MoShiro/backend/src/MoShiro.Backend.Application/Auth/WebAuthnRegistration.cs

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

namespace MoShiro.Backend.Application;

/// <summary>注册成功后要保存的凭证。</summary>
public sealed record RegisteredCredential(byte[] CredentialId, byte[] PublicKeySpki, uint SignCount);

/// <summary>
/// 解析并校验 WebAuthn 注册（<c>navigator.credentials.create</c> 的结果）。只接受 ES256；attestation 一律当 <c>none</c> 处理：
/// 不验设备证明，信任浏览器的实现（docs/access.md）。CBOR 解析只在 Backend 里，节点不带。
/// </summary>
public static class WebAuthnRegistration
{
    public static RegisteredCredential? Verify(
        ReadOnlySpan<byte> attestationObject, ReadOnlySpan<byte> clientDataJson, string rpId, ReadOnlySpan<byte> expectedChallenge,
        Func<string, bool> originAllowed, bool requireUserVerification)
    {
        if (!WebAuthnAssertion.ClientDataValid(clientDataJson, "webauthn.create", expectedChallenge, originAllowed))
        {
            return null;
        }

        try
        {
            if (new MiniCbor(attestationObject.ToArray()).ReadValue() is not Dictionary<object, object> attestation
                || attestation.GetValueOrDefault("authData") is not byte[] authData || authData.Length < 55)
            {
                return null;
            }

            if (!CryptographicOperations.FixedTimeEquals(authData.AsSpan(0, 32), SHA256.HashData(Encoding.UTF8.GetBytes(rpId))))
            {
                return null;
            }

            byte flags = authData[32];
            const byte attestedData = 0x40;
            if ((flags & WebAuthnAssertion.FlagUserPresent) == 0 || (flags & attestedData) == 0 || (requireUserVerification && (flags & WebAuthnAssertion.FlagUserVerified) == 0))
            {
                return null;
            }

            uint signCount = (uint)((authData[33] << 24) | (authData[34] << 16) | (authData[35] << 8) | authData[36]);
            int idLength = (authData[53] << 8) | authData[54];
            if (idLength is 0 or > 1023 || authData.Length < 55 + idLength)
            {
                return null;
            }

            byte[] credentialId = authData[55..(55 + idLength)];
            var reader = new MiniCbor(authData[(55 + idLength)..]);
            if (reader.ReadValue() is not Dictionary<object, object> cose
                || cose.GetValueOrDefault(1L) is not long kty || kty != 2
                || cose.GetValueOrDefault(3L) is not long alg || alg != -7
                || cose.GetValueOrDefault(-1L) is not long curve || curve != 1
                || cose.GetValueOrDefault(-2L) is not byte[] { Length: 32 } x
                || cose.GetValueOrDefault(-3L) is not byte[] { Length: 32 } y)
            {
                return null;
            }

            using ECDsa key = ECDsa.Create(new ECParameters { Curve = ECCurve.NamedCurves.nistP256, Q = new ECPoint { X = x, Y = y } });
            return new RegisteredCredential(credentialId, key.ExportSubjectPublicKeyInfo(), signCount);
        }
        catch (Exception ex) when (ex is InvalidDataException or CryptographicException or ArgumentException or IndexOutOfRangeException)
        {
            return null;
        }
    }
}

/// <summary>够用的 CBOR 读取器（RFC 8949）：整数、字节串、文本串、数组、映射、布尔和 null。输入有上限，嵌套有深度限制。</summary>
internal sealed class MiniCbor
{
    private const int MaxDepth = 8;
    private const int MaxItems = 64;

    private readonly byte[] _data;
    private int _position;

    public MiniCbor(byte[] data)
    {
        _data = data;
    }

    public object? ReadValue(int depth = 0)
    {
        if (depth > MaxDepth || _position >= _data.Length)
        {
            throw new InvalidDataException("cbor too deep or truncated");
        }

        byte initial = _data[_position++];
        int major = initial >> 5;
        long argument = ReadArgument(initial & 0x1F);
        switch (major)
        {
            case 0:
                return argument;
            case 1:
                return -1 - argument;
            case 2:
                return Take(argument);
            case 3:
                return Encoding.UTF8.GetString(Take(argument));
            case 4:
                {
                    CheckCount(argument);
                    var list = new List<object?>();
                    for (long i = 0; i < argument; i++)
                    {
                        list.Add(ReadValue(depth + 1));
                    }

                    return list;
                }

            case 5:
                {
                    CheckCount(argument);
                    var map = new Dictionary<object, object>();
                    for (long i = 0; i < argument; i++)
                    {
                        object key = ReadValue(depth + 1) ?? throw new InvalidDataException("null map key");
                        map[key] = ReadValue(depth + 1)!;
                    }

                    return map;
                }

            case 7:
                return initial switch { 0xF4 => false, 0xF5 => true, 0xF6 => null, _ => throw new InvalidDataException("unsupported simple value") };
            default:
                throw new InvalidDataException("unsupported cbor type");
        }
    }

    private long ReadArgument(int info)
    {
        if (info < 24)
        {
            return info;
        }

        int length = info switch { 24 => 1, 25 => 2, 26 => 4, 27 => 8, _ => throw new InvalidDataException("indefinite lengths are not supported") };
        byte[] bytes = Take(length);
        long value = 0;
        foreach (byte b in bytes)
        {
            value = (value << 8) | b;
        }

        return value < 0 ? throw new InvalidDataException("argument too large") : value;
    }

    private byte[] Take(long length)
    {
        if (length < 0 || length > _data.Length - _position)
        {
            throw new InvalidDataException("cbor truncated");
        }

        byte[] result = _data[_position..(_position + (int)length)];
        _position += (int)length;
        return result;
    }

    private static void CheckCount(long count)
    {
        if (count > MaxItems)
        {
            throw new InvalidDataException("cbor collection too large");
        }
    }
}