SocialPub/PrivaPub/Api/Mastodon/Auth/PersonaExchange.cs

using Microsoft.IdentityModel.Tokens;

using OpenIddict.Abstractions;
using OpenIddict.Server;

using PrivaPub.Services;
using PrivaPub.StaticServices;

using System.Security.Claims;

using static OpenIddict.Abstractions.OpenIddictConstants;
using static OpenIddict.Server.OpenIddictServerEvents;

namespace PrivaPub.Api.Mastodon.Auth
{
	// One sign-in for the first-party client (decePubClient): it signs in on /clientapi, then exchanges that root JWT for one
	// persona's Mastodon token (RFC 8693, grant_type urn:ietf:params:oauth:grant-type:token-exchange, subject_token_type
	// urn:ietf:params:oauth:token-type:jwt, avatar_id). Only the seeded first-party application holds the grant permission,
	// and the issued token is the same kind the authorization code flow issues: subject the avatar, never the root.
	public static class PersonaExchange
	{
		public const string SubjectTokenType = "urn:ietf:params:oauth:token-type:jwt";
		public const string AvatarParameter = "avatar_id";
		//the root the subject token belongs to; it lives on the subject token's principal only, which is never stored or issued
		public const string RootClaim = "privapub_root";

		public static string FirstPartyClientId(IConfiguration configuration) =>
			configuration["ClientApi:FirstPartyClientId"] is { Length: > 0 } clientId ? clientId : "decepub";

		//the first-party application: public (a browser keeps no secret), token exchange only, no redirect
		public static async Task EnsureFirstPartyClient(IServiceProvider services, CancellationToken token)
		{
			using var scope = services.CreateScope();
			var applications = scope.ServiceProvider.GetRequiredService<IOpenIddictApplicationManager>();
			var clientId = FirstPartyClientId(scope.ServiceProvider.GetRequiredService<IConfiguration>());
			var descriptor = new OpenIddictApplicationDescriptor
			{
				ClientId = clientId,
				ClientType = ClientTypes.Public,
				ConsentType = ConsentTypes.Implicit,
				DisplayName = "decePubClient",
				Permissions =
				{
					Permissions.Endpoints.Token,
					Permissions.Endpoints.Revocation,
					Permissions.GrantTypes.TokenExchange
				}
			};
			foreach (var scopeName in new[] { "read", "write", "follow" })
				descriptor.Permissions.Add(Permissions.Prefixes.Scope + scopeName);

			var existing = await applications.FindByClientIdAsync(clientId, token);
			if (existing == default)
			{
				await applications.CreateAsync(descriptor, token);
				return;
			}
			await applications.PopulateAsync(existing, descriptor, token);
			await applications.UpdateAsync(existing, token);
		}
	}

	// Validates a root JWT given as a token exchange subject token, before OpenIddict tries it as one of its own tokens.
	// The principal it builds names the root under PersonaExchange.RootClaim for TokenController, which signs in a fresh
	// identity for the avatar.
	public sealed class RootJwtSubjectToken : IOpenIddictServerHandler<ValidateTokenContext>
	{
		public static OpenIddictServerHandlerDescriptor Descriptor { get; } = OpenIddictServerHandlerDescriptor.CreateBuilder<ValidateTokenContext>()
			.UseScopedHandler<RootJwtSubjectToken>()
			.SetOrder(OpenIddictServerHandlers.Protection.ValidateIdentityModelToken.Descriptor.Order - 500)
			.SetType(OpenIddictServerHandlerType.Custom)
			.Build();

		readonly IConfiguration _configuration;
		readonly DbEntities _dbEntities;

		public RootJwtSubjectToken(IConfiguration configuration, DbEntities dbEntities)
		{
			_configuration = configuration;
			_dbEntities = dbEntities;
		}

		public async ValueTask HandleAsync(ValidateTokenContext context)
		{
			if (context.ValidTokenTypes.Count != 1 || !context.ValidTokenTypes.Contains(PersonaExchange.SubjectTokenType))
				return;

			var root = await RootJwt.Root(context.Token, _configuration, _dbEntities, context.Transaction.CancellationToken);
			if (root == default)
			{
				context.Reject(Errors.InvalidGrant, "The session token is not valid.");
				return;
			}

			var identity = new ClaimsIdentity(TokenValidationParameters.DefaultAuthenticationType, Claims.Name, Claims.Role);
			identity.SetClaim(PersonaExchange.RootClaim, root.ID);
			var principal = new ClaimsPrincipal(identity)
				.SetTokenType(PersonaExchange.SubjectTokenType)
				.SetCreationDate(DateTimeOffset.UtcNow);
			if (context.ValidPresenters.Count > 0)
				principal.SetPresenters(context.ValidPresenters);
			context.Principal = principal;
		}
	}
}