NIP-55

Android Signer Application

draft optional

This NIP describes a method for 2-way communication between an Android signer and any Nostr client on Android. The Android signer is an Android Application and the client can be a web client or an Android application.

Usage for Android applications

The Android signer uses Intents and Content Resolvers to communicate between applications.

To be able to use the Android signer in your application you should add this to your AndroidManifest.xml:

<queries>
  <intent>
    <action android:name="android.intent.action.VIEW" />
    <category android:name="android.intent.category.BROWSABLE" />
    <data android:scheme="nostrsigner" />
  </intent>
</queries>

Then you can use this function to check if there's a signer application installed:

fun isExternalSignerInstalled(context: Context): Boolean {
  val intent =
    Intent().apply {
      action = Intent.ACTION_VIEW
      data = Uri.parse("nostrsigner:")
    }
  val infos = context.packageManager.queryIntentActivities(intent, 0)
  return infos.size > 0
}

Using Intents

To get the result back from the Signer Application 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.

val launcher = rememberLauncherForActivityResult(
    contract = ActivityResultContracts.StartActivityForResult(),
    onResult = { result ->
        if (result.resultCode != Activity.RESULT_OK) {
            Toast.makeText(
                context,
                "Sign request rejected",
                Toast.LENGTH_SHORT
            ).show()
        } else {
            val signature = activityResult.data?.getStringExtra("signature")
            // Do something with signature ...
        }
    }
)

Create the Intent using the nostrsigner scheme:

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

Set the Signer package name:

intent.`package` = "com.example.signer"

Send the Intent:

launcher.launch(intent)

Methods

get_public_key

params:

val intent = Intent(Intent.ACTION_VIEW, Uri.parse("nostrsigner:"))
intent.`package` = "com.example.signer"
intent.putExtra("type", "get_public_key")
// You can send some default permissions for the user to authorize for ever
val permissions = listOf(
  Permission(
      type = "sign_event",
      kind = 22242
  ),
  Permission(
      type = "nip44_decrypt"
  )
)
intent.putExtra("permissions", permissions.toJson())
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.example.signer"
intent.putExtra("type", "sign_event")
// To handle results when not waiting between intents
intent.putExtra("id", event.id)
// Send the current logged in user npub
intent.putExtra("current_user", npub)

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.example.signer"
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: