R3 two-factor sign-in: TOTP with recovery codes, asked for only after a correct password

Completion program R3 (docs/ROADMAP.md, 2026-10-17).

- Services/TwoFactor.cs: TOTP (RFC 6238, 30 s, 6 digits, SHA-1, one step of slack, Otp.NET 1.4.1) and ten recovery
  codes (SHA-256 kept, pulled when used). RootUser.Totp holds the secret, the last step taken (moved only by a
  conditional update, so a code works once even for two sign-ins at the same moment) and the codes' hashes.
- /clientapi/user/totp/setup (the password) answers the secret, its otpauth:// URI and a QR code as an SVG data URI
  (QRCoder 1.8.0); /totp/enable takes the app's first code, shows the recovery codes once and ends every other session
  (this device gets a new one); /totp/disable takes the password and a code. GET /clientapi/user/account says whether
  it is on and how many recovery codes are left.
- Sign-in (/clientapi/user/login, the invitation login, /oauth/login) asks for LoginForm.OneTimeCode with a 428, and
  only once the password was right; the OAuth page has the field. Recovery by email leaves two-factor on.
- PrivaPub admin reset-totp <root>: the way back for a login that lost its authenticator and its codes.
- scenarios/accounts.sh checks it all, with codes computed in Python; the OAuth page with its antiforgery token.

Verified: dotnet build PrivaPub.sln -c Release; the pasture (gts, mailpit): accounts 45/45.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WpVCoDeM1qUTRp4fXQdfof

PrivaPub/Services/TwoFactor.cs

@@ -0,0 +1,84 @@
+using MongoDB.Entities;
+
+using OtpNet;
+
+using PrivaPub.Models.User;
+
+using QRCoder;
+
+using System.Security.Cryptography;
+
+namespace PrivaPub.Services
+{
+	// Two-factor sign-in by TOTP (completion program R3): the authenticator app's six digits, a step of 30 seconds and one
+	// step of slack either way, each code taken once (RootTotp.LastStep, moved forward only by a conditional update, so two
+	// sign-ins with one code can't both pass); or one of ten recovery codes, each kept as its SHA-256 and pulled when used.
+	public static class TwoFactor
+	{
+		public const string CodeNeeded = "Enter the code from your authenticator app, or a recovery code.";
+		public const string CodeInvalid = "That code is not valid.";
+		public const string Issuer = "PrivaPub";
+		const string RecoveryAlphabet = "abcdefghjkmnpqrstuvwxyz23456789";//no 0/o, 1/l/i
+
+		public static string NewSecret() => Base32Encoding.ToString(KeyGeneration.GenerateRandomKey(20)).TrimEnd('=');
+
+		public static string OtpAuthUri(string secret, string userName) =>
+			$"otpauth://totp/{Uri.EscapeDataString($"{Issuer}:{userName}")}?secret={secret}&issuer={Issuer}&algorithm=SHA1&digits=6&period=30";
+
+		public static string QrDataUri(string text)
+		{
+			using var generator = new QRCodeGenerator();
+			using var data = generator.CreateQrCode(text, QRCodeGenerator.ECCLevel.M);
+			var svg = new SvgQRCode(data).GetGraphic(6);
+			return "data:image/svg+xml;base64," + Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(svg));
+		}
+
+		public static List<string> NewRecoveryCodes(int count = 10) =>
+			Enumerable.Range(0, count).Select(_ =>
+			{
+				var chars = new char[10];
+				for (var i = 0; i < chars.Length; i++)
+					chars[i] = RecoveryAlphabet[RandomNumberGenerator.GetInt32(RecoveryAlphabet.Length)];
+				return $"{new string(chars, 0, 5)}-{new string(chars, 5, 5)}";
+			}).ToList();
+
+		public static string Normalize(string code) => (code ?? string.Empty).Trim().Replace(" ", "").Replace("-", "").ToLowerInvariant();
+
+		public static string HashRecovery(string code) => RecoveryJob.Hash(Normalize(code));
+
+		/// <summary>The step a six-digit code matches in the secret's window, or null.</summary>
+		public static long? Step(string secret, string code)
+		{
+			var digits = Normalize(code);
+			if (digits.Length != 6 || !digits.All(char.IsAsciiDigit) || string.IsNullOrEmpty(secret))
+				return null;
+			var totp = new Totp(Base32Encoding.ToBytes(secret));
+			return totp.VerifyTotp(digits, out var step, VerificationWindow.RfcSpecifiedNetworkDelay) ? step : null;
+		}
+
+		/// <summary>Takes a code for a login with two-factor on: a fresh authenticator code, or an unused recovery code.</summary>
+		public static async Task<bool> Consume(RootUser user, string code, CancellationToken token)
+		{
+			if (user?.Totp is not { EnabledAt: not null } totp)
+				return false;
+			if (Step(totp.Secret, code) is { } step)
+			{
+				if (step <= totp.LastStep)
+					return false;//used already
+				var moved = await DB.Default.Update<RootUser>()
+					.Match(u => u.ID == user.ID && u.Totp.LastStep < step)
+					.Modify(u => u.Totp.LastStep, step)
+					.ExecuteAsync(token);
+				return moved.ModifiedCount == 1;
+			}
+			var hash = HashRecovery(code);
+			if (!totp.RecoveryCodeHashes.Contains(hash))
+				return false;
+			var pulled = await DB.Default.Update<RootUser>()
+				.Match(u => u.ID == user.ID && u.Totp.RecoveryCodeHashes.Contains(hash))
+				.Modify(b => b.Pull(u => u.Totp.RecoveryCodeHashes, hash))
+				.ExecuteAsync(token);
+			return pulled.ModifiedCount == 1;
+		}
+	}
+}