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/AndroidExternalSigner.kt

@@ -0,0 +1,151 @@
+package su.reya.coop
+
+import android.app.Activity
+import android.content.Context
+import android.content.Intent
+import androidx.core.net.toUri
+import rust.nostr.sdk.PublicKey
+import rust.nostr.sdk.UnsignedEvent
+
+class AndroidExternalSigner(
+    private val context: Context,
+    private val launcher: ExternalSignerLauncher,
+) : ExternalSignerHandler {
+    private var cachedPackageName: String? = null
+    private var cachedPublicKey: PublicKey? = null
+
+    private data class ContentResolverResult(
+        val result: String,
+        val event: String? = null,
+    )
+
+    private fun queryContentResolver(
+        type: String,
+        payload: String,
+        pubkey: PublicKey? = null,
+        currentUser: PublicKey? = null,
+    ): ContentResolverResult? {
+        val uri = "content://$cachedPackageName.${type.uppercase()}".toUri()
+        val cursor = context.contentResolver.query(
+            uri,
+            listOf(payload, pubkey, currentUser) as Array<out String?>?,
+            null, null, null,
+        ) ?: return null
+
+        return cursor.use {
+            if (it.getColumnIndex("rejected") > -1) return null
+            if (it.moveToFirst()) {
+                val resultIndex = it.getColumnIndex("result")
+                val result = if (resultIndex > -1) it.getString(resultIndex) else null
+
+                val eventIndex = it.getColumnIndex("event")
+                val event = if (eventIndex > -1) it.getString(eventIndex) else null
+
+                ContentResolverResult(result = result!!, event = event)
+            } else null
+        }
+    }
+
+    private suspend fun request(
+        type: String,
+        payload: String,
+        pubkey: PublicKey? = null,
+        currentUser: PublicKey? = null,
+    ): String? {
+        // Try Content Resolver first
+        queryContentResolver(type, payload, pubkey, currentUser)?.let {
+            return it.result
+        }
+
+        // Fall back to Intent
+        val intent = Intent(Intent.ACTION_VIEW, "nostrsigner:$payload".toUri()).apply {
+            `package` = cachedPackageName
+            putExtra("type", type)
+            if (pubkey != null) putExtra("pubkey", pubkey.toHex())
+            if (currentUser != null) putExtra("current_user", currentUser.toHex())
+        }
+
+        val result = launcher.launch(intent)
+        if (result.resultCode != Activity.RESULT_OK) return null
+
+        val data = result.data ?: return null
+        if (data.getBooleanExtra("rejected", false)) return null
+
+        return data.getStringExtra("result")
+    }
+
+    override fun isAvailable(): Boolean {
+        val intent = Intent(Intent.ACTION_VIEW, "nostrsigner:".toUri())
+        return context.packageManager.queryIntentActivities(intent, 0).isNotEmpty()
+    }
+
+    override suspend fun getPublicKey(permissions: String?): ExternalSignerResult? {
+        val intent = Intent(Intent.ACTION_VIEW, "nostrsigner:".toUri()).apply {
+            putExtra("type", "get_public_key")
+            if (permissions != null) putExtra("permissions", permissions)
+        }
+
+        val result = launcher.launch(intent)
+        if (result.resultCode != Activity.RESULT_OK) return null
+
+        val data = result.data ?: return null
+        if (data.getBooleanExtra("rejected", false)) return null
+
+        val pubkey = data.getStringExtra("result") ?: return null
+        val packageName = data.getStringExtra("package") ?: return null
+
+        return ExternalSignerResult(pubkey, packageName)
+    }
+
+    override suspend fun signEvent(event: UnsignedEvent, currentUser: PublicKey): String? {
+        // Try Content Resolver first
+        queryContentResolver(
+            "SIGN_EVENT",
+            event.asJson(),
+            null,
+            currentUser
+        )?.let { result ->
+            if (result.event != null) return result.event
+        }
+
+        // Fall back to Intent
+        val intent = Intent(Intent.ACTION_VIEW, "nostrsigner:${event.asJson()}".toUri()).apply {
+            `package` = cachedPackageName
+            putExtra("type", "sign_event")
+            putExtra("current_user", currentUser.toHex())
+            if (event.id() != null) putExtra("id", event.id()!!.toHex())
+        }
+
+        val result = launcher.launch(intent)
+        if (result.resultCode != Activity.RESULT_OK) return null
+
+        val data = result.data ?: return null
+        if (data.getBooleanExtra("rejected", false)) return null
+
+        return data.getStringExtra("event")
+    }
+
+    override suspend fun nip04Encrypt(plaintext: String, pubkey: PublicKey): String? {
+        return request("nip04_encrypt", plaintext, pubkey)
+    }
+
+    override suspend fun nip04Decrypt(ciphertext: String, pubkey: PublicKey): String? {
+        return request("nip04_decrypt", ciphertext, pubkey)
+    }
+
+    override suspend fun nip44Encrypt(
+        plaintext: String,
+        pubkey: PublicKey,
+        currentUser: PublicKey
+    ): String? {
+        return request("nip44_encrypt", plaintext, pubkey, currentUser)
+    }
+
+    override suspend fun nip44Decrypt(
+        ciphertext: String,
+        pubkey: PublicKey,
+        currentUser: PublicKey
+    ): String? {
+        return request("nip44_decrypt", ciphertext, pubkey, currentUser)
+    }
+}

