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.Server/Controllers/ChannelsController.cs

@@ -1,6 +1,7 @@
 using System.Security.Claims;
 using EchoHub.Core.Constants;
 using EchoHub.Core.Contracts;
+using EchoHub.Core.Services;
 using EchoHub.Core.DTOs;
 using EchoHub.Core.Models;
 using EchoHub.Server.Data;
@@ -65,7 +66,8 @@ public class ChannelsController : ControllerBase
             return Unauthorized(new ErrorResponse("Authentication required."));
 
         var result = await _channelService.CreateChannelAsync(
-            Guid.Parse(userIdClaim), request.Name, request.Topic, request.IsPublic, request.Password);
+            Guid.Parse(userIdClaim), request.Name, request.Topic, request.IsPublic, request.Password,
+            request.EncryptionSalt, request.WrappedRoomKey);
         if (!result.IsSuccess)
             return MapChannelError(result);
 
@@ -75,6 +77,43 @@ public class ChannelsController : ControllerBase
         return Created($"/api/channels/{result.Channel.Name}", result.Channel);
     }
 
+    /// <summary>
+    /// Public crypto metadata for a channel: whether it is end-to-end encrypted and the
+    /// PBKDF2 salt clients need to derive their join credential. Never returns the
+    /// wrapped room key — that is only handed out after a successful join.
+    /// </summary>
+    [HttpGet("{channel}/crypto")]
+    public async Task<IActionResult> GetChannelCrypto(string channel)
+    {
+        var crypto = await _channelService.GetChannelCryptoAsync(channel);
+        if (crypto is null)
+            return NotFound(new ErrorResponse($"Channel '{channel}' does not exist."));
+
+        return Ok(crypto);
+    }
+
+    /// <summary>
+    /// Changes an encrypted channel's passphrase by re-wrapping its room key.
+    /// The caller proves knowledge of the old passphrase via the old auth key;
+    /// history is never re-encrypted (the room content key does not change).
+    /// </summary>
+    [HttpPost("{channel}/rekey")]
+    public async Task<IActionResult> RekeyChannel(string channel, [FromBody] RekeyChannelRequest request)
+    {
+        var userIdClaim = User.FindFirstValue(ClaimTypes.NameIdentifier);
+        if (userIdClaim is null)
+            return Unauthorized(new ErrorResponse("Authentication required."));
+
+        var result = await _channelService.RekeyChannelAsync(
+            Guid.Parse(userIdClaim), channel,
+            request.OldPassword, request.NewPassword,
+            request.NewEncryptionSalt, request.NewWrappedRoomKey);
+        if (!result.IsSuccess)
+            return MapChannelError(result);
+
+        return Ok(result.Channel);
+    }
+
     [HttpPut("{channel}/topic")]
     public async Task<IActionResult> UpdateTopic(string channel, [FromBody] UpdateTopicRequest request)
     {
@@ -131,34 +170,68 @@ public class ChannelsController : ControllerBase
 
         var file = Request.Form.Files[0];
 
-        // Detect file type early so we can apply the correct size limit
-        using var stream = file.OpenReadStream();
-        var isImage = FileValidationHelper.IsValidImage(stream);
-        var isAudio = !isImage && FileValidationHelper.IsAudioFile(file.FileName);
-
-        var maxSize = isImage ? HubConstants.MaxImageSizeBytes
-            : isAudio ? HubConstants.MaxAudioFileSizeBytes
-            : HubConstants.MaxFileSizeBytes;
-
-        if (file.Length > maxSize)
-            return BadRequest(new ErrorResponse($"File size exceeds maximum of {maxSize / (1024 * 1024)} MB."));
-
-        var (fileId, filePath) = await _fileStorage.SaveFileAsync(stream, file.FileName);
-
-        var messageType = isImage ? MessageType.Image
-            : isAudio ? MessageType.Audio
-            : MessageType.File;
+        MessageType messageType;
         string content;
+        string fileId;
 
