feat(app): an authenticator's key copied, marked sensitive, and its code six digits

A phone without a camera whose authenticator doesn't open otpauth: had no
way but typing the key. It is copied again, the key alone (ADR 0017
amended, the developer's decision of 3 Nov 2026): on Android through each
app's SensitiveClipboard.kt, marked EXTRA_IS_SENSITIVE, so Gboard and the
system show no preview and keep it out of their histories; elsewhere through
Flutter's clipboard. Emptied 60 seconds later if the clipboard still holds
it. The recovery phrase and codes stay never copied.

The code's field counted "/7", taking an optional space for a seventh
character: it now takes six digits alone, a pasted space dropped, and counts
nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/train/halkyon_train_flutter/android/app/src/main/kotlin/app/halkyon/train/MainActivity.kt

@@ -69,6 +69,7 @@ class MainActivity : FlutterActivity() {
     val messenger = flutterEngine.dartExecutor.binaryMessenger
     // Secrets halkyon_app shows kept out of screenshots and recents.
     ScreenGuard.attach(this, messenger)
+    SensitiveClipboard.attach(this, messenger)
     // The recording's side of the system (RecorderService).
     MethodChannel(messenger, "app.halkyon.train/recorder").setMethodCallHandler { call, result ->
       when (call.method) {

apps/train/halkyon_train_flutter/android/app/src/main/kotlin/app/halkyon/train/SensitiveClipboard.kt

@@ -0,0 +1,46 @@
+package app.halkyon.train
+
+import android.content.ClipData
+import android.content.ClipboardManager
+import android.content.Context
+import android.os.Build
+import android.os.PersistableBundle
+import io.flutter.plugin.common.BinaryMessenger
+import io.flutter.plugin.common.MethodChannel
+
+/**
+ * Copies an authenticator's key for halkyon_app's PlatformSecretClipboard,
+ * marked sensitive (ClipDescription.EXTRA_IS_SENSITIVE, read by the system
+ * from Android 13 on and by Gboard before it), so it shows no preview and
+ * stays out of clipboard histories; and empties the clipboard again if it
+ * still holds the key, which Android lets an app read only while in front.
+ */
+object SensitiveClipboard {
+  private const val IS_SENSITIVE = "android.content.extra.IS_SENSITIVE"
+
+  fun attach(context: Context, messenger: BinaryMessenger) {
+    val clipboard = context.getSystemService(Context.CLIPBOARD_SERVICE) as ClipboardManager
+    MethodChannel(messenger, "app.halkyon/clipboard").setMethodCallHandler { call, result ->
+      when (call.method) {
+        "copySensitive" -> {
+          val clip = ClipData.newPlainText("", call.arguments as String)
+          clip.description.extras = PersistableBundle().apply { putBoolean(IS_SENSITIVE, true) }
+          clipboard.setPrimaryClip(clip)
+          result.success(null)
+        }
+        "clearIf" -> {
+          val held = clipboard.primaryClip?.takeIf { it.itemCount > 0 }?.getItemAt(0)?.text
+          if (held?.toString() == call.arguments as String) {
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
+              clipboard.clearPrimaryClip()
+            } else {
+              clipboard.setPrimaryClip(ClipData.newPlainText("", ""))
+            }
+          }
+          result.success(null)
+        }
+        else -> result.notImplemented()
+      }
+    }
+  }
+}