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