Yesterday, Ars Technica reported on Confer, an AI company created by Moxie Marlinspike, who also created the secure messaging app/protocol Signal.
In Confer’s blog post “Making end-to-end encrypted AI chat feel like logging in,” Moxie writes about how Confer uses passkey PRFs to encrypt chats. As the blog post describes, the PRF extension allows Confer to derive a new key deterministically from a user’s passkey, so they can then use this new key to encrypt other material.
While passkey PRFs are powerful, and have legitimate uses, they carry a real risk: passkey PRFs increase the blast radius of passkey loss.
When a passkey PRF is used to encrypt material associated with a user, and that user loses their passkey, the encrypted material is also lost. Today, if a user loses a passkey used to log in to a service, they can usually recover their account somehow, but if materials tied to that account were encrypted with a passkey PRF, those materials are gone even when the user regains account access.
Passkey loss can happen for many reasons! Loss of a device or hardware token for keys that aren’t synced, lockout from a platform (such as an Apple ID) for passkeys kept in a platform-specific keystore, or even a user’s death for passkeys whose decryption mechanism expects biometrics. The fact that people have repeatedly lost cryptocurrency keys worth enormous sums of money shows that even with strong financial incentives, mistakes happen and keys get lost.
So while passkey PRFs are powerful (they let you tie encryption material to an authenticated user’s private key), they also require platforms to prepare users for what they’d lose if their passkey is lost.
The Confer article lists alternatives, such as giving the user a long passphrase to remember (which many will forget or store insecurely), encrypting based on a user’s password (which is phishable), or using a device-key (which blocks cross-device access). These trade-offs make passkey PRFs compelling! It’s just worth remembering it’s not a free lunch.
Copyright Andrew Lilley Brinker. Made with in California