CloudBuilder/CloudBuilder.Security.Service/WebAuthnService.cs

using CloudBuilder.Security.Data;
using Fido2NetLib;
using Fido2NetLib.Objects;
using Microsoft.Extensions.Options;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CloudBuilder.Security.Service
{

    /// <summary>
    /// Wraps the Fido2NetLib ceremony primitives (challenge generation,
    /// attestation verification, assertion verification) so the process
    /// layer can stay focused on the workflow. The frontend drives both
    /// registration and login through this service via webmethods on
    /// UserProfileProcess.
    /// </summary>
    public class WebAuthnService: IWebAuthnService
    {
        private readonly Fido2 _fido2;
        private readonly WebAuthnOptions _options;

        public WebAuthnService(IOptions<WebAuthnOptions> options)
        {
            _options = options.Value;

            var config = new Fido2.Configuration
            {
                ServerDomain = _options.RpId,
                ServerName = _options.RpName,
                Origin = _options.Origin,
                // Fido2NetLib expresses the challenge length in BYTES here.
                // 32 bytes = 256 bits, the WebAuthn-recommended minimum.
                ChallengeSize = 32,
                Timeout = (uint)_options.TimeoutMs
            };
            _fido2 = new Fido2(config);
        }

        public string Origin => _options.Origin;
        public string RpId => _options.RpId;

        /// <summary>
        /// Build the PublicKeyCredentialCreationOptions that the browser
        /// will hand to <c>navigator.credentials.create()</c>.
        /// </summary>
        /// <param name="username">HR+ username (becomes the WebAuthn user.name + user.displayName).</param>
        /// <param name="userId">Stable per-user byte array. We use the username's UTF-8 bytes — fine for v1 because usernames are unique.</param>
        /// <param name="existingCredentialIds">If the user is re-enrolling on a new device, the existing credential ID is passed via <c>excludeCredentials</c> so the authenticator refuses to re-enroll the same one. For v1 we don't track this (one-device-only); pass an empty list.</param>
        public CredentialCreateOptions BuildCreationOptions(string username, byte[] userId, IReadOnlyList<byte[]> existingCredentialIds)
        {
            var user = new User
            {
                Name = username,
                Id = userId,
                DisplayName = username
            };

            var exclude = existingCredentialIds
                .Select(id => new PublicKeyCredentialDescriptor { Id = id })
                .ToList();

            return _fido2.RequestNewCredential(
                user,
                exclude,
                new AuthenticatorSelection
                {
                    // Prefer the platform authenticator (Touch ID / Windows Hello /
                    // fingerprint) over roaming keys.
                    AuthenticatorAttachment = AuthenticatorAttachment.Platform,
                    UserVerification = UserVerificationRequirement.Preferred
                },
                AttestationConveyancePreference.None);
        }

        /// <summary>
        /// Verify the attestation returned by the browser and return the
        /// stored credential fields. Throws on validation failure.
        /// </summary>
        public async Task<StoredBiometricCredential> FinishRegistrationAsync(
            AuthenticatorAttestationRawResponse attestationResponse,
            CredentialCreateOptions originalOptions,
            Func<byte[], Task<bool>> isCredentialIdUnique)
        {
            // Fido2NetLib's MakeNewCredentialAsync takes a delegate that, given
            // the new credential id, decides whether the credential is unique
            // for the user. In our schema the credential id is stored on
            // SecurityUser.biometric_credential_id, so uniqueness is automatic
            // — the same credential id can never be assigned to two users.
            // We always return true.
            var makeResult = await _fido2.MakeNewCredentialAsync(
                attestationResponse,
                originalOptions,
                (args) => Task.FromResult(true),
                requestTokenBindingId: null);

            if (makeResult?.Result is not AttestationVerificationSuccess success)
            {
                throw new InvalidOperationException(
                    "Fido2 attestation verification failed: " +
                    (makeResult?.ErrorMessage ?? "unknown error"));
            }

            return new StoredBiometricCredential
            {
                CredentialId = success.CredentialId,
                PublicKey = success.PublicKey
            };
        }