-        if (isImage)
+        if (channelDto.IsEncrypted)
         {
-            var (w, h) = ImageToAsciiService.GetDimensions(size);
-            using var imageStream = System.IO.File.OpenRead(filePath);
-            content = _asciiService.ConvertToAscii(imageStream, w, h);
+            // E2E-encrypted channel: the blob is ciphertext the server cannot inspect.
+            // The client declares the type and supplies pre-rendered, room-encrypted
+            // content (ASCII art for images, encrypted filename otherwise).
+            messageType = Request.Form["type"].ToString().ToLowerInvariant() switch
+            {
+                "image" => MessageType.Image,
+                "audio" => MessageType.Audio,
+                _ => MessageType.File,
+            };
+
+            var declaredMax = messageType switch
+            {
+                MessageType.Image => HubConstants.MaxImageSizeBytes,
+                MessageType.Audio => HubConstants.MaxAudioFileSizeBytes,
+                _ => HubConstants.MaxFileSizeBytes,
+            };
+            if (file.Length > declaredMax)
+                return BadRequest(new ErrorResponse($"File size exceeds maximum of {declaredMax / (1024 * 1024)} MB."));
+
+            var clientContent = Request.Form["content"].ToString();
+            content = string.IsNullOrEmpty(clientContent) ? file.FileName : clientContent;
+
+            using var encryptedStream = file.OpenReadStream();
+            (fileId, _) = await _fileStorage.SaveFileAsync(encryptedStream, file.FileName);
         }
         else
         {
-            content = file.FileName;
+            // Detect file type early so we can apply the correct size limit
+            using var stream = file.OpenReadStream();
+            var isImage = FileValidationHelper.IsValidImage(stream);
+            var isAudio = !isImage && FileValidationHelper.IsAudioFile(file.FileName);
+
+            var maxSize = isImage ? HubConstants.MaxImageSizeBytes
+                : isAudio ? HubConstants.MaxAudioFileSizeBytes
+                : HubConstants.MaxFileSizeBytes;
+
+            if (file.Length > maxSize)
+                return BadRequest(new ErrorResponse($"File size exceeds maximum of {maxSize / (1024 * 1024)} MB."));
+
+            string filePath;
+            (fileId, filePath) = await _fileStorage.SaveFileAsync(stream, file.FileName);
+
+            messageType = isImage ? MessageType.Image
+                : isAudio ? MessageType.Audio
+                : MessageType.File;
+
+            if (isImage)
+            {
+                var (w, h) = ImageToAsciiService.GetDimensions(size);
+                using var imageStream = System.IO.File.OpenRead(filePath);
+                content = _asciiService.ConvertToAscii(imageStream, w, h);
+            }
+            else
+            {
+                content = file.FileName;
+            }
         }
 
         var attachmentUrl = $"/api/files/{fileId}";
@@ -219,6 +292,10 @@ public class ChannelsController : ControllerBase
         if (channelDto is null)
             return NotFound(new ErrorResponse($"Channel '{channelName}' does not exist."));
 
+        if (channelDto.IsEncrypted)
+            return BadRequest(new ErrorResponse(
+                "Sending images by URL is not available in end-to-end encrypted channels — download the image and /send the file instead."));
+
         if (string.IsNullOrWhiteSpace(request.Url))
             return BadRequest(new ErrorResponse("URL is required."));
 

src/EchoHub.Server/Controllers/UsersController.cs

@@ -1,6 +1,7 @@
 using System.Security.Claims;
 using EchoHub.Core.Constants;
 using EchoHub.Core.Contracts;
+using EchoHub.Core.Services;
 using EchoHub.Core.DTOs;
 using EchoHub.Server.Services;
 using Microsoft.AspNetCore.Authorization;

src/EchoHub.Server/Data/EchoHubDbContext.cs

@@ -45,6 +45,8 @@ public class EchoHubDbContext : DbContext
             entity.Property(c => c.Name).IsRequired().HasMaxLength(100);
             entity.Property(c => c.Topic).HasMaxLength(500);
             entity.Property(c => c.PasswordHash).HasMaxLength(100);
+            entity.Property(c => c.EncryptionSalt).HasMaxLength(64);
+            entity.Property(c => c.WrappedRoomKey).HasMaxLength(200);
 
             entity.HasMany(c => c.Messages)
                   .WithOne(m => m.Channel)

src/EchoHub.Server/Data/Migrations/20260716012917_AddChannelEncryptionEnvelope.Designer.cs

