feat: implement scoped join attempts and tickets (#10)

Closes #10

src/FinalFactory.Rendezvous.Server/Browser/SessionBrowserCursorCodec.cs

@@ -1,5 +1,4 @@
 using System.Security.Cryptography;
-using System.Text;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using FinalFactory.Rendezvous.Contracts;
@@ -10,12 +9,10 @@ namespace FinalFactory.Rendezvous.Server.Browser;
 internal sealed class SessionBrowserCursorCodec : IDisposable
 {
     private const string Prefix = "rvc1";
-    private readonly byte[] _key = RandomNumberGenerator.GetBytes(32);
-    private bool _disposed;
+    private readonly EphemeralCursorProtector _protector = new();
 
     public string Encode(VisibleListingQuery query, SessionListingId after, DateTimeOffset now)
     {
-        ObjectDisposedException.ThrowIf(_disposed, this);
         BrowserCursorPayload payload = new()
         {
             GameId = query.Scope.GameId.Value,
@@ -26,19 +23,14 @@ internal sealed class SessionBrowserCursorCodec : IDisposable
             AfterListingId = after.ToString(),
             ExpiresAtUnixSeconds = now.AddMinutes(5).ToUnixTimeSeconds(),
         };
-        string encoded = EncodeBytes(JsonSerializer.SerializeToUtf8Bytes(payload, ContractJson.Options));
-        string content = $"{Prefix}.{encoded}";
-        byte[] signature = HMACSHA256.HashData(_key, Encoding.ASCII.GetBytes(content));
+        byte[] encoded = JsonSerializer.SerializeToUtf8Bytes(payload, ContractJson.Options);
         try
         {
-            string cursor = $"{content}.{EncodeBytes(signature)}";
-            return ContractValidation.IsCursorValid(cursor)
-                ? cursor
-                : throw new InvalidOperationException("The browser cursor exceeds its contract limit.");
+            return _protector.Protect(Prefix, encoded);
         }
         finally
         {
-            CryptographicOperations.ZeroMemory(signature);
+            CryptographicOperations.ZeroMemory(encoded);
         }
     }
 
@@ -57,31 +49,7 @@ internal sealed class SessionBrowserCursorCodec : IDisposable
             return true;
         }
 
-        if (_disposed || !ContractValidation.IsCursorValid(cursor))
-        {
-            return false;
-        }
-
-        string[] segments = cursor.Split('.');
-        if (segments.Length != 3 || !string.Equals(segments[0], Prefix, StringComparison.Ordinal))
-        {
-            return false;
-        }
-
-        byte[] expected = HMACSHA256.HashData(
-            _key,
-            Encoding.ASCII.GetBytes($"{segments[0]}.{segments[1]}"));
-        if (!TryDecodeBytes(segments[2], out byte[] supplied))
-        {
-            CryptographicOperations.ZeroMemory(expected);
-            return false;
-        }
-
-        bool validSignature = supplied.Length == expected.Length
-            && CryptographicOperations.FixedTimeEquals(supplied, expected);
-        CryptographicOperations.ZeroMemory(supplied);
-        CryptographicOperations.ZeroMemory(expected);
-        if (!validSignature || !TryDecodeBytes(segments[1], out byte[] encodedPayload))
+        if (!_protector.TryUnprotect(Prefix, cursor, out byte[] encodedPayload))
         {
             return false;
         }
@@ -118,64 +86,30 @@ internal sealed class SessionBrowserCursorCodec : IDisposable
         return true;
     }
 
-    public void Dispose()
-    {
-        if (!_disposed)
-        {
-            _disposed = true;
-            CryptographicOperations.ZeroMemory(_key);
-        }
-    }
+    public void Dispose() => _protector.Dispose();
 
     public override string ToString() => "[SessionBrowserCursorCodec: key and cursors redacted]";
-
-    private static string EncodeBytes(ReadOnlySpan<byte> bytes) => Convert
-        .ToBase64String(bytes)
-        .TrimEnd('=')
-        .Replace('+', '-')
-        .Replace('/', '_');
-
-    private static bool TryDecodeBytes(string value, out byte[] bytes)
-    {
-        bytes = [];
-        if (string.IsNullOrEmpty(value)
-            || value.Any(static character =>
-                character is not (>= 'A' and <= 'Z')
-                and not (>= 'a' and <= 'z')
-                and not (>= '0' and <= '9')
-                and not '-'
-                and not '_'))
-        {
-            return false;
-        }
-
-        string padded = value.Replace('-', '+').Replace('_', '/');
-        padded += (padded.Length % 4) switch { 0 => "", 2 => "==", 3 => "=", _ => "!" };
-        try
-        {
-            bytes = Convert.FromBase64String(padded);
-            return true;
-        }
-        catch (FormatException)
-        {
-            return false;
-        }
-    }
 }
 
 internal sealed class BrowserCursorPayload
 {
     [JsonRequired]
     public string GameId { get; set; } = string.Empty;
+
     [JsonRequired]
     public string EnvironmentId { get; set; } = string.Empty;
+
     [JsonRequired]
     public uint ProtocolVersion { get; set; }
+
     public string? RegionId { get; set; }
+
     [JsonRequired]
     public bool ExcludeFull { get; set; }
+
     [JsonRequired]
     public string AfterListingId { get; set; } = string.Empty;
+
     [JsonRequired]
     public long ExpiresAtUnixSeconds { get; set; }
 }