Image: rsnel.github.io · Passkeys grouped per website
Cross-device passkey sign-in from an Android 9 phone
The maker wanted to sign in on a laptop by scanning a FIDO QR code with passkeys from Bitwarden. Claude would only build a skeleton, the working protocol came after many dead ends with DeepSeek, and Claude then polished the app.
Credits
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The full story
PassKeyChain is an open-source Android app that holds passkeys you own and signs you in with them, on the phone and on a computer by scanning a FIDO QR code. It imports passkeys from Bitwarden and exports them in open formats, with no account, cloud or telemetry. The maker wanted it after seeing a colleague sign in on a laptop by scanning a QR code, and found no way to do the same with passkeys stored in Bitwarden on their phone.
The maker asked Claude to build the app, but Claude found the task too daunting and would only create a skeleton. The protocol is complex: the phone has to send an encrypted Bluetooth message as proof of presence. The maker got a working implementation with DeepSeek's free chat after many hallucinations and dead ends, and Claude then polished the app.
The repository includes a full specification with test vectors. Signing in on a computer goes through a tunnel server, Google's by default, which only sees encrypted data.
How it was done
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Sources
From the maker's own write-up or code. We summarise what they report; we haven't tested it ourselves.
- Hacker News: Show HN: Cross-device passkey auth on Android 9, synced with Bitwarden
The app's website explaining what it does and how QR sign-in works.
- Hacker News discussion (2 points)HN submitter: riksnelPosted
The maker's Show HN post on how the app came about, with discussion.