@@ -0,0 +1,279 @@
+// <auto-generated />
+using System;
+using EchoHub.Server.Data;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+
+#nullable disable
+
+namespace EchoHub.Server.Data.Migrations
+{
+    [DbContext(typeof(EchoHubDbContext))]
+    [Migration("20260716012917_AddChannelEncryptionEnvelope")]
+    partial class AddChannelEncryptionEnvelope
+    {
+        /// <inheritdoc />
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder.HasAnnotation("ProductVersion", "10.0.3");
+
+            modelBuilder.Entity("EchoHub.Core.Models.Channel", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<long>("CreatedAt")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<Guid>("CreatedByUserId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("EncryptionSalt")
+                        .HasMaxLength(64)
+                        .HasColumnType("TEXT");
+
+                    b.Property<bool>("IsPublic")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PasswordHash")
+                        .HasMaxLength(100)
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Topic")
+                        .HasMaxLength(500)
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("WrappedRoomKey")
+                        .HasMaxLength(200)
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Name")
+                        .IsUnique();
+
+                    b.ToTable("Channels");
+                });
+
+            modelBuilder.Entity("EchoHub.Core.Models.ChannelMembership", b =>
+                {
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("ChannelId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<long>("JoinedAt")
+                        .HasColumnType("INTEGER");
+
+                    b.HasKey("UserId", "ChannelId");
+
+                    b.HasIndex("ChannelId");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("ChannelMemberships");
+                });
+
+            modelBuilder.Entity("EchoHub.Core.Models.Message", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("AttachmentFileName")
+                        .HasMaxLength(255)
+                        .HasColumnType("TEXT");
+
+                    b.Property<long?>("AttachmentFileSize")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("AttachmentUrl")
+                        .HasMaxLength(500)
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("ChannelId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Content")
+                        .IsRequired()
+                        .HasMaxLength(16000)
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("EmbedJson")
+                        .HasMaxLength(32000)
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("SenderUserId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("SenderUsername")
+                        .IsRequired()
+                        .HasMaxLength(50)
+                        .HasColumnType("TEXT");
+
+                    b.Property<long>("SentAt")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<int>("Type")
+                        .HasColumnType("INTEGER");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("ChannelId");
+
+                    b.HasIndex("SentAt");
+
+                    b.ToTable("Messages");
+                });
+
+            modelBuilder.Entity("EchoHub.Core.Models.RefreshToken", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<long>("CreatedAt")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<long>("ExpiresAt")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<long?>("RevokedAt")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("TokenHash")
+                        .IsRequired()
+                        .HasMaxLength(128)
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("TokenHash");
+
+                    b.HasIndex("UserId");
+
+                    b.ToTable("RefreshTokens");
+                });
+
+            modelBuilder.Entity("EchoHub.Core.Models.User", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("AvatarAscii")
+                        .HasMaxLength(10000)
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Bio")
+                        .HasMaxLength(500)
+                        .HasColumnType("TEXT");
+
+                    b.Property<long>("CreatedAt")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("DisplayName")
+                        .HasMaxLength(100)
+                        .HasColumnType("TEXT");
+
+                    b.Property<bool>("IsBanned")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<bool>("IsMuted")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<long>("LastSeenAt")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<long?>("MutedUntil")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("NicknameColor")
+                        .HasMaxLength(7)
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("PasswordHash")
+                        .IsRequired()
+                        .HasColumnType("TEXT");
+
+                    b.Property<int>("Role")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<int>("Status")
+                        .HasColumnType("INTEGER");
+
+                    b.Property<string>("StatusMessage")
+                        .HasMaxLength(100)
+                        .HasColumnType("TEXT");
+
+                    b.Property<string>("Username")
+                        .IsRequired()
+                        .HasMaxLength(50)
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("Username")
+                        .IsUnique();
+
+                    b.ToTable("Users");
+                });
+
+            modelBuilder.Entity("EchoHub.Core.Models.ChannelMembership", b =>
+                {
+                    b.HasOne("EchoHub.Core.Models.Channel", null)
+                        .WithMany()
+                        .HasForeignKey("ChannelId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.HasOne("EchoHub.Core.Models.User", null)
+                        .WithMany()
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
+            modelBuilder.Entity("EchoHub.Core.Models.Message", b =>
+                {
+                    b.HasOne("EchoHub.Core.Models.Channel", "Channel")
+                        .WithMany("Messages")
+                        .HasForeignKey("ChannelId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("Channel");
+                });
+
+            modelBuilder.Entity("EchoHub.Core.Models.RefreshToken", b =>
+                {
+                    b.HasOne("EchoHub.Core.Models.User", "User")
+                        .WithMany()
+                        .HasForeignKey("UserId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.Navigation("User");
+                });
+
+            modelBuilder.Entity("EchoHub.Core.Models.Channel", b =>
+                {
+                    b.Navigation("Messages");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}

src/EchoHub.Server/Data/Migrations/20260716012917_AddChannelEncryptionEnvelope.cs

@@ -0,0 +1,40 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace EchoHub.Server.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddChannelEncryptionEnvelope : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "EncryptionSalt",
+                table: "Channels",
+                type: "TEXT",
+                maxLength: 64,
+                nullable: true);
+
+            migrationBuilder.AddColumn<string>(
+                name: "WrappedRoomKey",
+                table: "Channels",
+                type: "TEXT",
+                maxLength: 200,
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "EncryptionSalt",
+                table: "Channels");
+
+            migrationBuilder.DropColumn(
+                name: "WrappedRoomKey",
+                table: "Channels");
+        }
+    }
+}

src/EchoHub.Server/Data/Migrations/EchoHubDbContextModelSnapshot.cs

@@ -29,6 +29,10 @@ namespace EchoHub.Server.Data.Migrations
                     b.Property<Guid>("CreatedByUserId")
                         .HasColumnType("TEXT");
 
+                    b.Property<string>("EncryptionSalt")
+                        .HasMaxLength(64)
+                        .HasColumnType("TEXT");
+
                     b.Property<bool>("IsPublic")
                         .HasColumnType("INTEGER");
 
@@ -45,6 +49,10 @@ namespace EchoHub.Server.Data.Migrations
                         .HasMaxLength(500)
                         .HasColumnType("TEXT");
 
+                    b.Property<string>("WrappedRoomKey")
+                        .HasMaxLength(200)
+                        .HasColumnType("TEXT");
+
                     b.HasKey("Id");
 
                     b.HasIndex("Name")

src/EchoHub.Server/Hubs/ChatHub.cs

@@ -14,9 +14,12 @@ public class ChatHub : Hub<IEchoHubClient>
     private readonly IChatService _chatService;
     private readonly ILogger<ChatHub> _logger;
 
-    public ChatHub(IChatService chatService, ILogger<ChatHub> logger)
+    private readonly IChannelService _channelService;
+
+    public ChatHub(IChatService chatService, IChannelService channelService, ILogger<ChatHub> logger)
     {
         _chatService = chatService;
+        _channelService = channelService;
         _logger = logger;
     }
 
@@ -67,7 +70,12 @@ public class ChatHub : Hub<IEchoHubClient>
                 return new JoinChannelResult(false, [], error, passwordRequired);
 
             await Groups.AddToGroupAsync(Context.ConnectionId, channelName.ToLowerInvariant().Trim());
-            return new JoinChannelResult(true, history);
+
+            // Members of encrypted channels receive the key envelope so they can unwrap
+            // the room content key with their passphrase (the server can't).
+            var (encryptionSalt, wrappedRoomKey) = await _channelService.GetChannelKeyEnvelopeAsync(channelName);
+            return new JoinChannelResult(true, history,
+                EncryptionSalt: encryptionSalt, WrappedRoomKey: wrappedRoomKey);
         }
         catch (Exception ex)
         {

src/EchoHub.Server/Program.cs

@@ -2,6 +2,7 @@ using System.Text;
 using System.Threading.RateLimiting;
 using EchoHub.Core.Constants;
 using EchoHub.Core.Contracts;
+using EchoHub.Core.Services;
 using EchoHub.Core.Models;
 using EchoHub.Server.Auth;
 using EchoHub.Server.Data;

src/EchoHub.Server/Services/ChannelService.cs

@@ -41,14 +41,16 @@ public class ChannelService : IChannelService
             .Skip(offset)
             .Take(limit)
             .Select(c => new ChannelDto(
-                c.Id, c.Name, c.Topic, c.IsPublic, c.Messages.Count, c.CreatedAt, c.PasswordHash != null))
+                c.Id, c.Name, c.Topic, c.IsPublic, c.Messages.Count, c.CreatedAt,
+                c.PasswordHash != null, c.WrappedRoomKey != null))
             .ToListAsync();
 
         return new PaginatedResponse<ChannelDto>(channels, total, offset, limit);
     }
 
     public async Task<ChannelOperationResult> CreateChannelAsync(
-        Guid creatorUserId, string name, string? topic, bool isPublic, string? password = null)
+        Guid creatorUserId, string name, string? topic, bool isPublic,
+        string? password = null, string? encryptionSalt = null, string? wrappedRoomKey = null)
     {
         if (string.IsNullOrWhiteSpace(name))
             return ChannelOperationResult.Fail(ChannelError.ValidationFailed, "Channel name is required.");
@@ -63,6 +65,12 @@ public class ChannelService : IChannelService
         if (passwordError is not null)
             return ChannelOperationResult.Fail(ChannelError.ValidationFailed, passwordError);
 
