Amber

Amber is an Android application that signs Nostr events on behalf of other applications. Rather than each client keeping its own copy of the user's private key, the key stays in Amber: a client sends a signing request, and Amber returns the signed event. The account identity is configured once, in one place, instead of being pasted into every client that needs it.

In practice, an installed Nostr client asks Amber to approve an action — publishing a note, sending a message, reacting — and the user decides whether to grant it. Requests are handled per application, so a client used only for reading need not be given the same latitude as one that posts.

Amber is not itself a client: it displays no feeds and requires at least one Nostr application that supports external signing to be useful. It is published for Android, by greenart7c3, under the MIT license.