feat: Implement end-to-end encryption for channels

- Added EncryptionSalt and WrappedRoomKey properties to Channel model.
- Introduced RoomCrypto class for client-side encryption and decryption.
- Updated ChannelService to handle encrypted channels, including creation and rekeying.
- Modified ChannelsController to expose crypto metadata and rekey functionality.
- Enhanced IrcCommandHandler to block joining encrypted channels over IRC.
- Updated database schema with migration for new encryption fields.
- Refactored file validation and image processing services to accommodate encrypted channels.
- Added unit tests for RoomCrypto functionality and updated existing tests for channel services.

src/EchoHub.Core/Contracts/IChannelService.cs

@@ -6,15 +6,20 @@ public interface IChannelService
 {
     // Channel CRUD
     Task<PaginatedResponse<ChannelDto>> GetChannelsAsync(Guid userId, int offset, int limit);
-    Task<ChannelOperationResult> CreateChannelAsync(Guid creatorUserId, string name, string? topic, bool isPublic, string? password = null);
+    Task<ChannelOperationResult> CreateChannelAsync(Guid creatorUserId, string name, string? topic, bool isPublic,
+        string? password = null, string? encryptionSalt = null, string? wrappedRoomKey = null);
     Task<ChannelOperationResult> UpdateTopicAsync(Guid callerUserId, string channelName, string? topic);
     Task<ChannelOperationResult> SetChannelPasswordAsync(Guid callerUserId, string channelName, string? password);
+    Task<ChannelOperationResult> RekeyChannelAsync(Guid callerUserId, string channelName,
+        string oldPassword, string newPassword, string newEncryptionSalt, string newWrappedRoomKey);
     Task<ChannelOperationResult> DeleteChannelAsync(Guid callerUserId, string channelName);
 
     // Channel queries
     Task<(string? Topic, bool Exists)> GetChannelTopicAsync(string channelName);
     Task<List<ChannelListItem>> GetChannelListAsync();
     Task<ChannelDto?> GetChannelByNameAsync(string channelName);
+    Task<ChannelCryptoDto?> GetChannelCryptoAsync(string channelName);
+    Task<(string? EncryptionSalt, string? WrappedRoomKey)> GetChannelKeyEnvelopeAsync(string channelName);
 
     // Membership
     Task<(bool Success, string? Error, bool PasswordRequired)> EnsureChannelMembershipAsync(Guid userId, string channelName, string? password = null);

src/EchoHub.Core/DTOs/ChatDtos.cs

@@ -22,7 +22,8 @@ public record ChannelDto(
     bool IsPublic,
     int MessageCount,
     DateTimeOffset CreatedAt,
-    bool IsProtected = false);
+    bool IsProtected = false,
+    bool IsEncrypted = false);
 
 public record UserDto(
     Guid Id,
@@ -34,13 +35,41 @@ public record UserDto(
 
 public record SendMessageRequest(string ChannelName, string Content);
 
-public record CreateChannelRequest(string Name, string? Topic = null, bool IsPublic = true, string? Password = null);
+public record CreateChannelRequest(
+    string Name,
+    string? Topic = null,
+    bool IsPublic = true,
+    string? Password = null,
+    string? EncryptionSalt = null,
+    string? WrappedRoomKey = null);
+
+/// <summary>
+/// Public crypto metadata for a channel — enough for a client to derive its join
+/// credential from a passphrase. Never includes the wrapped room key.
+/// </summary>
+public record ChannelCryptoDto(bool IsEncrypted, string? EncryptionSalt);
+
+/// <summary>
+/// Passphrase change for an encrypted channel: the client proves knowledge of the old
+/// passphrase (old auth key), then supplies the re-wrapped room key under the new one.
+/// </summary>
+public record RekeyChannelRequest(
+    string OldPassword,
+    string NewPassword,
+    string NewEncryptionSalt,
+    string NewWrappedRoomKey);
 
 public record UpdateTopicRequest(string? Topic);
 
 public record SendUrlRequest(string Url);
 
