using System.Security.Cryptography;
using Fleet.Identity;
using Fleet.Server;

namespace MoShiro.Backend.Application;

/// <summary>
/// 服务器的身份密钥（P-256）：操作员用 passkey 把公钥签进操作员集合，之后节点只认带这把密钥签名的服务器，
/// 并在这条链路上用应用层加密（TLS 在 Cloudflare 或被冒充的证书那里终止时，中间人拿不出这把密钥）。
/// 私钥存在 Backend 的设置里：它保护的是「面板被冒充」，不是「面板被攻破」；面板被攻破时它也会泄露，那时要由 passkey 签一个换掉它的新集合版本。
/// </summary>
public sealed class ServerIdentityService : IServerIdentity, IDisposable
{
    private const string Key = "server.identity_key";

    private readonly ISettingsStore _settings;
    private readonly object _gate = new();
    private ECDsa? _key;
    private byte[]? _spki;

    public ServerIdentityService(ISettingsStore settings)
    {
        _settings = settings;
    }

    public byte[] PublicKeySpki
    {
        get
        {
            EnsureLoaded();
            return _spki!;
        }
    }

    /// <summary>公钥的指纹（SHA-256，小写十六进制）：页面上给人核对。</summary>
    public string Fingerprint => Convert.ToHexStringLower(SHA256.HashData(PublicKeySpki));

    public byte[] Sign(ReadOnlySpan<byte> payload)
    {
        EnsureLoaded();
        return _key!.SignData(payload, HashAlgorithmName.SHA256, DSASignatureFormat.IeeeP1363FixedFieldConcatenation);
    }

    /// <summary>启动时调用：读出或生成密钥。节点可能在启动完成之前就连上来，所以 <see cref="Sign"/> 也会按需加载。</summary>
    public ValueTask EnsureAsync(CancellationToken ct)
    {
        EnsureLoaded(ct);
        return ValueTask.CompletedTask;
    }

    private void EnsureLoaded(CancellationToken ct = default)
    {
        if (_key is not null)
        {
            return;
        }

        lock (_gate)
        {
            if (_key is not null)
            {
                return;
            }

            string? stored = _settings.GetAsync(Key, ct).AsTask().GetAwaiter().GetResult();
            var key = ECDsa.Create(ECCurve.NamedCurves.nistP256);
            if (stored is not null)
            {
                key.ImportPkcs8PrivateKey(Convert.FromBase64String(stored), out _);
            }
            else
            {
                _settings.SetAsync(Key, Convert.ToBase64String(key.ExportPkcs8PrivateKey()), ct).AsTask().GetAwaiter().GetResult();
            }

            _spki = key.ExportSubjectPublicKeyInfo();
            _key = key;
        }
    }

    public void Dispose() => _key?.Dispose();
}