Windows Hello in Webauthn-rs

Recently I’ve been working again on webauthn-rs, as a member of the community wants to start using it in production for a service. So far the development of the library has been limited to the test devices that I own, but now this pushes me toward implementing true fido compliance.

A really major part of this though was that a lot of their consumers use windows, which means support windows hello.

A background on webauthn

Webauthn itself is not a specification for the cryptographic operations required for authentication using an authenticator device, but a specification that wraps other techniques to allow a variety of authenticators to be used exposing their “native” features.

The authentication side of webauthn is reasonably simple in this way. The server stores a public key credential associated to a user. During authentication the server provides a challenge which the authenticator signs using it’s private key. The server can then verify using it’s copy of the challenge, and the public key that the authentication must have come from that credentials. Of course like anything there is a little bit of magic in here around how the authenticators store credentials that allows other properties to be asserted, but that’s beyond the scope of this post.

The majority of the webauthn specification is around the process of registering credentials and requesting specific properties to exist in the credentials. Some of these properties are optional hints (resident keys, authenticator attachment) and some of these properties are enforced (user verification so that the credential is a true MFA). Beyond these there is also a process for the authenticator to provide information about it’s source and trust. This process is attestation and has multiple different formats and details associated.

It’s interesting to note that for most deployments of webauthn, attestation is not required by the attestation conveyance preference, and generally provides little value to these deployments. For many sites you only need to know that a webauthn authenticator is in use. However attestation allows environments with strict security requirements to verify and attest the legitimacy of, and make and model of authenticators in use. (An interesting part of webauthn is how much of it seems to be Google and Microsoft internal requirements leaking into a specification, just saying).

This leads to what is effectively, most of the code in webauthn-rs - attestation.rs.

Windows Hello

Windows Hello is Microsoft’s equivalent to TouchID on iOS. Using a Trusted Platform Module (TPM) as a tamper-resistant secure element, it allows devices such as a Windows Surface to perform cryptographic operations. As Microsoft is attempting to move to a passwordless future (which honestly, I’m on board for and want to support in Kanidm), this means they want to support Webauthn on as many of their devices as possible. Microsoft even defines in their hardware requirements for Windows 10 Home, Pro, Education and Enterprise that as of July 28, 2016, all new device models, lines or series … a component which implements the TPM 2.0 must be present and enabled by default from this effective date.. This is pretty major as this means that slowly MS have been ensuring that all consumer and enterprise devices are steadily moving to a point where passwordless is a viable future. Microsoft state that they use TPMv2 for many reasons, but a defining one is: The TPM 1.2 spec only allows for the use of RSA and the SHA-1 hashing algorithm which is now considered broken.

Of course, if you have noticed this means that TPM’s are involved. Webauthn supports a TPM attestation path, and that means I have to implement it.

Once more into the abyss

Reading the Webauthn spec for TPM attestation it pointed me to the TPMv2.0 specification part1, part2 and part3. I will spare you from this as there is a sum total of 861 pages between these documents, and the Webauthn spec while it only references a few parts, manages to then create a set of expanding references within these documents. To make it even more enjoyable, text search is mostly broken in these documents, meaning that trying to determine the field contents and types involves a lot of manual-eyeball work.

TPM’s structures are packed C structs which means that they can be very tricky to parse. They use u16 identifiers to switch on unions, and other fun tricks that we love to see from C programs. These u16’s often have some defined constants which are valid choices, such as TPM_ALG_ID, which allows switching on which cryptographic algorithms are in use. Some stand out parts of this section were as follows.

Unabashed optimism:

TPM_ALG_ERROR 0x0000 // Should not occur

Broken Crypto

TPM_ALG_SHA1 0x0004 // The SHA1 Algorithm

Being the boomer equivalent of JWT

TPM_ALG_NULL 0x0010 // The NULL Algorithm

And supporting the latest in modern cipher suites

TPM_ALG_XOR 0x000A // TCG TPM 2.0 library specification - the XOR encryption algorithm.

ThE XOR eNcRyPtIoN aLgoRitHm.

Some of the structures are even quite fun to implement, such as TPMT_SIGNATURE, where a matrix of how to switch on it is present where the first two bytes when interpreted as a u16, define a TPM_ALG_ID where, if it the two bytes are not in a set of the TPM_ALG_ID then the whole blob including leading two bytes is actually just a blob of hash. It would certainly be unfortunate if in the interest of saving two bytes that my hash accidentally emited data where the first two bytes were accidentally a TPM_ALG_ID causing a parser to overflow.

I think the cherry on all of this though, is that despite Microsoft requiring TPMv2.0 to move away from RSA and SHA-1, that when I checked the attestation signatures for a Windows Hello device I had to implement the following:

COSEContentType::INSECURE_RS1 => {
    hash::hash(hash::MessageDigest::sha1(), input)
        .map(|dbytes| Vec::from(dbytes.as_ref()))
        .map_err(|e| WebauthnError::OpenSSLError(e))
}

Conclusion

Saying this, I’m happy that Windows Hello is now in Webauthn-rs. The actual Webauthn authentication flows DO use secure algorithms (RSA2048 + SHA256 and better), it is only in the attestation path that some components are signed by SHA1. So please use webauthn-rs, and do use Windows Hello with it!