init 1.0.0

2026-06-19 00:42:00 +02:00
commit 5ca960746b
67 changed files with 3261 additions and 0 deletions
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+using System.Security.Cryptography;
+using System.Text;
+using System.Text.Json;
+
+namespace LehrerApp.Sync.Crypto;
+
+/// <summary>
+/// AES-256-GCM Verschlüsselung + PBKDF2 Schlüsselableitung.
+///
+/// Pairing-Flow:
+///   Sender:   EncryptedSyncKey = AES(syncKey, PBKDF2(code))
+///   Empfänger: syncKey = AES_Decrypt(EncryptedSyncKey, PBKDF2(code))
+///   → Schlüssel verlässt nie den Server, nur der Code wird geteilt.
+/// </summary>
+public static class SyncCrypto
+{
+    private const int KeySize   = 32;   // 256 bit
+    private const int NonceSize = 12;   // 96 bit – GCM Standard
+    private const int TagSize   = 16;   // 128 bit Auth-Tag
+    private const int Pbkdf2Iter = 100_000;
+
+    // ── Schlüssel ──────────────────────────────────────────────────────────────
+
+    public static byte[] GenerateKey()
+    {
+        var k = new byte[KeySize];
+        RandomNumberGenerator.Fill(k);
+        return k;
+    }
+    public static string KeyToBase64(byte[] key) => Convert.ToBase64String(key);
+    public static byte[] KeyFromBase64(string b64) => Convert.FromBase64String(b64);
+
+    /// <summary>Leitet Schlüssel aus Einmal-Code ab. PBKDF2 erschwert Brute-Force.</summary>
+    public static byte[] DeriveKeyFromCode(string code)
+    {
+        var bytes = Encoding.UTF8.GetBytes(code.Trim().ToUpperInvariant());
+        var salt  = Encoding.UTF8.GetBytes("LehrerApp-PairingCode-v1");
+        return Rfc2898DeriveBytes.Pbkdf2(bytes, salt, Pbkdf2Iter, HashAlgorithmName.SHA256, 32);
+    }
+
+    public static string EncryptKeyWithCode(byte[] syncKey, string code) =>
+        Convert.ToBase64String(Encrypt(syncKey, DeriveKeyFromCode(code)));
+
+    public static byte[] DecryptKeyWithCode(string encryptedB64, string code) =>
+        Decrypt(Convert.FromBase64String(encryptedB64), DeriveKeyFromCode(code));
+
+    // ── Ver-/Entschlüsselung ───────────────────────────────────────────────────
+
+    /// <summary>Format: [Nonce 12B][Ciphertext][Tag 16B]</summary>
+    public static byte[] Encrypt(byte[] plaintext, byte[] key)
+    {
+        var nonce = new byte[NonceSize];
+        RandomNumberGenerator.Fill(nonce);
+        var ciphertext = new byte[plaintext.Length];
+        var tag        = new byte[TagSize];
+        using var aes  = new AesGcm(key, TagSize);
+        aes.Encrypt(nonce, plaintext, ciphertext, tag);
+        var result = new byte[NonceSize + ciphertext.Length + TagSize];
+        nonce.CopyTo(result, 0);
+        ciphertext.CopyTo(result, NonceSize);
+        tag.CopyTo(result, NonceSize + ciphertext.Length);
+        return result;
+    }
+
+    public static byte[] Decrypt(byte[] encrypted, byte[] key)
+    {
+        if (encrypted.Length < NonceSize + TagSize)
+            throw new CryptographicException("Ungültiges Datenformat.");
+        var nonce      = encrypted[..NonceSize];
+        var tag        = encrypted[^TagSize..];
+        var ciphertext = encrypted[NonceSize..^TagSize];
+        var plaintext  = new byte[ciphertext.Length];
+        using var aes  = new AesGcm(key, TagSize);
+        aes.Decrypt(nonce, ciphertext, tag, plaintext);
+        return plaintext;
+    }
+
+    // ── JSON-Komfort ───────────────────────────────────────────────────────────
+
+    public static string EncryptObject<T>(T obj, byte[] key) =>
+        Convert.ToBase64String(Encrypt(JsonSerializer.SerializeToUtf8Bytes(obj), key));
+    public static T? DecryptObject<T>(string b64, byte[] key) =>
+        JsonSerializer.Deserialize<T>(Decrypt(Convert.FromBase64String(b64), key));
+
+    // ── Schlüsselspeicherung ───────────────────────────────────────────────────
+
+    public static void SaveKey(byte[] key, string path)
+    {
+        Directory.CreateDirectory(Path.GetDirectoryName(path)!);
+        File.WriteAllText(path, KeyToBase64(key));
+        if (!OperatingSystem.IsWindows())
+            File.SetUnixFileMode(path, UnixFileMode.UserRead | UnixFileMode.UserWrite);
+    }
+    public static byte[]? LoadKey(string path) =>
+        File.Exists(path) ? KeyFromBase64(File.ReadAllText(path).Trim()) : null;
+}