapped public key, but if any part of the public key is tampered with the TPM will detect this via a hash in the wrapped private error and give a binding error on load.

The current code really only does version 1 of the FIDO protocol. Ideally it needs upgrading to version 2. However, there’s not really much point because for all the crypto agility, most TPMs on the market today can only do NIST P-256 curves, so you wouldn’t gain that much.

Conclusions

Using this scheme you’re ready to play with FIDO/U2F as long as you have a laptop with a functional TPM 2.0 and a working USB gadget subsystem. If you want to play, please remember to make sure you have the gadget patch applied.

1. In fact, the only reason you can’t use a real tty/pty pair is because the Firefox rust engine will try to verify that the hidraw device is backed by a FIDO descriptor, which it does using special ioctls
2. webauthn.io does both version 1 and version 2 of the protocol, so make sure you’re using version 1 by setting the Attestation to None and the Authenticator Type to Unspecified