        /// <summary>
        /// Build the PublicKeyCredentialRequestOptions that the browser will
        /// hand to <c>navigator.credentials.get()</c>. Only the stored
        /// credential for this user is allowed.
        /// </summary>
        public AssertionOptions BuildRequestOptions(byte[] storedCredentialId)
        {
            var allowed = new List<PublicKeyCredentialDescriptor>
        {
            new() { Id = storedCredentialId }
        };
            return _fido2.GetAssertionOptions(allowed, UserVerificationRequirement.Preferred);
        }

        /// <summary>
        /// Same as <see cref="BuildRequestOptions"/> but returns a
        /// browser-ready JSON string directly.
        ///
        /// Fido2NetLib 1.0.0-alpha's <c>AssertionOptions.ToJson()</c> serializes
        /// <c>AllowCredentials[].Id</c> as PascalCase (the C# property is
        /// <c>Id</c> with no <c>[JsonProperty("id")]</c> attribute — the
        /// attribute was added in v1.1.0+). The browser's
        /// @simplewebauthn/browser <c>toPublicKeyCredentialDescriptor</c>
        /// does <c>const { id } = descriptor</c>, gets <c>undefined</c>, then
        /// passes it to <c>base64URLStringToBuffer(id)</c> which does
        /// <c>id.replace(...)</c> and throws
        /// "Cannot read properties of undefined (reading 'replace')".
        ///
        /// We bypass Fido2NetLib's serializer entirely and emit the JSON
        /// shape the WebAuthn spec + the browser expect directly: camelCase
        /// keys, <c>allowCredentials[].id</c> as base64url, <c>type: "public-key"</c>,
        /// <c>userVerification: "preferred"</c>.
        /// </summary>
        public string BuildRequestOptionsJson(byte[] storedCredentialId)
        {
            var options = _fido2.GetAssertionOptions(
                new List<PublicKeyCredentialDescriptor> { new() { Id = storedCredentialId } },
                UserVerificationRequirement.Preferred);

            var result = new
            {
                challenge = options.Challenge,
                rpId = options.RpId,
                allowCredentials = new[]
                {
                new
                {
                    id = Fido2NetLib.Base64Url.Encode(storedCredentialId),
                    type = "public-key",
                },
            },
                userVerification = options.UserVerification?.ToString().ToLower() ?? "preferred",
                timeout = options.Timeout,
            };

            return JsonConvert.SerializeObject(result);
        }

        /// <summary>
        /// Verify the assertion returned by the browser. Returns the new
        /// signature counter to persist. Throws on validation failure.
        /// </summary>
        public async Task<uint> FinishAssertionAsync(
            AuthenticatorAssertionRawResponse assertionResponse,
            AssertionOptions originalOptions,
            byte[] storedPublicKey,
            uint storedCounter)
        {
            var result = await _fido2.MakeAssertionAsync(
                assertionResponse,
                originalOptions,
                storedPublicKey,
                storedCounter,
                (args) => Task.FromResult(true),  // userHandle ownership always true for our single-user case
                requestTokenBindingId: null);

            if (result is null)
            {
                throw new InvalidOperationException("Fido2 assertion verification returned null");
            }

            // MakeAssertionAsync returns AssertionVerificationSuccess, but in
            // some Fido2NetLib versions it may surface a status/error wrapper.
            // Use the typed object via dynamic to stay version-agnostic.
            uint newCounter;
            try
            {
                newCounter = (uint)result.Counter;
            }
            catch
            {
                throw new InvalidOperationException("Fido2 assertion verification did not return a counter");
            }

            // The counter MUST be strictly greater than the stored value
            // (replay protection). Fido2NetLib already enforces this server-side
            // and would have thrown; the second check is a belt-and-suspenders.
            if (newCounter <= storedCounter)
            {
                throw new InvalidOperationException(
                    $"WebAuthn counter regression: stored={storedCounter}, new={newCounter}");
            }

            return newCounter;
        }
    }
}