Cambium

About Cambium

Cambium is an Android bridge that lets compatible Nostr apps request signatures from a paired hardware signer. It registers as an external signer for apps that use Android’s NIP-55 signing interface, then forwards signing requests to a separate NIP-46 hardware signer. In this setup, Cambium acts as a proxy rather than holding the user’s identity keys itself. The hardware signer remains responsible for approving and producing cryptographic signatures. The bridge supports requests to retrieve a public key, sign events, encrypt or decrypt messages, handle certain private payment events, and select the current identity. Multiple identities can be paired and kept separate. Pairing can be completed by scanning a QR code or entering a bunker connection URI. The paired signer communicates through Nostr relays, and payloads are encrypted using NIP-44. This enables Android clients that support external signers to use a remote hardware signer even when those clients do not connect to that signer directly. Cambium can ask for per-app approval and keeps local approval or denial choices for connected apps, which can be reviewed and forgotten. Website signing requests can be handled through supported links with one-time approval and a validated secure callback or clipboard result. There is also a content-provider route for requests already approved, and an optional foreground service can keep the connection warm. Queues prioritize user work, and the app reports when its paired signer stops responding. The phone stores pairing information and a temporary client key, not the identity secret, according to the product description. Pairing data is kept in encrypted Android Keystore-backed storage. An optional feature can unlock a paired hardware board after a restart, gated by the phone’s screen lock; this is separate from signing. Cambium supports 64-bit Android 8.1 and later. It is intended for users who already operate a compatible hardware signer and want Android applications to route signing requests to it.

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