Amber: Nostr event signer for Android

Amber is a nostr event signer for Android. It allows users to keep their nsec segregated in a single, dedicated app. The goal of Amber is to have your smartphone act as a NIP-46 signing device without any need for servers or additional hardware. "Private keys should be exposed to as few systems as possible as each system adds to the attack surface," as the rationale of said NIP states. In addition to native apps, Amber aims to support all current nostr web applications without requiring any extensions or web servers.

Current Features

Offline
Use nip-46 or make an addendum in nip-46
Improve the ui (currently its showing a text with the raw json of the event)
Check if we can use Amber to sign the events of web applications
Change the sign button to just copy the signature of the event
Use content provider to sign events in background when you checked the remember my choice option on android
Support for multiple accounts

Usage for Android applications

Amber uses Intents and Content Resolvers to communicate between applications.

To be able to use Amber in your application you should add the following package visibility needs:

<queries>
    <package android:name="com.greenart7c3.nostrsigner"/>
</queries>

Using Intents

To get the result back from Amber you should use registerForActivityResult or rememberLauncherForActivityResult in Kotlin. If you are using another framework check the documentation of your framework or a third party library to get the result.

Create the Intent using the nostrsigner scheme:

val intent = Intent(Intent.ACTION_VIEW, Uri.parse("nostrsigner:$content"))

Set the Amber package name

intent.`package` = "com.greenart7c3.nostrsigner"

Methods

get_public_key

params:

val intent = Intent(Intent.ACTION_VIEW, Uri.parse("nostrsigner:"))
intent.`package` = "com.greenart7c3.nostrsigner"

// You can send some default permissions for the user authorize for ever
val permissions = listOf(
  Permission(
      "sign_event",
      22242
  ),
  Permission(
      "nip04_encrypt"
  ),
  Permission(
      "nip04_decrypt"
  ),
  Permission(
      "nip44_encrypt"
  ),
  Permission(
      "nip44_decrypt"
  ),
  Permission(
      "decrypt_zap_event"
  ),
)
intent.putExtra("permissions", permissions.toJson())
 
intent.putExtra("type", "get_public_key")
context.startActivity(intent)

result:

If the user approved intent it will return the npub in the signature field

val npub = intent.data?.getStringExtra("signature")
// The package name of the signer application
val packageName = intent.data?.getStringExtra("package")

sign_event

params:

val intent = Intent(Intent.ACTION_VIEW, Uri.parse("nostrsigner:$eventJson"))
intent.`package` = "com.greenart7c3.nostrsigner"
intent.putExtra("type", "sign_event")
// to control the result in your application in case you are not waiting the result before sending another intent
intent.putExtra("id", event.id)
// Send the current logged in user npub
intent.putExtra("current_user", account.keyPair.pubKey.toNpub())

// If you are sending multiple intents without awaiting you can add some intent flags to sign all events without opening multiple screens
intent.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP or Intent.FLAG_ACTIVITY_CLEAR_TOP)

context.startActivity(intent)

result:

If the user approved intent it will return the signature, id and event fields

val signature = intent.data?.getStringExtra("signature")
// the id you sent
val id = intent.data?.getStringExtra("id")
val signedEventJson = intent.data?.getStringExtra("event")

nip04_encrypt

params:

val intent = Intent(Intent.ACTION_VIEW, Uri.parse("nostrsigner:$plaintext"))
intent.`package` = "com.greenart7c3.nostrsigner"
intent.putExtra("type", "nip04_encrypt")
// to control the result in your application in case you are not waiting the result before sending another intent
intent.putExtra("id", "some_id")
// Send the current logged in user npub
intent.putExtra("current_user", account.keyPair.pubKey.toNpub())
// Send the hex pubKey that will be used for encrypting the data
intent.putExtra("pubKey", pubKey)

context.startActivity(intent)

result: