feat: enhance message decryption for IRC clients and improve attachment handling

src/EchoHub.Core/Contracts/IMessageEncryptionService.cs

@@ -2,6 +2,12 @@ namespace EchoHub.Core.Contracts;
 
 public interface IMessageEncryptionService
 {
+    /// <summary>
+    /// Prefix marking transport/at-rest encrypted content. <see cref="Decrypt"/> is a
+    /// pass-through for values without it.
+    /// </summary>
+    const string CiphertextPrefix = "$ENC$v1$";
+
     /// <summary>
     /// Whether database content should be encrypted at rest (server setting).
     /// </summary>

src/EchoHub.Server.Irc/IrcBroadcaster.cs

@@ -16,8 +16,16 @@ public class IrcBroadcaster : IChatBroadcaster
 
     public async Task SendMessageToChannelAsync(string channelName, MessageDto message)
     {
-        // Decrypt content for IRC clients (they can't handle app-layer encryption)
-        var decryptedMessage = message with { Content = _encryption.Decrypt(message.Content) };
+        // Decrypt content and attachment previews for IRC clients (they can't handle
+        // app-layer encryption). E2E room ciphertext ($RC1$) passes through untouched;
+        // the formatter drops previews that are still ciphertext.
+        var decryptedMessage = message with
+        {
+            Content = _encryption.Decrypt(message.Content),
+            Attachments = message.Attachments?
+                .Select(a => a with { AsciiPreview = _encryption.DecryptNullable(a.AsciiPreview) })
+                .ToList(),
+        };
         var lines = IrcMessageFormatter.FormatMessage(decryptedMessage);
 
         foreach (var conn in _gateway.GetConnectionsInChannel(channelName))

src/EchoHub.Server.Irc/IrcMessageFormatter.cs

@@ -1,7 +1,9 @@
 using System.Text;
 using System.Text.RegularExpressions;
+using EchoHub.Core.Contracts;
 using EchoHub.Core.DTOs;
 using EchoHub.Core.Models;
+using EchoHub.Core.Security;
 
 namespace EchoHub.Server.Irc;
 
@@ -34,7 +36,7 @@ public static partial class IrcMessageFormatter
                 {
                     case AttachmentKind.Image:
                         lines.Add($"{prefix} PRIVMSG {ircChannel} :[Image: {attachment.FileName}] {attachment.Url}");
-                        if (attachment.AsciiPreview is not null)
+                        if (attachment.AsciiPreview is not null && !IsCiphertext(attachment.AsciiPreview))
                         {
                             foreach (var line in attachment.AsciiPreview.Split('\n'))
                             {
@@ -66,6 +68,15 @@ public static partial class IrcMessageFormatter
         return lines;
     }
 
+    /// <summary>
+    /// True when a preview is still encrypted — transport ($ENC$v1$) if a broadcast path
+    /// forgot to decrypt it, or E2E room ciphertext ($RC1$) the server cannot decrypt.
+    /// Emitting it would flood IRC clients with a multi-KB base64 blob.
+    /// </summary>
+    private static bool IsCiphertext(string text) =>
+        text.StartsWith(IMessageEncryptionService.CiphertextPrefix, StringComparison.Ordinal)
+        || text.StartsWith(RoomCrypto.CiphertextPrefix, StringComparison.Ordinal);
+
     /// <summary>
     /// Format a link embed as IRC PRIVMSG lines (text-only, no ASCII thumbnail).
     /// </summary>

src/EchoHub.Server/Services/MessageEncryptionService.cs

@@ -8,7 +8,7 @@ namespace EchoHub.Server.Services;
 
 public class MessageEncryptionService : IMessageEncryptionService
 {
-    private const string EncryptionPrefix = "$ENC$v1$";
+    private const string EncryptionPrefix = IMessageEncryptionService.CiphertextPrefix;
     private const int NonceSizeBytes = 12;
     private const int TagSizeBytes = 16;
 

src/EchoHub.Tests/Irc/IrcBroadcasterTests.cs

@@ -89,6 +89,27 @@ public class IrcBroadcasterTests
         Assert.DoesNotContain(output, l => l.Contains("$ENC$"));
     }
 
+    [Fact]
+    public async Task SendMessage_DecryptsAttachmentAsciiPreview()
+    {
+        var (_, stream) = AddConnectionWithCapture("bob", "general");
+
+        var attachment = new AttachmentDto(
+            AttachmentKind.Image, "/api/files/abc", "photo.png", 1234,
+            _encryption.Encrypt("line1\nline2"));
+        var message = new MessageDto(
+            Guid.NewGuid(), _encryption.Encrypt("look at this"), "alice", null, "general",
+            DateTimeOffset.UtcNow, [attachment]);
+
+        await _broadcaster.SendMessageToChannelAsync("general", message);
+
+        var output = stream.GetOutputLines();
+        Assert.Contains(output, l => l.Contains("[Image: photo.png]"));
+        Assert.Contains(output, l => l.Contains("line1"));
+        Assert.Contains(output, l => l.Contains("line2"));
+        Assert.DoesNotContain(output, l => l.Contains("$ENC$"));
+    }
+
     [Fact]
     public async Task SendMessage_SkipsSender()
     {

src/EchoHub.Tests/Irc/IrcMessageFormatterTests.cs

@@ -138,6 +138,19 @@ public class IrcMessageFormatterTests
         Assert.Equal(2, asciiLines.Count);
     }
 
+    [Theory]
+    [InlineData("$ENC$v1$abc123$def456")] // transport ciphertext a broadcast path forgot to decrypt
+    [InlineData("$RC1$abc123def456")]     // E2E room ciphertext the server cannot decrypt
+    public void FormatMessage_ImageMessage_SkipsCiphertextPreview(string ciphertextPreview)
+    {
+        var msg = CreateImageMessage(ciphertextPreview, "photo.jpg", "https://example.com/photo.jpg");
+        var lines = IrcMessageFormatter.FormatMessage(msg);
+
+        // Only the [Image: ...] header line — never the ciphertext blob
+        Assert.Single(lines);
+        Assert.Contains("[Image: photo.jpg]", lines[0]);
+    }
+
     [Fact]
     public void FormatMessage_FileMessage_FormatsCorrectly()
     {