Rendezvous/src/FinalFactory.Rendezvous.Server/JoinAttempts/JoinAttemptCursorCodec.cs

using System.Security.Cryptography;
using System.Text.Json;
using System.Text.Json.Serialization;
using FinalFactory.Rendezvous.Contracts;
using FinalFactory.Rendezvous.Server.Browser;

namespace FinalFactory.Rendezvous.Server.JoinAttempts;

internal sealed class JoinAttemptCursorCodec : IDisposable
{
    private const string Prefix = "rvj1";
    private readonly EphemeralCursorProtector _protector = new();

    public string Encode(
        SessionListingId listingId,
        JoinAttemptId after,
        DateTimeOffset now)
    {
        JoinAttemptCursorPayload payload = new()
        {
            ListingId = listingId.ToString(),
            AfterAttemptId = after.ToString(),
            ExpiresAtUnixSeconds = now.AddMinutes(5).ToUnixTimeSeconds(),
        };
        byte[] encoded = JsonSerializer.SerializeToUtf8Bytes(payload, ContractJson.Options);
        try
        {
            return _protector.Protect(Prefix, encoded);
        }
        finally
        {
            CryptographicOperations.ZeroMemory(encoded);
        }
    }

    public bool TryDecode(
        string? cursor,
        SessionListingId listingId,
        DateTimeOffset now,
        out JoinAttemptId? after)
    {
        after = null;
        if (cursor is null)
        {
            return true;
        }

        if (!_protector.TryUnprotect(Prefix, cursor, out byte[] encodedPayload))
        {
            return false;
        }

        JoinAttemptCursorPayload? payload;
        try
        {
            payload = JsonSerializer.Deserialize<JoinAttemptCursorPayload>(
                encodedPayload,
                ContractJson.Options);
        }
        catch (JsonException)
        {
            payload = null;
        }
        finally
        {
            CryptographicOperations.ZeroMemory(encodedPayload);
        }

        if (payload is null
            || payload.ExpiresAtUnixSeconds <= now.ToUnixTimeSeconds()
            || !string.Equals(payload.ListingId, listingId.ToString(), StringComparison.Ordinal)
            || !JoinAttemptId.TryParse(payload.AfterAttemptId, out JoinAttemptId attemptId))
        {
            return false;
        }

        after = attemptId;
        return true;
    }

    public void Dispose() => _protector.Dispose();

    public override string ToString() => "[JoinAttemptCursorCodec: key and cursors redacted]";
}

internal sealed class JoinAttemptCursorPayload
{
    [JsonRequired]
    public string ListingId { get; set; } = string.Empty;

    [JsonRequired]
    public string AfterAttemptId { get; set; } = string.Empty;

    [JsonRequired]
    public long ExpiresAtUnixSeconds { get; set; }
}