WatchedTogether/Jellyfin.Plugin.WatchedTogether/Auth/SharedAccountAuthenticationProvider.cs

using System;
using System.Threading.Tasks;
using Jellyfin.Database.Implementations.Entities;
using MediaBrowser.Controller.Authentication;
using MediaBrowser.Model.Cryptography;
using Microsoft.Extensions.Logging;

namespace Jellyfin.Plugin.WatchedTogether.Auth;

/// <summary>
/// Authenticates a shared account against the passwords of each of its members.
/// </summary>
/// <remarks>
/// <para>
/// Jellyfin selects a provider per user via <c>User.AuthenticationProviderId</c>, so this provider
/// only ever sees shared accounts that provisioning assigned to it. Implementing
/// <see cref="IRequiresResolvedUser"/> means Jellyfin hands us the already-resolved shared account
/// rather than us having to look it up by name.
/// </para>
/// <para>
/// Verification reads each member's stored hash directly instead of calling
/// <c>IUserManager.AuthenticateUser</c>. Going through the normal flow would trip every member's
/// failed-attempt counter each time a <em>different</em> member's password was the one that
/// matched, eventually locking out members who did nothing wrong. Reading the live hash also means
/// member password changes take effect immediately, with no second copy of any credential stored.
/// </para>
/// </remarks>
public class SharedAccountAuthenticationProvider : IAuthenticationProvider, IRequiresResolvedUser
{
    private readonly ICryptoProvider _cryptoProvider;
    private readonly Lazy<Services.IGroupService> _groupService;
    private readonly Lazy<Services.IDynamicGroupService> _dynamicGroupService;
    private readonly ILogger<SharedAccountAuthenticationProvider> _logger;

    /// <summary>
    /// Initializes a new instance of the <see cref="SharedAccountAuthenticationProvider"/> class.
    /// </summary>
    /// <param name="cryptoProvider">The crypto provider used to verify stored password hashes.</param>
    /// <param name="groupService">A deferred handle to the group service.</param>
    /// <param name="dynamicGroupService">A deferred handle to the on-demand group creation service.</param>
    /// <param name="logger">The logger.</param>
    /// <remarks>
    /// The group services are taken as <see cref="Lazy{T}"/> to break a container-level cycle.
    /// Jellyfin's <c>UserManager</c> constructor-injects every <see cref="IAuthenticationProvider"/>,
    /// so resolving those services eagerly here would require <c>IUserManager</c> while it is still
    /// being built and the host would refuse to start. Deferring the lookup to the first
    /// authentication is safe: nobody can log in until the host is fully up.
    /// </remarks>
    public SharedAccountAuthenticationProvider(
        ICryptoProvider cryptoProvider,
        Lazy<Services.IGroupService> groupService,
        Lazy<Services.IDynamicGroupService> dynamicGroupService,
        ILogger<SharedAccountAuthenticationProvider> logger)
    {
        _cryptoProvider = cryptoProvider;
        _groupService = groupService;
        _dynamicGroupService = dynamicGroupService;
        _logger = logger;
    }

    /// <inheritdoc />
    public string Name => "Watched Together Shared Account";

    /// <inheritdoc />
    public bool IsEnabled => true;

    /// <inheritdoc />
    /// <remarks>
    /// Jellyfin calls the <see cref="IRequiresResolvedUser"/> overload instead, so this exists only
    /// to satisfy the interface.
    /// </remarks>
    public Task<ProviderAuthenticationResult> Authenticate(string username, string password)
        => Authenticate(username, password, null);

    /// <inheritdoc />
    public async Task<ProviderAuthenticationResult> Authenticate(string username, string password, User? resolvedUser)
    {
        // Jellyfin passes a null user when no account matches the typed name, and offers the login
        // to every enabled provider. That is the hook for "type alice+bob and the account appears":
        // a real user with that exact name always resolves first and never reaches this branch.
        if (resolvedUser is null)
        {
            var created = await _dynamicGroupService.Value
                .TryCreateFromLoginAsync(username, password)
                .ConfigureAwait(false);

            if (created is null)
            {
                throw new AuthenticationException("Invalid username or password.");
            }

            return new ProviderAuthenticationResult { Username = created.SharedUsername };
        }

        var group = _groupService.Value.GetGroupForSharedUser(resolvedUser.Id);
        if (group is null)
        {
            // Either not one of ours, or the group is disabled. Either way this account has no
            // member passwords to check, so it cannot be unlocked.
            _logger.LogWarning(
                "Rejected login for {Username}: no enabled Watched Together group owns this account",
                resolvedUser.Username);
            throw new AuthenticationException("Invalid username or password.");
        }

        var members = _groupService.Value.GetEligibleMembers(group);
        if (members.Count == 0)
        {
            _logger.LogWarning(
                "Rejected login for {Username}: group has no eligible members",
                resolvedUser.Username);
            throw new AuthenticationException("Invalid username or password.");
        }

        foreach (var member in members)
        {
            if (!VerifyPassword(member, password))
            {
                continue;
            }

            _logger.LogInformation(
                "Shared account {SharedUsername} unlocked by member {MemberUsername}",
                resolvedUser.Username,
                member.Username);

            return new ProviderAuthenticationResult
            {
                Username = resolvedUser.Username
            };
        }

        _logger.LogWarning(
            "Rejected login for {Username}: no member password matched",
            resolvedUser.Username);
        throw new AuthenticationException("Invalid username or password.");
    }

    /// <inheritdoc />
    /// <remarks>
    /// A shared account always has a password in the sense that matters to Jellyfin: some member
    /// credential is required. Returning <c>false</c> would let clients offer a passwordless login.
    /// </remarks>
    public bool HasPassword(User user) => true;

    /// <inheritdoc />
    /// <remarks>
    /// Shared accounts have no password of their own to change - members change their own passwords
    /// in the normal way and the effect is picked up on the next login.
    /// </remarks>
    public Task ChangePassword(User user, string newPassword)
    {
        throw new NotSupportedException(
            "A Watched Together shared account has no password of its own. Members change their own passwords instead.");
    }

    /// <summary>
    /// Verifies a submitted password against a member's live stored hash.
    /// </summary>
    /// <param name="member">The member whose stored credential to check.</param>
    /// <param name="password">The submitted password.</param>
    /// <returns><c>true</c> if the password matches.</returns>
    private bool VerifyPassword(User member, string password)
    {
        if (string.IsNullOrEmpty(member.Password))
        {
            // A member with no password set cannot contribute a credential to the group.
            return false;
        }

        try
        {
            var hash = PasswordHash.Parse(member.Password);
            return _cryptoProvider.Verify(hash, password);
        }
        catch (Exception ex) when (ex is FormatException or ArgumentException)
        {
            // Never log the hash or the submitted password.
            _logger.LogError(
                ex,
                "Could not parse the stored password hash for member {MemberId}; skipping",
                member.Id);
            return false;
        }
    }
}