composeApp/src/androidMain/AndroidManifest.xml

@@ -11,6 +11,14 @@
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_DATA_SYNC" />
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
 
+    <queries>
+        <intent>
+            <action android:name="android.intent.action.VIEW" />
+            <category android:name="android.intent.category.BROWSABLE" />
+            <data android:scheme="nostrsigner" />
+        </intent>
+    </queries>
+
     <application
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"

composeApp/src/androidMain/kotlin/su/reya/coop/ExternalSigner.kt

@@ -0,0 +1,46 @@
+package su.reya.coop
+
+import android.content.Intent
+import androidx.activity.ComponentActivity
+import androidx.activity.result.ActivityResultLauncher
+import androidx.activity.result.contract.ActivityResultContracts
+import androidx.core.net.toUri
+import kotlinx.coroutines.suspendCancellableCoroutine
+import kotlin.coroutines.resume
+
+class AndroidExternalSignerLauncher(activity: ComponentActivity) : ExternalSignerLauncher {
+    private var callback: ((String?) -> Unit)? = null
+
+    private val launcher: ActivityResultLauncher<Intent> =
+        activity.registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
+            val intent = result.data
+            val result = intent?.getStringExtra("signature")
+                ?: intent?.getStringExtra("public_key")
+                ?: intent?.getStringExtra("content")
+                ?: intent?.dataString
+
+            callback?.invoke(result)
+            callback = null
+        }
+
+    override suspend fun launch(
+        content: String,
+        type: String,
+        pubkey: String?,
+        id: String?
+    ): String? =
+        suspendCancellableCoroutine { continuation ->
+            callback = { continuation.resume(it) }
+
+            val intent = Intent(Intent.ACTION_VIEW, "nostrsigner:$content".toUri())
+            intent.putExtra("type", type)
+            pubkey?.let { intent.putExtra("pubkey", it) }
+            id?.let { intent.putExtra("id", it) }
+
+            try {
+                launcher.launch(intent)
+            } catch (e: Exception) {
+                callback?.invoke(null)
+            }
+        }
+}

composeApp/src/androidMain/kotlin/su/reya/coop/MainActivity.kt

@@ -2,6 +2,8 @@ package su.reya.coop
 
 import android.content.Intent
 import android.os.Bundle
+import android.os.Process
+import android.util.Log
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
@@ -14,50 +16,28 @@ import su.reya.coop.coop.storage.SecretStore
 import kotlin.system.exitProcess
 
 class MainActivity : ComponentActivity() {
-    private val viewModel: NostrViewModel by viewModels {
+    private lateinit var externalSignerLauncher: AndroidExternalSignerLauncher
-        object : ViewModelProvider.Factory {
-            override fun <T : ViewModel> create(modelClass: Class<T>): T {
-                val secretStore = SecretStore(this@MainActivity)
-                return NostrViewModel(NostrManager.instance, secretStore) as T
-            }
-        }
-    }
 
     override fun onCreate(savedInstanceState: Bundle?) {
-        Thread.setDefaultUncaughtExceptionHandler { thread, throwable ->
-            throwable.printStackTrace()
-            android.util.Log.e(
-                "CoopCrash",
-                "Uncaught exception in thread ${thread.name}",
-                throwable
-            )
-
-            // Start the Crash Activity
-            val intent = Intent(this, CrashActivity::class.java).apply {
-                putExtra("error", throwable.stackTraceToString())
-                addFlags(Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK)
-            }
-            startActivity(intent)
-
-            // Exit
-            android.os.Process.killProcess(android.os.Process.myPid())
-            exitProcess(1)
-        }
-
         val splashScreen = installSplashScreen()
+        setupCrashHandler()
         enableEdgeToEdge()
 
         super.onCreate(savedInstanceState)
 
-        val serviceIntent = Intent(this, NostrForegroundService::class.java)
+        // Initialize the nostr service and external signer
-        startForegroundService(serviceIntent)
+        setupExternalSigner()
+        startNostrService()
+
+        // Initialize the ViewModel
+        val viewModel: NostrViewModel by viewModels { NostrViewModelFactory(this) }
 
         // Keep the splash screen visible until the signer check is complete
         splashScreen.setKeepOnScreenCondition {
             viewModel.signerRequired.value == null
         }
 
-        // Bind the lifecycle of the ViewModel to the Activity's lifecycle'
+        // Bind the lifecycle of the ViewModel to the Activity's lifecycle
         viewModel.bindLifecycle(ProcessLifecycleOwner.get().lifecycle)
 
         setContent {
@@ -65,8 +45,44 @@ class MainActivity : ComponentActivity() {
         }
     }
 
