MTOTP: Wouldn't it be nice if you were the 2FA device?

Summary

mTOTP It takes a special kind of geek to not carry a 2FA device. One who becomes the 2FA. mTOTP is an experimental, manual variant of TOTP designed to be computed by a human without electronic devices. It explores the limits of time-based authentication under strict human constraints and makes no claims of cryptographic equivalence to standard TOTP. Table of Contents Introduction mTOTP is a human‑executable OTP scheme designed to be: deterministic mentally doable (with practice) auditable and explainable reproducible by both humans and software This protocol intentionally allows OTPs to be calculated for future times. Rather than treating this as a limitation, it makes it a requirement: the user must know when they intend to authenticate, and the verifier checks against that agreed moment. Time is therefore not an approximation, but an explicit part of the protocol - Turning authentication time from reactive to intentional. This document describes the exact algorithm used by the tool, written for humans first. This protocol is designed for human execution first, with software acting as a helper and verifier. Clarity, determinism, and mental tractability are intentional design goals. Demo PAM demo Overview An mTOTP is generated from: a secret numeric key a planned login time The algorithm uses: a key‑derived digit S‑box digitwise modular arithmetic a simple diffusion step a deterministic fold into a 6‑digit OTP No randomness is involved during generation. Example Inputs Secret key (10 digits): 1234598760 (If your key is shorter than 10 digits, pad or derive it consistently before use.) Planned login time: 2026‑01‑17 17:00 Step 1 - Build Time Vector Convert the planned login time into the format, take into account that you are calculating for the server-side set time: YYMMDDHHMM Example: 2026‑01‑17 17:00 → 2601171700 Result: T = 2601171700 Step 2 - Build Sbox from the Secret Key The S‑box is a digit substitution table (0–9 → 0–9) derived only from the secret key. S-bo...