Cambium

Cambium is an Android NIP-55 signer that holds no user keys. Every signing
request is proxied to a paired Heartwood hardware signer over NIP-46 (Nostr
relays), and the response is handed straight back to the requesting app.

Amethyst, Primal and most other Amber-compatible clients cannot log in to a
remote NIP-46 bunker directly, but they all support NIP-55 external signers.
Cambium fills that gap: it registers as a signer, but it is not one.

No user secrets ever touch the phone. Compromising the phone exposes only the
ability to request signatures — which the hardware signer's policy engine and
physical confirmation button still gate — never the identity key itself.

No Google Play services, no Firebase, no analytics. Runs on any Android 8.1+
device, including GrapheneOS.