+    private fun setupExternalSigner() {
+        val launcher = AndroidExternalSignerLauncher(this)
+        ExternalSignerLauncherProvider.launcher = launcher
+    }
+
+    private fun startNostrService() {
+        val intent = Intent(this, NostrForegroundService::class.java)
+        startForegroundService(intent)
+    }
+
+    private fun setupCrashHandler() {
+        Thread.setDefaultUncaughtExceptionHandler { thread, throwable ->
+            throwable.printStackTrace()
+
+            Log.e("CoopCrash", "Uncaught exception in thread ${thread.name}", throwable)
+
+            val intent = Intent(this, CrashActivity::class.java).apply {
+                putExtra("error", throwable.stackTraceToString())
+                addFlags(Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK)
+            }
+            startActivity(intent)
+
+            Process.killProcess(Process.myPid())
+            exitProcess(1)
+        }
+    }
+
     override fun onNewIntent(intent: Intent) {
         super.onNewIntent(intent)
         setIntent(intent)
     }
 }
+
+class NostrViewModelFactory(
+    private val activity: ComponentActivity
+) : ViewModelProvider.Factory {
+    override fun <T : ViewModel> create(modelClass: Class<T>): T {
+        val secretStore = SecretStore(activity)
+        return NostrViewModel(NostrManager.instance, secretStore) as T
+    }
+}

composeApp/src/androidMain/kotlin/su/reya/coop/screens/OnboardingScreen.kt

@@ -1,5 +1,7 @@
 package su.reya.coop.screens
 
+import android.content.Context
+import android.content.Intent
 import androidx.compose.foundation.Canvas
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -13,13 +15,16 @@ import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material3.Button
 import androidx.compose.material3.ButtonDefaults
 import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.FilledTonalButton
 import androidx.compose.material3.MaterialTheme
-import androidx.compose.material3.OutlinedButton
 import androidx.compose.material3.Scaffold
 import androidx.compose.material3.SnackbarHost
+import androidx.compose.material3.SnackbarResult
 import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.geometry.Size
@@ -27,6 +32,7 @@ import androidx.compose.ui.graphics.ColorFilter
 import androidx.compose.ui.graphics.drawscope.rotate
 import androidx.compose.ui.graphics.drawscope.translate
 import androidx.compose.ui.graphics.painter.Painter
+import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.text.LinkAnnotation
 import androidx.compose.ui.text.SpanStyle
 import androidx.compose.ui.text.TextLinkStyles
@@ -34,10 +40,13 @@ import androidx.compose.ui.text.buildAnnotatedString
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.unit.dp
+import androidx.core.net.toUri
 import coop.composeapp.generated.resources.Res
 import coop.composeapp.generated.resources.coop
+import kotlinx.coroutines.launch
 import org.jetbrains.compose.resources.painterResource
 import su.reya.coop.LocalNavigator
+import su.reya.coop.LocalNostrViewModel
 import su.reya.coop.LocalSnackbarHostState
 import su.reya.coop.Screen
 import su.reya.coop.shared.getExpressiveFontFamily
@@ -45,8 +54,11 @@ import su.reya.coop.shared.getExpressiveFontFamily
 @OptIn(ExperimentalMaterial3ExpressiveApi::class)
 @Composable
 fun OnboardingScreen() {
+    val context = LocalContext.current
     val snackbarHostState = LocalSnackbarHostState.current
     val navigator = LocalNavigator.current
+    val viewModel = LocalNostrViewModel.current
+    val scope = rememberCoroutineScope()
 
     val logoPainter = painterResource(Res.drawable.coop)
     val expressiveFont = getExpressiveFontFamily()
@@ -142,7 +154,38 @@ fun OnboardingScreen() {
                                 )
                             }
                             Spacer(modifier = Modifier.size(8.dp))
-                            OutlinedButton(
+                            FilledTonalButton(
+                                onClick = {
+                                    scope.launch {
+                                        if (isExternalSignerInstalled(context)) {
+                                            viewModel.importIdentity("external")
+                                        } else {
+                                            val result = snackbarHostState.showSnackbar(
+                                                message = "External signer not installed. Please install Amber or alternatives.",
+                                                actionLabel = "Install"
+                                            )
+
+                                            if (result == SnackbarResult.ActionPerformed) {
+                                                val intent = Intent(
+                                                    Intent.ACTION_VIEW,
+                                                    "https://zapstore.dev/apps/com.greenart7c3.nostrsigner".toUri()
+                                                )
+                                                context.startActivity(intent)
+                                            }
+                                        }
+                                    }
+                                },
+                                modifier = Modifier
+                                    .fillMaxWidth()
+                                    .size(ButtonDefaults.MediumContainerHeight),
+                            ) {
+                                Text(
+                                    text = "Connect via Amber",
+                                    style = MaterialTheme.typography.titleMedium,
+                                )
+                            }
+                            Spacer(modifier = Modifier.size(8.dp))
+                            TextButton(
                                 onClick = { navigator.navigate(Screen.Import) },
                                 modifier = Modifier
                                     .fillMaxWidth()
@@ -216,3 +259,8 @@ fun LogoRepeatingBackground(
         }
     }
 }
+
+fun isExternalSignerInstalled(context: Context): Boolean {
+    val intent = Intent(Intent.ACTION_VIEW, "nostrsigner:".toUri())
+    return context.packageManager.queryIntentActivities(intent, 0).isNotEmpty()
+}

