feat: Implement message encryption and decryption support

- Added IMessageEncryptionService and its implementation MessageEncryptionService for handling message encryption.
- Updated ChannelsController and ChatService to encrypt messages before storing and sending.
- Introduced encryption key retrieval endpoint in ServerController.
- Modified EchoHubDbContext to accommodate increased message content and embed JSON lengths for encrypted data.
- Created migrations to support encryption-related database changes.
- Enhanced FirstRunSetup to ensure encryption key is generated if not present.
- Updated appsettings.example.json to include encryption configuration.
- Added comprehensive unit tests for encryption service and compatibility tests between client and server encryption.

src/EchoHub.Tests/ClientEncryptionServiceTests.cs

@@ -0,0 +1,233 @@
+using System.Security.Cryptography;
+using EchoHub.Client.Services;
+using Xunit;
+
+namespace EchoHub.Tests;
+
+public class ClientEncryptionServiceTests
+{
+    private static string GenerateKey() =>
+        Convert.ToBase64String(RandomNumberGenerator.GetBytes(32));
+
+    private static ClientEncryptionService CreateInitialized(string? key = null)
+    {
+        var service = new ClientEncryptionService();
+        service.SetKey(key ?? GenerateKey());
+        return service;
+    }
+
+    // ── Initialization ───────────────────────────────────────────────
+
+    [Fact]
+    public void IsInitialized_DefaultFalse()
+    {
+        var service = new ClientEncryptionService();
+        Assert.False(service.IsInitialized);
+    }
+
+    [Fact]
+    public void IsInitialized_TrueAfterSetKey()
+    {
+        var service = CreateInitialized();
+        Assert.True(service.IsInitialized);
+    }
+
+    [Fact]
+    public void EncryptDatabaseEnabled_AlwaysFalse()
+    {
+        var service = CreateInitialized();
+        Assert.False(service.EncryptDatabaseEnabled);
+    }
+
+    [Fact]
+    public void SetKey_WrongLength_Throws()
+    {
+        var service = new ClientEncryptionService();
+        var shortKey = Convert.ToBase64String(RandomNumberGenerator.GetBytes(16));
+
+        Assert.Throws<InvalidOperationException>(() => service.SetKey(shortKey));
+    }
+
+    [Fact]
+    public void SetKey_InvalidBase64_Throws()
+    {
+        var service = new ClientEncryptionService();
+        Assert.Throws<FormatException>(() => service.SetKey("not-valid-base64!!!"));
+    }
+
+    // ── Encrypt before initialization ────────────────────────────────
+
+    [Fact]
+    public void Encrypt_NotInitialized_PassesThrough()
+    {
+        var service = new ClientEncryptionService();
+        var result = service.Encrypt("hello");
+
+        Assert.Equal("hello", result);
+    }
+
+    [Fact]
+    public void Decrypt_NotInitialized_PassesThrough()
+    {
+        var service = new ClientEncryptionService();
+        var result = service.Decrypt("$ENC$v1$something$else");
+
+        Assert.Equal("$ENC$v1$something$else", result);
+    }
+
+    // ── Encrypt after initialization ─────────────────────────────────
+
+    [Fact]
+    public void Encrypt_ProducesEncryptedFormat()
+    {
+        var service = CreateInitialized();
+        var encrypted = service.Encrypt("Hello, world!");
+
+        Assert.StartsWith("$ENC$v1$", encrypted);
+    }
+
+    [Fact]
+    public void Encrypt_DifferentNonceEachTime()
+    {
+        var service = CreateInitialized();
+        var a = service.Encrypt("same message");
+        var b = service.Encrypt("same message");
+
+        Assert.NotEqual(a, b);
+    }
+
+    [Fact]
+    public void Encrypt_EmptyString_Works()
+    {
+        var service = CreateInitialized();
