admin/Azaion.Services/RefreshTokenService.cs

using System.Security.Cryptography;
using System.Text;
using Azaion.Common;
using Azaion.Common.Configs;
using Azaion.Common.Database;
using Azaion.Common.Entities;
using LinqToDB;
using Microsoft.Extensions.Options;

namespace Azaion.Services;

/// <summary>
/// AZ-531 — issues, rotates, and validates opaque refresh tokens. Reuse-detection
/// kills the entire session family per OAuth 2.1 §6.1.
/// </summary>
public interface IRefreshTokenService
{
    /// <summary>
    /// Mint a fresh refresh token at login; starts a new session family. Returns
    /// the opaque token (NEVER persisted; only its sha256 lands in the DB) and
    /// the session row that backs it.
    /// </summary>
    Task<(string OpaqueToken, Session Session)> IssueForNewLogin(Guid userId, CancellationToken ct = default);

    /// <summary>
    /// Rotate <paramref name="opaqueToken"/>. On success returns the new token +
    /// the new session row. On reuse-detection or invalid token throws
    /// <see cref="BusinessException"/> with <see cref="ExceptionEnum.InvalidRefreshToken"/>;
    /// reuse also revokes every active row in the same family.
    /// </summary>
    Task<(string OpaqueToken, Session Session)> Rotate(string opaqueToken, CancellationToken ct = default);
}

public class RefreshTokenService(IDbFactory dbFactory, IOptions<SessionConfig> sessionConfig) : IRefreshTokenService
{
    private const int OpaqueTokenBytes = 32; // 256 bits → 43-char base64url string.

    private readonly SessionConfig _cfg = sessionConfig.Value;

    public async Task<(string OpaqueToken, Session Session)> IssueForNewLogin(Guid userId, CancellationToken ct = default)
    {
        var (opaque, hash) = GenerateToken();
        var now = DateTime.UtcNow;
        var session = new Session
        {
            Id              = Guid.NewGuid(),
            UserId          = userId,
            RefreshHash     = hash,
            FamilyId        = Guid.NewGuid(), // self-rooted family
            IssuedAt        = now,
            LastUsedAt      = now,
            ExpiresAt       = now.AddHours(_cfg.RefreshSlidingHours),
            FamilyStartedAt = now,
        };
        // family_id should equal id for the root row so SELECT family_id from
        // any row returns a stable handle even if id is renamed later.
        session.FamilyId = session.Id;

        await dbFactory.RunAdmin(async db => await db.InsertAsync(session, token: ct));
        return (opaque, session);
    }

    public async Task<(string OpaqueToken, Session Session)> Rotate(string opaqueToken, CancellationToken ct = default)
    {
        if (string.IsNullOrWhiteSpace(opaqueToken))
            throw new BusinessException(ExceptionEnum.InvalidRefreshToken);

        var hash = HashToken(opaqueToken);
        var now  = DateTime.UtcNow;

        return await dbFactory.RunAdmin(async db =>
        {
            // Use a serializable transaction so two concurrent refreshes can't both
            // observe the row as un-rotated and both succeed.
            await using var tx = await db.BeginTransactionAsync(System.Data.IsolationLevel.Serializable, ct);

            var current = await db.Sessions.FirstOrDefaultAsync(s => s.RefreshHash == hash, token: ct);
            if (current == null)
                throw new BusinessException(ExceptionEnum.InvalidRefreshToken);

            // Reuse detection: presenting an already-rotated token kills the family.
            if (current.RevokedAt.HasValue)
            {
                if (current.RevokedReason == SessionRevokedReasons.Rotated)
                {
                    await db.Sessions
                        .Where(s => s.FamilyId == current.FamilyId && s.RevokedAt == null)
                        .Set(s => s.RevokedAt,     now)
                        .Set(s => s.RevokedReason, SessionRevokedReasons.ReuseDetected)
                        .UpdateAsync(token: ct);
                    await tx.CommitAsync(ct);
                }
                throw new BusinessException(ExceptionEnum.InvalidRefreshToken);
            }

            // Sliding expiry — each rotation restarts the window from `now`.
            if (current.ExpiresAt < now)
                throw new BusinessException(ExceptionEnum.InvalidRefreshToken);

            // Absolute expiry — the family cannot live past this regardless of rotations.
            if ((now - current.FamilyStartedAt).TotalHours > _cfg.RefreshAbsoluteHours)
                throw new BusinessException(ExceptionEnum.InvalidRefreshToken);

            var (newOpaque, newHash) = GenerateToken();
            var newSession = new Session
            {
                Id              = Guid.NewGuid(),
                UserId          = current.UserId,
                RefreshHash     = newHash,
                FamilyId        = current.FamilyId,
                IssuedAt        = now,
                LastUsedAt      = now,
                ExpiresAt       = now.AddHours(_cfg.RefreshSlidingHours),
                FamilyStartedAt = current.FamilyStartedAt,
                ParentSessionId = current.Id,
            };

            await db.Sessions
                .Where(s => s.Id == current.Id && s.RevokedAt == null)
                .Set(s => s.RevokedAt,     now)
                .Set(s => s.RevokedReason, SessionRevokedReasons.Rotated)
                .Set(s => s.LastUsedAt,    now)
                .UpdateAsync(token: ct);

            await db.InsertAsync(newSession, token: ct);
            await tx.CommitAsync(ct);
            return (newOpaque, newSession);
        });
    }

    private static (string Opaque, string Hash) GenerateToken()
    {
        var raw = RandomNumberGenerator.GetBytes(OpaqueTokenBytes);
        var opaque = Base64Url(raw);
        var hash = HashToken(opaque);
        return (opaque, hash);
    }

    private static string HashToken(string opaque)
    {
        var bytes = Encoding.ASCII.GetBytes(opaque);
        var digest = SHA256.HashData(bytes);
        return Convert.ToHexString(digest);
    }

    private static string Base64Url(byte[] bytes) =>
        Convert.ToBase64String(bytes).TrimEnd('=').Replace('+', '-').Replace('/', '_');
}