shared/src/commonMain/kotlin/su/reya/coop/ExternalSigner.kt

@@ -0,0 +1,56 @@
+package su.reya.coop
+
+import rust.nostr.sdk.AsyncNostrSigner
+import rust.nostr.sdk.Event
+import rust.nostr.sdk.PublicKey
+import rust.nostr.sdk.UnsignedEvent
+
+interface ExternalSignerLauncher {
+    suspend fun launch(
+        content: String,
+        type: String,
+        pubkey: String? = null,
+        id: String? = null
+    ): String?
+}
+
+object ExternalSignerLauncherProvider {
+    var launcher: ExternalSignerLauncher? = null
+}
+
+/**
+ * A cross-platform implementation of AsyncNostrSigner that delegates
+ * to a platform-specific launcher (NIP-55 on Android).
+ */
+class ExternalSigner(private val launcher: ExternalSignerLauncher) : AsyncNostrSigner {
+    override suspend fun getPublicKeyAsync(): PublicKey? {
+        val result = launcher.launch("", "get_public_key")
+        return result?.let { PublicKey.parse(it) }
+    }
+
+    override suspend fun signEventAsync(unsignedEvent: UnsignedEvent): Event? {
+        val result =
+            launcher.launch(unsignedEvent.asJson(), "sign_event", id = unsignedEvent.id()?.toHex())
+        return result?.let { Event.fromJson(it) }
+    }
+
+    override suspend fun nip04EncryptAsync(publicKey: PublicKey, content: String): String {
+        return launcher.launch(content, "nip04_encrypt", publicKey.toHex())
+            ?: throw Exception("Encryption failed")
+    }
+
+    override suspend fun nip04DecryptAsync(publicKey: PublicKey, encryptedContent: String): String {
+        return launcher.launch(encryptedContent, "nip04_decrypt", publicKey.toHex())
+            ?: throw Exception("Decryption failed")
+    }
+
+    override suspend fun nip44EncryptAsync(publicKey: PublicKey, content: String): String {
+        return launcher.launch(content, "nip44_encrypt", publicKey.toHex())
+            ?: throw Exception("Encryption failed")
+    }
+
+    override suspend fun nip44DecryptAsync(publicKey: PublicKey, payload: String): String {
+        return launcher.launch(payload, "nip44_decrypt", publicKey.toHex())
+            ?: throw Exception("Decryption failed")
+    }
+}

shared/src/commonMain/kotlin/su/reya/coop/NostrViewModel.kt

@@ -372,6 +372,7 @@ class NostrViewModel(
     private suspend fun createSigner(secret: String): AsyncNostrSigner {
         return when {
             secret.startsWith("nsec1") -> Keys.parse(secret)
+
             secret.startsWith("bunker://") -> {
                 val appKeys = getOrInitAppKeys()
                 val bunker = NostrConnectUri.parse(secret)
@@ -379,6 +380,12 @@ class NostrViewModel(
                 NostrConnect(uri = bunker, appKeys, timeout, null)
             }
 
+            secret == "external" -> {
+                val launcher = ExternalSignerLauncherProvider.launcher
+                    ?: throw IllegalStateException("External signer not supported on this platform")
+                ExternalSigner(launcher)
+            }
+
             else -> throw IllegalArgumentException("Invalid secret format")
         }
     }
@@ -476,10 +483,10 @@ class NostrViewModel(
         try {
             val signer = createSigner(secret)
             nostr.setSigner(signer)
-            secretStore.set("user_signer", secret)
         } catch (e: Exception) {
             showError("Error: ${e.message}")
         } finally {
+            secretStore.set("user_signer", secret)
             _signerRequired.value = false
             _isLoggedIn.value = false
         }