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

@@ -0,0 +1,166 @@
+package su.reya.coop
+
+import android.app.Activity
+import android.content.Context
+import android.content.Intent
+import androidx.core.net.toUri
+import rust.nostr.sdk.AsyncNostrSigner
+import rust.nostr.sdk.Event
+import rust.nostr.sdk.PublicKey
+import rust.nostr.sdk.UnsignedEvent
+
+data class SignerResult(
+    val result: String,
+    val event: Event? = null,
+)
+
+class ExternalSigner(
+    private val context: Context,
+    private val packageName: String,
+    private val currentUser: PublicKey,
+    private val launcher: ExternalSignerLauncher,
+) : AsyncNostrSigner {
+    private fun queryContentResolver(
+        type: String,
+        payload: String,
+        pubkey: PublicKey? = null
+    ): SignerResult? {
+        val uri = "content://$packageName.${type.uppercase()}".toUri()
+        val projection = mutableListOf<String>()
+        projection.add(payload)
+        if (pubkey != null) projection.add(pubkey.toHex())
+        projection.add(currentUser.toHex())
+
+        val cursor = context.contentResolver.query(
+            uri,
+            projection.toList() as Array<out String?>?,
+            null, null, null,
+        ) ?: return null
+
+        return cursor.use {
+            if (it.getColumnIndex("rejected") > -1) return null
+
+            if (it.moveToFirst()) {
+                val resultIndex = it.getColumnIndex(payload)
+                val result = if (resultIndex > -1) it.getString(resultIndex) else null
+
+                val eventIndex = it.getColumnIndex("event")
+                val event = if (eventIndex > -1) it.getString(eventIndex) else null
+
+                SignerResult(result = result!!, event = Event.fromJson(event!!))
+            } else {
+                null
+            }
+        }
+    }
+
+    private suspend fun requestViaIntent(
+        type: String,
+        payload: String,
+        extras: Map<String, String> = emptyMap()
+    ): String {
+        val intent = Intent(Intent.ACTION_VIEW, "nostrsigner:$payload".toUri()).apply {
+            `package` = packageName
+            putExtra("type", type)
+            putExtra("current_user", currentUser.toHex())
+            extras.forEach { (k, v) -> putExtra(k, v) }
+        }
+
+        val result = launcher.launch(intent)
+
+        if (result.resultCode != Activity.RESULT_OK) {
+            throw Exception("Signer returned error (resultCode=${result.resultCode})")
+        }
+
+        val data = result.data ?: throw Exception("Signer returned no data")
+
+        if (data.getBooleanExtra("rejected", false)) {
+            throw Exception("User rejected the request")
+        }
+
+        return data.getStringExtra("result") ?: throw Exception("Signer returned no result")
+    }
+
+    private suspend fun request(
+        type: String,
+        payload: String,
+        pubkey: PublicKey? = null,
+        currentUser: PublicKey? = null,
+        extras: Map<String, String> = emptyMap()
+    ): String {
+        // Try silent Content Resolver first
+        queryContentResolver(type, payload, pubkey)?.let { return it.result }
+
+        // Fall back to Intent
+        val allExtras = extras.toMutableMap().apply {
+            if (pubkey != null) put("pubkey", pubkey.toHex())
+            if (currentUser != null) put("current_user", currentUser.toHex())
+        }
+
+        return requestViaIntent(type, payload, allExtras)
+    }
+
+    override suspend fun getPublicKeyAsync(): PublicKey {
+        return currentUser
+    }
+
+    override suspend fun signEventAsync(unsignedEvent: UnsignedEvent): Event? {
+        val eventJson = unsignedEvent.asJson()
+
+        // Try Content Resolver first
+        val contentResult = queryContentResolver("sign_event", eventJson)
+        contentResult?.event?.let { return it }
+
+        // Fall back to Intent
+        val intent = Intent(Intent.ACTION_VIEW, "nostrsigner:$eventJson".toUri()).apply {
+            `package` = packageName
+            putExtra("type", "sign_event")
+            putExtra("current_user", currentUser.toHex())
+            putExtra("id", unsignedEvent.id()?.toHex() ?: "")
+        }
+
+        val result = launcher.launch(intent)
+        if (result.resultCode != Activity.RESULT_OK) return null
+
+        val data = result.data ?: return null
+        if (data.getBooleanExtra("rejected", false)) return null
+
+        val signedEventJson = data.getStringExtra("event")
+
+        return signedEventJson?.let { Event.fromJson(it) }
+    }
+
+    override suspend fun nip04EncryptAsync(publicKey: PublicKey, content: String): String {
+        return request(
+            "nip04_encrypt",
+            payload = content,
+            pubkey = publicKey
+        )
+    }
+
+    override suspend fun nip04DecryptAsync(publicKey: PublicKey, encryptedContent: String): String {
+        return request(
+            "nip04_decrypt",
+            payload = encryptedContent,
+            pubkey = publicKey
+        )
+    }
+
+    override suspend fun nip44EncryptAsync(publicKey: PublicKey, content: String): String {
+        return request(
+            "nip44_encrypt",
+            payload = content,
+            pubkey = publicKey,
+            currentUser = currentUser
+        )
+    }
+
+    override suspend fun nip44DecryptAsync(publicKey: PublicKey, payload: String): String {
+        return request(
+            "nip44_decrypt",
+            payload = payload,
+            pubkey = publicKey,
+            currentUser = currentUser
+        )
+    }
+}