+        // The E2E envelope (client-generated) only makes sense on password-gated channels
+        var hasEnvelope = !string.IsNullOrWhiteSpace(encryptionSalt) && !string.IsNullOrWhiteSpace(wrappedRoomKey);
+        if (hasEnvelope && password is null)
+            return ChannelOperationResult.Fail(ChannelError.ValidationFailed,
+                "Encrypted channels require a password.");
+
         using var scope = _scopeFactory.CreateScope();
         var db = scope.ServiceProvider.GetRequiredService<EchoHubDbContext>();
 
@@ -77,6 +85,8 @@ public class ChannelService : IChannelService
             IsPublic = isPublic,
             CreatedByUserId = creatorUserId,
             PasswordHash = password is not null ? BCrypt.Net.BCrypt.HashPassword(password) : null,
+            EncryptionSalt = hasEnvelope ? encryptionSalt : null,
+            WrappedRoomKey = hasEnvelope ? wrappedRoomKey : null,
         };
 
         db.Channels.Add(channel);
@@ -91,7 +101,7 @@ public class ChannelService : IChannelService
         await db.SaveChangesAsync();
 
         var dto = new ChannelDto(channel.Id, channel.Name, channel.Topic, channel.IsPublic, 0, channel.CreatedAt,
-            channel.PasswordHash != null);
+            channel.PasswordHash != null, channel.WrappedRoomKey != null);
         return ChannelOperationResult.Success(dto);
     }
 
@@ -119,12 +129,14 @@ public class ChannelService : IChannelService
 
         var messageCount = await db.Messages.CountAsync(m => m.ChannelId == dbChannel.Id);
         var dto = new ChannelDto(dbChannel.Id, dbChannel.Name, dbChannel.Topic, dbChannel.IsPublic, messageCount, dbChannel.CreatedAt,
-            dbChannel.PasswordHash != null);
+            dbChannel.PasswordHash != null, dbChannel.WrappedRoomKey != null);
         return ChannelOperationResult.Success(dto);
     }
 
     /// <summary>
     /// Sets, changes, or clears (null) a channel's join password. Creator or admin only.
+    /// Not available on end-to-end encrypted channels — those change passphrase via
+    /// <see cref="RekeyChannelAsync"/> so the room key envelope stays consistent.
     /// </summary>
     public async Task<ChannelOperationResult> SetChannelPasswordAsync(Guid callerUserId, string channelName, string? password)
     {
@@ -141,6 +153,10 @@ public class ChannelService : IChannelService
         if (dbChannel is null)
             return ChannelOperationResult.Fail(ChannelError.NotFound, $"Channel '{channelName}' does not exist.");
 
+        if (dbChannel.WrappedRoomKey is not null)
+            return ChannelOperationResult.Fail(ChannelError.Protected,
+                "This channel is end-to-end encrypted — change its passphrase from the EchoHub client (/passwd).");
+
         var caller = await db.Users.FindAsync(callerUserId);
         if (dbChannel.CreatedByUserId != callerUserId && (caller is null || caller.Role < ServerRole.Admin))
             return ChannelOperationResult.Fail(ChannelError.Forbidden,
@@ -151,7 +167,55 @@ public class ChannelService : IChannelService
 
