Optional app PIN, settings header, pink selection ring

- A PIN (4 to 8 digits, salted SHA-256 in prefs) set in Settings shows
  a logo splash with a PIN prompt at start and whenever the app returns
  after leaving the screen; Back there leaves the app.
- Settings gets the back-arrow header shared with the folder grid
  (Ui.header) and scrolls, since the PIN fields overflow the screen.
- Tile and list focus rings use the icon pink.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/app/src/main/java/mygram/droid/Prefs.java b/app/src/main/java/mygram/droid/Prefs.java
index 33ce878..0ff295d 100644
--- a/app/src/main/java/mygram/droid/Prefs.java
+++ b/app/src/main/java/mygram/droid/Prefs.java
@@ -17,6 +17,11 @@ package mygram.droid;
 import android.content.Context;
 import android.content.SharedPreferences;
 
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
+
 /** Settings stored on the device. */
 final class Prefs {
 	private static SharedPreferences prefs;
@@ -57,4 +62,46 @@ final class Prefs {
 	static void setTimes(int photo, int background) {
 		prefs.edit().putInt("photo", photo).putInt("background", background).apply();
 	}
+
+	static boolean validPin(String pin) {
+		return pin.matches("[0-9]{4,8}");
+	}
+
+	static boolean hasPin() {
+		return !prefs.getString("pin", "").isEmpty();
+	}
+
+	/** Stores "salt:sha256(salt + pin)", never the PIN; null removes it. */
+	static void setPin(String pin) {
+		if (pin == null) {
+			prefs.edit().remove("pin").apply();
+			return;
+		}
+		byte[] b = new byte[16];
+		new SecureRandom().nextBytes(b);
+		String salt = hex(b);
+		prefs.edit().putString("pin", salt + ":" + hash(salt, pin)).apply();
+	}
+
+	static boolean checkPin(String pin) {
+		String[] stored = prefs.getString("pin", "").split(":");
+		return stored.length == 2 && MessageDigest.isEqual(stored[1].getBytes(StandardCharsets.UTF_8),
+				hash(stored[0], pin).getBytes(StandardCharsets.UTF_8));
+	}
+
+	// ponytail: one SHA-256 round; a 4-8 digit PIN is a privacy screen, not a secret against someone with the device storage.
+	static String hash(String salt, String pin) {
+		try {
+			return hex(MessageDigest.getInstance("SHA-256").digest((salt + pin).getBytes(StandardCharsets.UTF_8)));
+		} catch (NoSuchAlgorithmException e) {
+			throw new IllegalStateException(e); // every Java platform ships SHA-256
+		}
+	}
+
+	private static String hex(byte[] b) {
+		StringBuilder s = new StringBuilder();
+		for (byte x : b)
+			s.append(String.format("%02x", x));
+		return s.toString();
+	}
 }