+        var encrypted = service.Encrypt("");
+
+        Assert.StartsWith("$ENC$v1$", encrypted);
+    }
+
+    // ── Decrypt ──────────────────────────────────────────────────────
+
+    [Fact]
+    public void Decrypt_RoundTrip()
+    {
+        var service = CreateInitialized();
+        var original = "Hello, world!";
+
+        var encrypted = service.Encrypt(original);
+        var decrypted = service.Decrypt(encrypted);
+
+        Assert.Equal(original, decrypted);
+    }
+
+    [Fact]
+    public void Decrypt_EmptyString_RoundTrip()
+    {
+        var service = CreateInitialized();
+        var encrypted = service.Encrypt("");
+        var decrypted = service.Decrypt(encrypted);
+
+        Assert.Equal("", decrypted);
+    }
+
+    [Fact]
+    public void Decrypt_Unicode_RoundTrip()
+    {
+        var service = CreateInitialized();
+        var original = "Hello 🌍 世界 مرحبا";
+
+        var encrypted = service.Encrypt(original);
+        var decrypted = service.Decrypt(encrypted);
+
+        Assert.Equal(original, decrypted);
+    }
+
+    [Fact]
+    public void Decrypt_PlaintextPassthrough()
+    {
+        var service = CreateInitialized();
+        var plaintext = "This is just plain text";
+
+        var result = service.Decrypt(plaintext);
+
+        Assert.Equal(plaintext, result);
+    }
+
+    [Fact]
+    public void Decrypt_WrongKey_ReturnsFailureMessage()
+    {
+        var encryptor = CreateInitialized();
+        var decryptor = CreateInitialized(); // different key
+
+        var encrypted = encryptor.Encrypt("secret message");
+        var result = decryptor.Decrypt(encrypted);
+
+        Assert.Contains("decryption failed", result);
+    }
+
+    [Fact]
+    public void Decrypt_MalformedContent_ReturnsOriginal()
+    {
+        var service = CreateInitialized();
+
+        // Malformed: prefix present but no valid separator after nonce
+        var result = service.Decrypt("$ENC$v1$noseperatorhere");
+
+        Assert.Equal("$ENC$v1$noseperatorhere", result);
+    }
+
+    [Fact]
+    public void Decrypt_TamperedCiphertext_ReturnsFailure()
+    {
+        var service = CreateInitialized();
+        var encrypted = service.Encrypt("original message");
+
+        var tampered = encrypted[..^5] + "XXXXX";
+        var result = service.Decrypt(tampered);
+
+        Assert.Contains("decryption failed", result);
+    }
+
+    // ── Nullable helpers ─────────────────────────────────────────────
+
+    [Fact]
+    public void EncryptNullable_Null_ReturnsNull()
+    {
+        var service = CreateInitialized();
+        Assert.Null(service.EncryptNullable(null));
+    }
+
+    [Fact]
+    public void DecryptNullable_Null_ReturnsNull()
+    {
+        var service = CreateInitialized();
+        Assert.Null(service.DecryptNullable(null));
+    }
+
+    // ── Key replacement ──────────────────────────────────────────────
+
+    [Fact]
+    public void SetKey_CanBeCalledMultipleTimes()
+    {
+        var service = new ClientEncryptionService();
+
+        var key1 = GenerateKey();
+        var key2 = GenerateKey();
+
+        service.SetKey(key1);
+        var encrypted1 = service.Encrypt("test");
+
+        service.SetKey(key2);
+        var encrypted2 = service.Encrypt("test");
+
+        // Both produce encrypted content
+        Assert.StartsWith("$ENC$v1$", encrypted1);
+        Assert.StartsWith("$ENC$v1$", encrypted2);
+
+        // Old key's content can't be decrypted with new key
+        var result = service.Decrypt(encrypted1);
+        Assert.Contains("decryption failed", result);
+
+        // New key's content decrypts fine
+        Assert.Equal("test", service.Decrypt(encrypted2));
+    }
+}