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

Nature and Goals of mTOTP

• Presented as an early, experimental “human-computable TOTP” under strict mental constraints, not production crypto.
• Goal: avoid ever revealing the underlying secret to a device while also not relying on any electronic token that can be compromised.
• Some find the idea intellectually neat and worth exploring; others see it more as a fun puzzle than a practical security tool.

Is It Actually 2FA?

• Many argue this is not a second factor: it reduces entirely to “something you know” plus mental computation, i.e., a password variant.
• Counter‑view: “factor” is about what you must have/know at login time; a brain-stored secret that never leaves your head can still serve as a second factor alongside, say, a password manager or SSH key.
• Ongoing disagreement over whether clonability (you can tell someone the secret) disqualifies it as “something you have.”

Security Properties and Weaknesses

• Keyspace is small (~10 billion); commenters note that 2–3 observed codes with timestamps plus brute force can recover the secret.
• Human computation can’t use high-cost key derivation; compensating with longer, random secrets quickly becomes impractical to memorize.
• Suggestions include longer passphrases, larger wordlists, or multiple rotating keys, but rotation introduces sync and complexity issues.
• Some see value mainly against phishing / replay (no static secret entered), others say the reduced search space and server-side secret storage undercut that.

Human Constraints and Usability

• Mental math each login is seen as too demanding for most users; many would likely offload it to an app, defeating the concept and leaking the secret.
• Time-based nature assumes users roughly plan login time; several doubt this is realistic behavior.
• The 6th digit is identified as a checksum / self-check, not real extra security.

Comparison to Existing 2FA Methods

• TOTP itself is “password + time-based computation,” but with a separate device and short-lived output that mitigates some attacks (password reuse, replay).
• Debate over storing TOTP seeds in password managers: convenient but collapses factors if that device is compromised.
• Hardware tokens / secure enclaves are regarded as stronger for “something you have,” but less flexible and harder to back up.
• Biometrics are criticized as non-revocable, privacy-sensitive, and often effectively tied to a device anyway.

Threat Models and Philosophy

• Several argue that what’s “correct” depends on actual attack vectors: remote password stuffing vs device compromise vs coercion (“rubber-hose”).
• Some see mTOTP as offering marginal practical security; others value it as a thought experiment probing the limits of human-only authentication.