XFESystem

using Org.BouncyCastle.Crypto.Prng;
using Org.BouncyCastle.Security;

namespace XFESystem.Tls;

/// <summary>The platform must reject unavailable entropy and untrustworthy UTC; no fallback is allowed.</summary>
public interface ITlsSecurityContext
{
    DateTime UtcNow { get; }
    void FillRandom(Span<byte> destination);
}

internal sealed class PlatformRandomGenerator(ITlsSecurityContext platform) : IRandomGenerator
{
    // Bouncy Castle may add supplemental public material. It must never replace
    // the independently seeded, platform-owned CSPRNG with this material.
    public void AddSeedMaterial(byte[] seed) { }
    public void AddSeedMaterial(ReadOnlySpan<byte> seed) { }
    public void AddSeedMaterial(long seed) { }
    public void NextBytes(byte[] bytes) => platform.FillRandom(bytes);
    public void NextBytes(byte[] bytes, int start, int len) => platform.FillRandom(bytes.AsSpan(start, len));
    public void NextBytes(Span<byte> bytes) => platform.FillRandom(bytes);
}

internal static class TlsSecurity
{
    public static DateTime ReadUtc(ITlsSecurityContext platform)
    {
        DateTime now = platform.UtcNow;
        ValidateUtc(now);
        return now;
    }

    internal static void ValidateUtc(DateTime now)
    {
        if (now.Kind != DateTimeKind.Utc || now < new DateTime(2024, 1, 1, 0, 0, 0, DateTimeKind.Utc))
            throw new InvalidOperationException("HTTPS requires a reliable UTC clock.");
    }

    public static SecureRandom CreateRandom(ITlsSecurityContext platform)
    {
        // Probe before the ClientHello. This is an availability check, not an
        // entropy estimate; the platform performs the actual source checks.
        Span<byte> probe = stackalloc byte[32];
        try { platform.FillRandom(probe); }
        finally { probe.Clear(); }
        return new SecureRandom(new PlatformRandomGenerator(platform));
    }
}