-public record JoinChannelResult(bool Success, List<MessageDto> History, string? Error = null, bool PasswordRequired = false);
+public record JoinChannelResult(
+    bool Success,
+    List<MessageDto> History,
+    string? Error = null,
+    bool PasswordRequired = false,
+    string? EncryptionSalt = null,
+    string? WrappedRoomKey = null);
 
 public record EmbedDto(
     string? SiteName,

src/EchoHub.Core/EchoHub.Core.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFramework>net10.0</TargetFramework>
@@ -6,4 +6,8 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="SixLabors.ImageSharp" Version="3.1.12" />
+  </ItemGroup>
+
 </Project>

src/EchoHub.Core/Models/Channel.cs

@@ -7,6 +7,12 @@ public class Channel
     public string? Topic { get; set; }
     public bool IsPublic { get; set; } = true;
     public string? PasswordHash { get; set; }
+
+    // End-to-end encryption envelope (client-generated; server cannot decrypt room content).
+    // EncryptionSalt: PBKDF2 salt for passphrase-derived keys. WrappedRoomKey: the room
+    // content key encrypted under the passphrase-derived key-encryption key.
+    public string? EncryptionSalt { get; set; }
+    public string? WrappedRoomKey { get; set; }
     public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
     public Guid CreatedByUserId { get; set; }
 