         var messageCount = await db.Messages.CountAsync(m => m.ChannelId == dbChannel.Id);
         var dto = new ChannelDto(dbChannel.Id, dbChannel.Name, dbChannel.Topic, dbChannel.IsPublic, messageCount, dbChannel.CreatedAt,
-            dbChannel.PasswordHash != null);
+            dbChannel.PasswordHash != null, dbChannel.WrappedRoomKey != null);
+        return ChannelOperationResult.Success(dto);
+    }
+
+    /// <summary>
+    /// Changes an encrypted channel's passphrase by swapping the join-gate hash and the
+    /// wrapped room key. The room content key itself never changes, so history stays
+    /// readable — the client re-wraps it under the new passphrase-derived key.
+    /// Creator only: admins cannot rekey a room whose passphrase they don't know.
+    /// </summary>
+    public async Task<ChannelOperationResult> RekeyChannelAsync(Guid callerUserId, string channelName,
+        string oldPassword, string newPassword, string newEncryptionSalt, string newWrappedRoomKey)
+    {
+        channelName = channelName.ToLowerInvariant().Trim();
+
+        string? validatedNew = newPassword;
+        var passwordError = ValidateChannelPassword(ref validatedNew);
+        if (passwordError is not null)
+            return ChannelOperationResult.Fail(ChannelError.ValidationFailed, passwordError);
+        if (validatedNew is null || string.IsNullOrWhiteSpace(newEncryptionSalt) || string.IsNullOrWhiteSpace(newWrappedRoomKey))
+            return ChannelOperationResult.Fail(ChannelError.ValidationFailed,
+                "New password, salt, and wrapped room key are required.");
+
+        using var scope = _scopeFactory.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<EchoHubDbContext>();
+
+        var dbChannel = await db.Channels.FirstOrDefaultAsync(c => c.Name == channelName);
+        if (dbChannel is null)
+            return ChannelOperationResult.Fail(ChannelError.NotFound, $"Channel '{channelName}' does not exist.");
+
+        if (dbChannel.WrappedRoomKey is null || dbChannel.PasswordHash is null)
+            return ChannelOperationResult.Fail(ChannelError.ValidationFailed,
+                "This channel is not end-to-end encrypted.");
+
+        if (dbChannel.CreatedByUserId != callerUserId)
+            return ChannelOperationResult.Fail(ChannelError.Forbidden,
+                "Only the channel creator can change the passphrase.");
+
+        if (!BCrypt.Net.BCrypt.Verify(oldPassword, dbChannel.PasswordHash))
+            return ChannelOperationResult.Fail(ChannelError.Forbidden, "The current passphrase is incorrect.");
+
+        dbChannel.PasswordHash = BCrypt.Net.BCrypt.HashPassword(validatedNew);
+        dbChannel.EncryptionSalt = newEncryptionSalt;
+        dbChannel.WrappedRoomKey = newWrappedRoomKey;
+        await db.SaveChangesAsync();
+
+        var messageCount = await db.Messages.CountAsync(m => m.ChannelId == dbChannel.Id);
+        var dto = new ChannelDto(dbChannel.Id, dbChannel.Name, dbChannel.Topic, dbChannel.IsPublic, messageCount, dbChannel.CreatedAt,
+            true, true);
         return ChannelOperationResult.Success(dto);
     }
 
@@ -179,7 +243,7 @@ public class ChannelService : IChannelService
         await db.SaveChangesAsync();
 
         var dto = new ChannelDto(dbChannel.Id, dbChannel.Name, dbChannel.Topic, dbChannel.IsPublic, 0, dbChannel.CreatedAt,
-            dbChannel.PasswordHash != null);
+            dbChannel.PasswordHash != null, dbChannel.WrappedRoomKey != null);
         return ChannelOperationResult.Success(dto);
     }
 
@@ -220,7 +284,32 @@ public class ChannelService : IChannelService
         if (c is null) return null;
 
         var messageCount = await db.Messages.CountAsync(m => m.ChannelId == c.Id);
-        return new ChannelDto(c.Id, c.Name, c.Topic, c.IsPublic, messageCount, c.CreatedAt, c.PasswordHash != null);
+        return new ChannelDto(c.Id, c.Name, c.Topic, c.IsPublic, messageCount, c.CreatedAt,
+            c.PasswordHash != null, c.WrappedRoomKey != null);
+    }
+
+    public async Task<ChannelCryptoDto?> GetChannelCryptoAsync(string channelName)
+    {
+        channelName = channelName.ToLowerInvariant().Trim();
+
+        using var scope = _scopeFactory.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<EchoHubDbContext>();
+
+        var c = await db.Channels.FirstOrDefaultAsync(ch => ch.Name == channelName);
+        if (c is null) return null;
+
+        return new ChannelCryptoDto(c.WrappedRoomKey != null, c.EncryptionSalt);
+    }
+
+    public async Task<(string? EncryptionSalt, string? WrappedRoomKey)> GetChannelKeyEnvelopeAsync(string channelName)
+    {
+        channelName = channelName.ToLowerInvariant().Trim();
+
+        using var scope = _scopeFactory.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<EchoHubDbContext>();
+
+        var c = await db.Channels.FirstOrDefaultAsync(ch => ch.Name == channelName);
+        return (c?.EncryptionSalt, c?.WrappedRoomKey);
     }
 
     public async Task<(bool Success, string? Error, bool PasswordRequired)> EnsureChannelMembershipAsync(