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

@@ -0,0 +1,34 @@
+package su.reya.coop
+
+import android.content.Intent
+import androidx.activity.result.ActivityResult
+import androidx.activity.result.ActivityResultLauncher
+import kotlinx.coroutines.CompletableDeferred
+
+class ExternalSignerLauncher {
+    private var launcher: ActivityResultLauncher<Intent>? = null
+    private var pendingResult: CompletableDeferred<ActivityResult>? = null
+
+    fun register(launcher: ActivityResultLauncher<Intent>) {
+        this.launcher = launcher
+    }
+
+    fun unregister() {
+        launcher = null
+        pendingResult?.cancel()
+        pendingResult = null
+    }
+
+    suspend fun launch(intent: Intent): ActivityResult {
+        val deferred = CompletableDeferred<ActivityResult>()
+        pendingResult = deferred
+        launcher?.launch(intent)
+            ?: throw IllegalStateException("ExternalSignerLauncher not registered")
+        return deferred.await()
+    }
+
+    fun onResult(result: ActivityResult) {
+        pendingResult?.complete(result)
+        pendingResult = null
+    }
+}

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

@@ -2,9 +2,12 @@ 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
+import androidx.activity.result.contract.ActivityResultContracts
 import androidx.activity.viewModels
 import androidx.core.splashscreen.SplashScreen.Companion.installSplashScreen
 import androidx.lifecycle.ProcessLifecycleOwner
@@ -14,11 +17,14 @@ import su.reya.coop.coop.storage.SecretStore
 import kotlin.system.exitProcess
 
 class MainActivity : ComponentActivity() {
+    private val externalSignerLauncher = ExternalSignerLauncher()
+
     private val viewModel: NostrViewModel by viewModels {
         object : ViewModelProvider.Factory {
             override fun <T : ViewModel> create(modelClass: Class<T>): T {
                 val secretStore = SecretStore(this@MainActivity)
-                return NostrViewModel(NostrManager.instance, secretStore) as T
+                val androidSigner = AndroidExternalSigner(this@MainActivity, externalSignerLauncher)
+                return NostrViewModel(NostrManager.instance, secretStore, androidSigner) as T
             }
         }
     }
@@ -26,10 +32,9 @@ class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {
         Thread.setDefaultUncaughtExceptionHandler { thread, throwable ->
             throwable.printStackTrace()
-            android.util.Log.e(
-                "CoopCrash",
-                "Uncaught exception in thread ${thread.name}",
-                throwable
+
+            Log.e(
+                "CoopCrash", "Uncaught exception in thread ${thread.name}", throwable
             )
 
             // Start the Crash Activity
@@ -40,10 +45,17 @@ class MainActivity : ComponentActivity() {
             startActivity(intent)
 
             // Exit
-            android.os.Process.killProcess(android.os.Process.myPid())
+            Process.killProcess(Process.myPid())
             exitProcess(1)
         }
 