src/EchoHub.Core/Security/RoomCrypto.cs

@@ -0,0 +1,132 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace EchoHub.Core.Security;
+
+/// <summary>
+/// Client-side envelope encryption for private (end-to-end encrypted) channels.
+///
+/// Design: at creation the client generates a random 256-bit room content key (RCK)
+/// that encrypts all room content. The RCK is stored on the server *wrapped*
+/// (AES-GCM encrypted) by a key derived from the passphrase, next to a BCrypt hash
+/// of a separately derived auth key used as the join gate. The passphrase, the
+/// key-encryption key, and the RCK never leave the client, so the server can gate
+/// joins and count/measure content without being able to read it. Changing the
+/// passphrase only re-wraps the RCK — history is never re-encrypted.
+///
+/// Derivation: PBKDF2-SHA256(passphrase, salt, 210000 iterations) → 64 bytes;
+/// first 32 bytes are the auth key (sent to the server as lowercase hex),
+/// last 32 bytes are the key-encryption key (never sent).
+/// </summary>
+public static class RoomCrypto
+{
+    public const string CiphertextPrefix = "$RC1$";
+
+    private const int Pbkdf2Iterations = 210_000;
+    private const int SaltSizeBytes = 16;
+    private const int KeySizeBytes = 32;
+    private const int NonceSizeBytes = 12;
+    private const int TagSizeBytes = 16;
+
+    public sealed record DerivedKeys(string AuthKeyHex, byte[] KeyEncryptionKey);
+
+    public static byte[] GenerateSalt() => RandomNumberGenerator.GetBytes(SaltSizeBytes);
+
+    public static byte[] GenerateRoomKey() => RandomNumberGenerator.GetBytes(KeySizeBytes);
+
+    /// <summary>
+    /// Derives the auth key (join gate credential) and key-encryption key from a passphrase.
+    /// </summary>
+    public static DerivedKeys DeriveKeys(string passphrase, byte[] salt)
+    {
+        var okm = Rfc2898DeriveBytes.Pbkdf2(
+            Encoding.UTF8.GetBytes(passphrase), salt, Pbkdf2Iterations,
+            HashAlgorithmName.SHA256, KeySizeBytes * 2);
+
+        var authKey = Convert.ToHexString(okm.AsSpan(0, KeySizeBytes)).ToLowerInvariant();
+        var kek = okm[KeySizeBytes..];
+        CryptographicOperations.ZeroMemory(okm.AsSpan(0, KeySizeBytes));
+        return new DerivedKeys(authKey, kek);
+    }
+
+    /// <summary>Encrypts UTF-8 text with the room key. Output: $RC1$base64(nonce||tag||ciphertext).</summary>
+    public static string EncryptText(string plaintext, byte[] key) =>
+        CiphertextPrefix + Convert.ToBase64String(EncryptBytes(Encoding.UTF8.GetBytes(plaintext), key));
+
+    /// <summary>
+    /// Decrypts text produced by <see cref="EncryptText"/>. Returns false when the input
+    /// is not room ciphertext or the key does not match.
+    /// </summary>
+    public static bool TryDecryptText(string content, byte[] key, out string plaintext)
+    {
+        plaintext = string.Empty;
+        if (!IsRoomCiphertext(content))
+            return false;
+
+        try
+        {
+            var blob = Convert.FromBase64String(content[CiphertextPrefix.Length..]);
+            plaintext = Encoding.UTF8.GetString(DecryptBytes(blob, key));
+            return true;
+        }
+        catch (Exception ex) when (ex is FormatException or CryptographicException or ArgumentException)
+        {
+            return false;
+        }
+    }
+
+    public static bool IsRoomCiphertext(string? content) =>
+        content is not null && content.StartsWith(CiphertextPrefix, StringComparison.Ordinal);
+
+    /// <summary>Encrypts a binary blob (file contents) with the room key: nonce||tag||ciphertext.</summary>
+    public static byte[] EncryptBytes(byte[] plaintext, byte[] key)
+    {
+        var nonce = RandomNumberGenerator.GetBytes(NonceSizeBytes);
+        var ciphertext = new byte[plaintext.Length];
+        var tag = new byte[TagSizeBytes];
+
+        using var aes = new AesGcm(key, TagSizeBytes);
+        aes.Encrypt(nonce, plaintext, ciphertext, tag);
+
+        var blob = new byte[NonceSizeBytes + TagSizeBytes + ciphertext.Length];
+        nonce.CopyTo(blob, 0);
+        tag.CopyTo(blob, NonceSizeBytes);
+        ciphertext.CopyTo(blob, NonceSizeBytes + TagSizeBytes);
+        return blob;
+    }
+
+    /// <summary>Decrypts a blob produced by <see cref="EncryptBytes"/>. Throws <see cref="CryptographicException"/> on key mismatch.</summary>
+    public static byte[] DecryptBytes(byte[] blob, byte[] key)
+    {
+        if (blob.Length < NonceSizeBytes + TagSizeBytes)
+            throw new CryptographicException("Ciphertext blob is too short.");
+
+        var nonce = blob.AsSpan(0, NonceSizeBytes);
+        var tag = blob.AsSpan(NonceSizeBytes, TagSizeBytes);
+        var ciphertext = blob.AsSpan(NonceSizeBytes + TagSizeBytes);
+        var plaintext = new byte[ciphertext.Length];
+
+        using var aes = new AesGcm(key, TagSizeBytes);
+        aes.Decrypt(nonce, ciphertext, tag, plaintext);
+        return plaintext;
+    }
+
+    /// <summary>Wraps the room content key under the key-encryption key for server storage.</summary>
+    public static string WrapRoomKey(byte[] roomKey, byte[] kek) =>
+        Convert.ToBase64String(EncryptBytes(roomKey, kek));
+
+    /// <summary>Unwraps the stored room content key. Returns false when the KEK (passphrase) is wrong.</summary>
+    public static bool TryUnwrapRoomKey(string wrappedRoomKey, byte[] kek, out byte[] roomKey)
+    {
+        roomKey = [];
+        try
+        {
+            roomKey = DecryptBytes(Convert.FromBase64String(wrappedRoomKey), kek);
+            return roomKey.Length == KeySizeBytes;
+        }
+        catch (Exception ex) when (ex is FormatException or CryptographicException or ArgumentException)
+        {
+            return false;
+        }
+    }
+}

src/EchoHub.Core/Services/FileValidationHelper.cs

@@ -1,4 +1,4 @@
-namespace EchoHub.Server.Services;
+namespace EchoHub.Core.Services;
 
 public static class FileValidationHelper
 {

src/EchoHub.Core/Services/ImageToAsciiService.cs

@@ -4,7 +4,7 @@ using SixLabors.ImageSharp;
 using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Processing;
 
-namespace EchoHub.Server.Services;
+namespace EchoHub.Core.Services;
 
 public class ImageToAsciiService
 {