using System.Buffers.Text;
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;

namespace Fleet.Identity;

/// <summary>
/// 验证一次 WebAuthn 断言（<c>navigator.credentials.get</c> 的结果）。只支持 ES256（P-256 的 ECDSA），纯函数，只用 BCL：
/// 面板用它验证 passkey 登录，节点以后用它验证 passkey 签出的委托（docs/access.md 第 7 节）。
/// 不解析 CBOR——注册时的 attestation 解析只在 Backend 里。
/// </summary>
public static class WebAuthnAssertion
{
    public const byte FlagUserPresent = 0x01;
    public const byte FlagUserVerified = 0x04;

    /// <param name="publicKeySpki">注册时保存的凭证公钥（SPKI）。</param>
    /// <param name="expectedChallenge">这次请求里服务端或签名人规定的挑战原文（不是 base64url）。</param>
    /// <param name="originAllowed">校验 <c>clientDataJSON.origin</c>。</param>
    /// <param name="signCount">authenticatorData 里的签名计数器。</param>
    public static bool Verify(
        ReadOnlySpan<byte> publicKeySpki, ReadOnlySpan<byte> authenticatorData, ReadOnlySpan<byte> clientDataJson, ReadOnlySpan<byte> signature,
        string rpId, ReadOnlySpan<byte> expectedChallenge, Func<string, bool> originAllowed, bool requireUserVerification, out uint signCount)
    {
        signCount = 0;
        if (authenticatorData.Length < 37 || !CryptographicOperations.FixedTimeEquals(authenticatorData[..32], SHA256.HashData(Encoding.UTF8.GetBytes(rpId))))
        {
            return false;
        }

        byte flags = authenticatorData[32];
        if ((flags & FlagUserPresent) == 0 || (requireUserVerification && (flags & FlagUserVerified) == 0))
        {
            return false;
        }

        if (!ClientDataValid(clientDataJson, "webauthn.get", expectedChallenge, originAllowed))
        {
            return false;
        }

        try
        {
            using ECDsa key = ECDsa.Create();
            key.ImportSubjectPublicKeyInfo(publicKeySpki, out _);
            byte[] signed = [.. authenticatorData, .. SHA256.HashData(clientDataJson)];
            if (!key.VerifyData(signed, signature, HashAlgorithmName.SHA256, DSASignatureFormat.Rfc3279DerSequence))
            {
                return false;
            }
        }
        catch (CryptographicException)
        {
            return false;
        }

        signCount = (uint)((authenticatorData[33] << 24) | (authenticatorData[34] << 16) | (authenticatorData[35] << 8) | authenticatorData[36]);
        return true;
    }

    /// <summary><c>clientDataJSON</c> 的 type、challenge、origin 都要对得上。注册和登录共用。</summary>
    public static bool ClientDataValid(ReadOnlySpan<byte> clientDataJson, string expectedType, ReadOnlySpan<byte> expectedChallenge, Func<string, bool> originAllowed)
    {
        try
        {
            using JsonDocument document = JsonDocument.Parse(clientDataJson.ToArray());
            JsonElement root = document.RootElement;
            if (root.ValueKind != JsonValueKind.Object
                || root.GetProperty("type").GetString() != expectedType
                || root.GetProperty("origin").GetString() is not { } origin || !originAllowed(origin)
                || root.GetProperty("challenge").GetString() is not { } challenge)
            {
                return false;
            }

            byte[] decoded = Base64Url.DecodeFromChars(challenge);
            return CryptographicOperations.FixedTimeEquals(decoded, expectedChallenge);
        }
        catch (Exception ex) when (ex is JsonException or KeyNotFoundException or FormatException or InvalidOperationException)
        {
            return false;
        }
    }
}