+        val resultLauncher = registerForActivityResult(
+            ActivityResultContracts.StartActivityForResult()
+        ) { result ->
+            externalSignerLauncher.onResult(result)
+        }
+        externalSignerLauncher.register(resultLauncher)
+
         val splashScreen = installSplashScreen()
         enableEdgeToEdge()
 

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

@@ -0,0 +1,24 @@
+package su.reya.coop
+
+import rust.nostr.sdk.PublicKey
+import rust.nostr.sdk.UnsignedEvent
+
+/**
+ * Platform interface for NIP-55 external signer communication.
+ * Implemented on Android; no-op/null on other platforms.
+ */
+interface ExternalSignerHandler {
+    fun isAvailable(): Boolean
+
+    suspend fun getPublicKey(permissions: String? = null): ExternalSignerResult?
+    suspend fun signEvent(event: UnsignedEvent, currentUser: PublicKey): String?
+    suspend fun nip04Encrypt(plaintext: String, pubkey: PublicKey): String?
+    suspend fun nip04Decrypt(ciphertext: String, pubkey: PublicKey): String?
+    suspend fun nip44Encrypt(plaintext: String, pubkey: PublicKey, currentUser: PublicKey): String?
+    suspend fun nip44Decrypt(ciphertext: String, pubkey: PublicKey, currentUser: PublicKey): String?
+}
+
+data class ExternalSignerResult(
+    val pubkey: String,
+    val packageName: String,
+)

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

@@ -0,0 +1,41 @@
+package su.reya.coop
+
+import rust.nostr.sdk.AsyncNostrSigner
+import rust.nostr.sdk.Event
+import rust.nostr.sdk.PublicKey
+import rust.nostr.sdk.UnsignedEvent
+
+class ExternalSignerProxy(
+    private val handler: ExternalSignerHandler,
+    private val packageName: String,
+    private val currentUser: PublicKey,
+) : AsyncNostrSigner {
+    override suspend fun getPublicKeyAsync(): PublicKey {
+        return currentUser
+    }
+
+    override suspend fun signEventAsync(unsignedEvent: UnsignedEvent): Event? {
+        val signedJson = handler.signEvent(unsignedEvent, currentUser) ?: return null
+        return Event.fromJson(signedJson)
+    }
+
+    override suspend fun nip04EncryptAsync(publicKey: PublicKey, content: String): String {
+        return handler.nip04Encrypt(content, publicKey)
+            ?: throw Exception("NIP-04 encrypt rejected")
+    }
+
+    override suspend fun nip04DecryptAsync(publicKey: PublicKey, encryptedContent: String): String {
+        return handler.nip04Decrypt(encryptedContent, publicKey)
+            ?: throw Exception("NIP-04 decrypt rejected")
+    }
+
+    override suspend fun nip44EncryptAsync(publicKey: PublicKey, content: String): String {
+        return handler.nip44Encrypt(content, publicKey, currentUser)
+            ?: throw Exception("NIP-44 encrypt rejected")
+    }
+
+    override suspend fun nip44DecryptAsync(publicKey: PublicKey, payload: String): String {
+        return handler.nip44Decrypt(payload, publicKey, currentUser)
+            ?: throw Exception("NIP-44 decrypt rejected")
+    }
+}

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

@@ -44,7 +44,8 @@ import kotlin.time.Duration.Companion.seconds
 
 class NostrViewModel(
     private val nostr: Nostr,
-    private val secretStore: SecretStorage
+    private val secretStore: SecretStorage,
+    private val externalSignerHandler: ExternalSignerHandler? = null,
 ) : ViewModel() {
     private val _isNotificationBannerDismissed = MutableStateFlow(false)
     val isNotificationBannerDismissed = _isNotificationBannerDismissed.asStateFlow()
@@ -374,6 +375,7 @@ 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)
@@ -381,6 +383,18 @@ class NostrViewModel(
                 NostrConnect(uri = bunker, appKeys, timeout, null)
             }
 
+            secret.startsWith("nip55://") -> {
+                val handler = externalSignerHandler
+                    ?: throw IllegalStateException("External signer not available on this platform")
+
+                // Format: nip55://packageName/hexPubkey
+                val parts = secret.removePrefix("nip55://").split("/", limit = 2)
+                val packageName = parts[0]
+                val pubkey = PublicKey.parse(parts[1])
+
+                ExternalSignerProxy(handler, packageName, pubkey)
+            }
+
             else -> throw IllegalArgumentException("Invalid secret format")
         }
     }