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Comment by palata | Hacker News Reader

Probably a naive question, but: if the passkey is synchronised between multiple devices, doesn't it just trivially render the signCount useless?

Say I have a passkey shared between my laptop and my smartphone. When I log in with the smartphone, the signCount is incremented and the new value is synchronised with the laptop, as suggested in the article.

Now say my passkey is compromised, and an attacker logs in from somewhere else. Won't the signCount just be incremented and synchronised with the smartphone and the laptop? How does signCount prevent that?

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You made a good point, esp. if your passkey vault is comprosed, e.g. Apple iCloud's credentials are leaked. signCount, incremented or not, would not help here in informing you of your hacked iCloud account – that would be dependent on iCloud's service itself for detecting and informing you of your compromised account.

I would still like to see big tech passkey providers implement signCount for the following 2 reasons:

1. It helps to push relying parties to implement signCount verification. Right now most relying parties do not implement it as many providers are returning `0` for `signCount`.

2. This would be an odd one, it helps against detecting leaked private keys of passkeys, if a malicious attacker, internal or external, manages to obtain the private key.

I'm a bit confused: how does signCount ever bring security in a shared-passkey scenario?

The only way I can see it be useful is if you have exactly one instance of the passkey (e.g. a security key), because if `signCount` got incremented without the security key being aware of it, then you have a problem.

Same reason how signCount is useful in a non-shared passkey. Yubikeys are not supposed to be cloneable afaik, but this helps to detect if somehow it got done.

Also, why not.

> Same reason how signCount is useful in a non-shared passkey.

Again: unless I am missing something, signCount is useless with a shared passkey. If your laptop expects signCount to be "2" and sees "5", it will just believe that your smartphone was used in the meantime. The counter doesn't say "it was used illegally", does it?

> Also, why not.

"Because it's useless" sounds like a good reason to me. Unless you explain why it is not useless, that is.

signCount would need to be synchronised as well as the passkey by whatever method you elect to use. If your synchronisation method has been persistently compromised you're hosed, but if the passkey is cloned as a one off, the server would continue to increment signCount everytime either copy is used, while the passkey in your possession would only increment when used by you ie. half as often. You'd run into trouble if the sync service can't tally multiple device uses in quick succession, which is the likely reason for the article - if I use my synced passkey on three separate devices in a few minutes, all three copies would have the same signCount, but it would be lower than the server's signCount. Either you'd have to prompt the user everytime this happens or record and sync a lot more information about every passkey use and let the sync service count them.

Say the user has two devices and hence two copies of the same passkey, let's call them A and B. They have a shared signCount.

Say an attacker manages to make a copy C of A. They have the signCount as part of it, right? So they can immediately connect to the server. The server will increment signCount and sync it with A and B, but C is already in and C knows that the signCount is probably lastSignCount+1.

The only way I could imagine signCount to be useful is if somehow the server synchronises it between A and B in a way that C - who got access for a while - cannot access. It would mean that C has access until A or B connects, and after that the next time C connects, it will be out of sync. This does not sound super useful, and it assumes that C cannot access the sync process even though it has unlimited access to the passkey (until A or B is used).

What am I missing? To me signCount doesn't bring anything here...

If C uses it without the knowledge of the original owner (A or B). With proper signCount check, C would have to increment it at its end; A or B would not have known.

When A logs in with an unincremented signCount. A and the relying party are now aware of a potential cloned authenticator and disable the compromised passkey.

I'm sorry but it seems far-fetched to me. For signCount to be useful with shared passkeys, the attacker who managed to copy the passkey and get full access until the true owner logs in again would have to not synchronise the signCount (which they can totally do because they have full access), and it would "only" let the true owner know that the passkey is compromised.

It seems strictly worse than just sending an email saying "your passkey was used from <IP-based geolocation>, wasn't it you?".

I haven't paid much attention to the passkey journey.

Why would you sync a passkey, rather than create a separate one for each device? Seems risky if it's compromised?

I don't; I use Yubikeys so each passkey is unique.

But I was assuming that the TooBigTech implementation was somehow sharing the passkeys?

convenience. They judge that people using synced passkeys is better than people finding passkeys annoying and not using passkeys at all.

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