diff --git a/SVSim.EmulatedEntrypoint/Controllers/ToolController.cs b/SVSim.EmulatedEntrypoint/Controllers/ToolController.cs
index ea54700..6c67755 100644
--- a/SVSim.EmulatedEntrypoint/Controllers/ToolController.cs
+++ b/SVSim.EmulatedEntrypoint/Controllers/ToolController.cs
@@ -4,6 +4,7 @@ using SVSim.Database.Repositories.Viewer;
 using SVSim.EmulatedEntrypoint.Extensions;
 using SVSim.EmulatedEntrypoint.Models.Dtos.Requests;
 using SVSim.EmulatedEntrypoint.Models.Dtos.Responses;
+using SVSim.EmulatedEntrypoint.Services;
 
 namespace SVSim.EmulatedEntrypoint.Controllers;
 
@@ -11,11 +12,16 @@ public class ToolController : SVSimController
 {
     private readonly ILogger<ToolController> _logger;
     private readonly IViewerRepository _viewerRepository;
+    private readonly ShadowverseSessionService _sessionService;
 
-    public ToolController(ILogger<ToolController> logger, IViewerRepository viewerRepository)
+    public ToolController(
+        ILogger<ToolController> logger,
+        IViewerRepository viewerRepository,
+        ShadowverseSessionService sessionService)
     {
         _logger = logger;
         _viewerRepository = viewerRepository;
+        _sessionService = sessionService;
     }
 
     /// <summary>
@@ -43,6 +49,13 @@ public class ToolController : SVSimController
                      ?? await _viewerRepository.RegisterAnonymousViewer(udid);
 
         HttpContext.SetViewer(viewer);
+
+        // Pre-store the SID the client will compute and use for its very next request. After
+        // signup the client switches to SID-only headers (no UDID), so without this mapping the
+        // translation middleware can't decrypt the next body. Formula mirrors the decompiled
+        // Cute/Certification.SessionId getter — see ShadowverseSessionService.ComputeClientSessionId.
+        _sessionService.StoreSessionForViewer(viewer.Id, udid);
+
         _logger.LogInformation("Signup resolved for udid={Udid} → viewer_id={ViewerId}, short_udid={ShortUdid}.",
             udid, viewer.Id, viewer.ShortUdid);
 
diff --git a/SVSim.EmulatedEntrypoint/Services/ShadowverseSessionService.cs b/SVSim.EmulatedEntrypoint/Services/ShadowverseSessionService.cs
index 54a883b..0ed8612 100644
--- a/SVSim.EmulatedEntrypoint/Services/ShadowverseSessionService.cs
+++ b/SVSim.EmulatedEntrypoint/Services/ShadowverseSessionService.cs
@@ -1,9 +1,19 @@
 using System.Collections.Concurrent;
+using System.Globalization;
+using System.Security.Cryptography;
+using System.Text;
 
 namespace SVSim.EmulatedEntrypoint.Services;
 
 public class ShadowverseSessionService
 {
+    /// <summary>
+    /// Salt the client's <c>Cute/Cryptographer.MakeMd5</c> appends to every input before hashing.
+    /// Must match the decompiled client exactly — the server computes SIDs that the client
+    /// also computes locally for its outgoing request headers, and any mismatch breaks decrypt.
+    /// </summary>
+    private const string MakeMd5Salt = "r!I@ws8e5i=";
+
     private readonly ConcurrentDictionary<string, Guid> _sessionIdToUdid;
 
     public ShadowverseSessionService()
@@ -25,4 +35,30 @@ public class ShadowverseSessionService
     {
         _sessionIdToUdid.AddOrUpdate(sid, _ => udid, (_, _) => udid);
     }
+
+    /// <summary>
+    /// Replicates the client's <c>Cute/Certification.SessionId</c> getter:
+    /// <c>MakeMd5(viewerId.ToString() + udid.ToString("D"))</c>. Returned as lowercase hex.
+    /// The client computes this once after signup and sends it as the SID header on every
+    /// subsequent request — the server must produce the same value to map back to the UDID.
+    /// </summary>
+    public string ComputeClientSessionId(long viewerId, Guid udid)
+    {
+        string input = viewerId.ToString(CultureInfo.InvariantCulture)
+                     + udid.ToString("D")
+                     + MakeMd5Salt;
+        byte[] hash = MD5.HashData(Encoding.UTF8.GetBytes(input));
+        return Convert.ToHexString(hash).ToLowerInvariant();
+    }
+
+    /// <summary>
+    /// Pre-stores the SID→UDID mapping the client will use for its first SID-only request
+    /// after <c>/tool/signup</c>. Without this, the translation middleware can't decrypt the
+    /// next request body (no UDID header, no mapping, falls back to <c>Guid.Empty</c>).
+    /// </summary>
+    public void StoreSessionForViewer(long viewerId, Guid udid)
+    {
+        string sid = ComputeClientSessionId(viewerId, udid);
+        StoreUdidForSessionId(sid, udid);
+    }
 }
\ No newline at end of file
diff --git a/SVSim.UnitTests/Services/ShadowverseSessionServiceTests.cs b/SVSim.UnitTests/Services/ShadowverseSessionServiceTests.cs
new file mode 100644
index 0000000..4f6e511
--- /dev/null
+++ b/SVSim.UnitTests/Services/ShadowverseSessionServiceTests.cs
@@ -0,0 +1,38 @@
+using NUnit.Framework;
+using SVSim.EmulatedEntrypoint.Services;
+
+namespace SVSim.UnitTests.Services;
+
+public class ShadowverseSessionServiceTests
+{
+    /// <summary>
+    /// Fixture captured live from a fresh signup against this server. The client computed this
+    /// exact SID locally and sent it on the next /check/game_start request. Pinning the formula
+    /// here means any future refactor of <see cref="ShadowverseSessionService.ComputeClientSessionId"/>
+    /// that drifts from <c>Cute/Cryptographer.MakeMd5(viewerId + udid)</c> will fail this test
+    /// before the user discovers it as a decrypt failure on game_start.
+    /// </summary>
+    [Test]
+    public void ComputeClientSessionId_matches_captured_fixture()
+    {
+        var svc = new ShadowverseSessionService();
+        const long viewerId = 1;
+        var udid = new System.Guid("62747917-93bc-454c-abb4-ef423b3c9317");
+
+        string sid = svc.ComputeClientSessionId(viewerId, udid);
+
+        Assert.That(sid, Is.EqualTo("dc4aac79d35fe15dfb6262e0071bb03c"));
+    }
+
+    [Test]
+    public void StoreSessionForViewer_makes_sid_resolvable_to_udid()
+    {
+        var svc = new ShadowverseSessionService();
+        const long viewerId = 1;
+        var udid = new System.Guid("62747917-93bc-454c-abb4-ef423b3c9317");
+
+        svc.StoreSessionForViewer(viewerId, udid);
+
+        Assert.That(svc.GetUdidFromSessionId("dc4aac79d35fe15dfb6262e0071bb03c"), Is.EqualTo(udid));
+    }
+}