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> Passkeys can be public/private keypairs, or they can just be secret passwords. (WebAuthn passkeys were designed by committee, so there’s always more than one

> Passkeys can be public/private keypairs, or they can just be secret passwords. (WebAuthn passkeys were designed by committee, so there’s always more than one way to do it.) I'm surprised by this. Last time I checked, a few years ago, I couldn't create a passkey in Chrome without using either the Chrome developer tools to create an emulated passkey, or an actual hardware device. Why would I use a TPM to store a shared secret? Has this changed, or did I miss something? > Instead, password managers recommend that sites/apps allow each user to have multiple passkeys. Sites/apps may or may not actually allow that, but, for now, that’s the only way to be sure a given user can login with both Google’s password manager and Apple’s password manager: give each password manager its own passkey for each site. Having a simple procedure for rotating/replacing keys would probably be useful for the "switch password manager/TPM" case. I.e. a protocol between two password managers and a browser to do batch authorization of a new key. The migration story is one reason I don't use passkeys.

eqvinox 1y ago
> Why would I use a TPM to store a shared secret? I don't understand your incredulity at this; a disk encryption key is a shared secret and commonly stored in a TPM. It kinda still makes sense because the TPM 'enforces' platform security. But a TPM isn't the same as a hardware token, and storing a shared secret on a hardware token does make less sense. But that doesn't prevent people from doing it...
- flowerthoughts 1y ago
  > a disk encryption key is a shared secret and commonly stored in a TPM. Thinking some more about this, you're right. If the stored shared secret is high entropy, and the user might choose a weak password, then using the TPM as a defense against password cracking is a useful reason for the seeming indirection. That assumes the TPM can e.g. rate limit extraction attempts and not be foiled by power-loss or sped up oscillators or similar externalities.
timmyc123 1y ago
> Why would I use a TPM to store a shared secret? There is no shared secret. A passkey is a public/private key pair.

eqvinox 1y ago

> Why would I use a TPM to store a shared secret? I don't understand your incredulity at this; a disk encryption key is a shared secret and commonly stored in a TPM. It kinda still makes sense because the TPM 'enforces' platform security. But a TPM isn't the same as a hardware token, and storing a shared secret on a hardware token does make less sense. But that doesn't prevent people from doing it...

flowerthoughts 1y ago
> a disk encryption key is a shared secret and commonly stored in a TPM. Thinking some more about this, you're right. If the stored shared secret is high entropy, and the user might choose a weak password, then using the TPM as a defense against password cracking is a useful reason for the seeming indirection. That assumes the TPM can e.g. rate limit extraction attempts and not be foiled by power-loss or sped up oscillators or similar externalities.

timmyc123 1y ago

> Why would I use a TPM to store a shared secret? There is no shared secret. A passkey is a public/private key pair.