Ultimately what he is suggesting to do is to bind a string of entropy to a website through facilitation of the browser and a Passkey.
A cryptographic seed is one of the most sensitive things. And here you choose to expose it to a website (even though it was specifically generated for that website). This is not something you do for authentication. The only reason to do this is to have javascript/wasm on a website perform sensitive cryptographic operations for you. You should never be doing this.
You can run the PassSeed code/mechanism on your own domain or localhost to ensure it's not subject to malicious host exfiltratuon. I agree that one should only trust a foreign host with low-security uses under this scheme.
Passkeys can be hijacked to serve as cryptographic seed material that is securely synced across all of a user’s devices, enabling the generation of a wide range of cryptographic keys. This allows Passkeys to power use cases far beyond what they have traditionally been constrained to. I’ve been calling this mechanism PassSeeds.
I’ll leave the details to the blog post, but here’s a short list of what PassSeeds enable:
- Need a user-custodied BLS12-381 key to engage in more advanced ZKP Verifiable Credential / proofing flows? Say less, you're covered.
- Want to create a petty cash Web wallet for Bitcoin transactions that relies on a secp256k1 key? Ask and ye shall receive.
- How about keys for decentralized social media identifiers and post signing that are of a type other than P-256? No problem, I got you!
I don’t understand why you want to enforce only using the public key instead of private key - while I believe you that as of now browsers do not disclose the public key anywhere, I’d also suspect that this is far more likely to be violated and accidentally disclosed by a bug than the private key, which theoretically cannot ever leave the TPM.
Would KDF(deterministic_sign(“well-known message”)) not also provide valid entropy?
Is it just impossible to force a nonce for a deterministic signature?
+1. I bet it's because of this confusing verbiage, the AI also got the gist of the article wrong, and lead me to believe that this article shows "post-hoc exploit" , when in fact there's no mention of the word 'exploit' in the article. See the screenshot linked below [1].
On a tangent, in the process I learnt that Firefox (at least on desktop) now has an "AI preview" feature where if you long-press on a URL, it brings up the pop-up. The first time, it notifies that the "AI" processing is local-only to preserve privacy.
To me, “hijacking” a passkey sounds like credential disclosure, which is quite worrying for a core team member to talk about. I know what you mean, but it’s probably the wrong term to be using if we want to emphasize that passkeys cannot be stolen.
What stops anyone else doing the ECDSA public key recovery hack by signing two identical messages and getting the public key, i.e. the thing you are using as a cryptographic seed?
In general, using a key for a purpose it was not designed for gets you into trouble. Treating a public key as private key seed material is almost certainly going to be a problem. Systems are just not designed to keep public keys secret, even if webauth does.
That would either mean you have arbitrary, malicious code executing in the bound origin (the origin was hacked and shipped malicious code), or you allowed random callers externally to take signatures out of the boundary - don't do either of these things, they are verboten. The whole point is that for the passkey you use as a PassSeed, you never do any signing other than locally for ECDSA recovery.
It seems malicious code on the phone can get the public key and thus derive the secret keys. This is weaker protection than PassKeys provide (would have to crack the hardware, not just software).
The interesting thing about Passkeys is that they are only ever output in the client create() call, and the platform does not retain them for disclosure after that, so if you don't send them out of the origin boundary, they are treated like a virtually secret value by the platform. It's ironic, because the WebAuthn/Passkey authors (who I know some of) explicitly treat the public key as a sensitive value, and built system assumptions around that, which is what makes this possible. It's a rather gross hack, can't deny it that, but there are a group of use cases for which it is a better fit than any of the far more ugly flows many non-P-256 self-custodied key use cases are accomplished with today.
> they are treated like a virtually secret value by the platform
"virtually" is the problem
for webauthn the public key isn't revealed to everyone for privacy reasons, not cryptographic reasons
the webauthn API is also only part of the cryptosystem
the platform authenticator (yubikey, windows hello, password manager, whatever) may have an API to list stored public keys without any authentication at all
It's a deliberate architectural decision that passkey authenticators not allow any retrieval or enumeration of key pairs - they don't even have internal APIs for it. This holds true for all known implementations, as it is a core principle of the system design.
The underlying CTAP implementations are only used by the platform to facilitate core activities, they are not used to expose key pairs to external parties. Please link to where any API offers up public keys to external userland actors, and any use of said APIs beyond core credential management. If this is assumed insecure/exposed, it would mean the system and its guarantees cannot be trusted as advertised, given both keys are supposed to be handled as a secure, opaque bundle, disclosed to no one beyond the bound origin at create time.
No need for the "oh dear"-ing before you provide evidence. I'm not aware of any command for fetch or enumeration of public keys in CTAP (was rather confident it doesn't provide any such thing). Care to link to what you were referring to?
Interesting, but the PRF / LargeBlob extensions already enable just such functionality (and more) without relying on the secrecy of a public key.
Why not just use those?
Edit: that's what I get for not reading far enough -- the article addresses this, though I would quibble with the confident assertion that the extensions are not available in major browsers, given I worked for a startup literal years ago which built major functionality on top of these extensions, which were available in (at least) all relevant mobile browsers.
Very fair (see my edit), though I would submit to you that this isn't a sufficient polyfill for PRF, since PRF allows for a _secondary secret_ alongside the public key, allowing the server to safely store the public key without storing the cryptographic seed material itself.
The inability to use a passkey for the purposes of both authentication and secret storage (at least, without building non-trivial additional cryptographic plumbing) seems to me a reason to just use and push for the continued adoption and acceleration of the purpose-built extensions, instead of reusing a _public_ key as private material.
You're right and I was misremembering (we had only developed against modern mobile browsers), though I am 100% certain we made use of these extensions on iOS Safari, so I honestly don't believe caniuse when they assert that it supports _neither_ extension. Per my recollection, iOS Safari supported the large blob extension quite early on.
Apologies for the brash statement earlier; that was wrong of me.
Yes, this is true, however, that means an external actor is able to execute arbitrary code in your origin, so they could also trick the user into signing malicious payloads with even the native passkey itself. There's more downside to exfiltration here, but having arbitrary code from an external party executing in your page is a more general cause for concern you'd need to mitigate regardless.
How it's better: automatically synced across all a user's devices, not subject to manual interactions with input fields (you can't programmatically request/regen passwords the same way you can with this).
I did use AI for the ECDSA public key recovery diagram, because I wasn't about to spend hours hand rolling that in Lunacy. It's correct in broad strokes, and anyone who wants to understand it more deeply can just look at the code, imo.
IMO automatic sync is a mess with the passkeys, it just muddies the whole guarantees around security based on possession, its not available unless you are signed in on the platform (eg. apple account) making the behavior inconsistent
I think if you are doing it in the browser then you bind the flow to the request origin making it phishing resistant compared to a static, origin agnostic storage
A cryptographic seed is one of the most sensitive things. And here you choose to expose it to a website (even though it was specifically generated for that website). This is not something you do for authentication. The only reason to do this is to have javascript/wasm on a website perform sensitive cryptographic operations for you. You should never be doing this.
Applications such as password managers can already integrate entropy from a passkey to encrypt their databases using the Challenge-Response protocol: https://docs.yubico.com/yesdk/users-manual/application-otp/c...