[crote]

What makes this 2FA? It's "something you know, plus mental labor", which makes it a password.

2FA is "something you have" (or ".. you are", for biometrics): it is supposed to prove that you currently physically posses the single copy of a token. The textbook example is a TOTP stored in a Yubikey.

Granted, this has been watered down a lot by the way-too-common practice of storing TOTP secrets in password managers, but that's how it is supposed to work.

Does your mTOTP prove you own the single copy? No, you could trivially tell someone else the secret key. Does it prove that you currently own it? No, you can pre-calculate a verification token for future use.

I still think it is a very neat idea on paper, but I'm not quite seeing the added value. The obvious next step is to do all the math in client-side code and just have the user enter the secret - doing this kind of mental math every time you log in is something only the most hardcore nerds get excited about.

[fxj]

TOTP is also just password + some computation. So where is the difference? There is a lot of security theatre around TOTP with the QR code and then need of an app but you can write a 8 liner in python that does the same when you extract the password out of the QR code.

   import base64
   import hmac
   import struct
   import time

   def totp(key, time_step=30, digits=6, digest='sha1'):
        key = base64.b32decode(key.upper() + '=' \* ((8 - len(key)) % 8))
        counter = struct.pack('>Q', int(time.time() / time_step))
        mac = hmac.new(key, counter, digest).digest()
        offset = mac[-1] & 0x0f
        binary = struct.unpack('>L', mac[offset:offset+4])[0] & 0x7fffffff
        return str(binary)[-digits:].zfill(digits)

https://dev.to/yusadolat/understanding-totp-what-really-happ...

[elderlybanana]

Yes, TOTP is a secret + computation, and generating it is trivial once you have the secret. The security difference is that the TOTP secret is separate from the user’s password and the output is short-lived. Each of the two factors address different threat models.

[crote]

You are supposed to store the password in a Secure Enclave, which you can only query for the current token value. You are also supposed to immediately destroy the QR code after importing it.

As I already mentioned, the fact that people often use it wrong undermines its security, but that doesn't change the intended outcome.

[gruez]

>You are supposed to store the password in a Secure Enclave,

That's at best a retcon, given given that the RFC was first published in 2008

>You are also supposed to immediately destroy the QR code after importing it.

Most TOTP apps support backups/restores, which defeats this.

[craftkiller]

> That's at best a retcon, given given that the RFC was first published in 2008

How so? Apple didn't invent the idea of a secure enclave. Here is a photo of one such device, similar to one I was issued for work back in ~2011: https://webobjects2.cdw.com/is/image/CDW/1732119

No option to get the secret key out. All you can get out is the final TOTP codes. If anything, having an end-user-programmable "secure enclave" is the only thing that has changed.

I think they probably meant "Secure Enclave" in the same way that people say "band-aid" instead of "adhesive bandage", "velcro" instead of "hook and loop fastener", and "yubikey" instead of "hardware security token".

[aja12]

>Most TOTP apps support backups/restores, which defeats this.

Citation needed? Yubico authenticator doesn't (the secure enclave is the Yubikey). I'd be very surprised if MS Authenticator and Authy (which I don't use but are the most popular apps that I know of) support such backups

[Rubberducky1324]

> Citation needed? Yubico authenticator doesn't (the secure enclave is the Yubikey). I'd be very surprised if MS Authenticator and Authy (which I don't use but are the most popular apps that I know of) support such backups

Google Authenticator has an export option that I've used in the past, so that one does it for sure. Authy allows cloud-based synchronization in any case, so exporting seems quite possible. MS Authenticator also allow cloud sync, so probably exporting is not difficult.

[aja12]

> cloud-based synchronization

Well I don't disagree that it might be possible to abuse cloud sync in some way to export the secrets, but it's not quite as egregious as just including the secrets by default in an app backup

Not perfect, but (imho) still better than SMS 2FA, mail 2FA, or lack of 2FA

[alt227]

IMO if it is possible to use a system wrongly which undermines its security, it is already broken.

[TeMPOraL]

On the contrary - perfect security is only possible if your system is an inert rock. Or not even then, as the users could still use the rock "wrong" by beating security maximalists over their heads with it.

Also honestly TIL that TOTP are somehow supposed to also enforce a single copy of the backing token being in existence. That's not just bad UX, that feels closer to security overreach.

People in tech, especially software and security folks, tend to miss the fact that most websites with 2FA already put a heavier security burden on their users than anything else in real life. There's generally no other situation in peoples' lives that would require you to safely store for years a document that cannot be recovered or replaced when destroyed[0]. 2FA backup codes have much stricter security standard than any government ID!

And then security people are surprised there's so much pushback on passkeys.

--

[0] - The problem really manifest when you add lack of any kind of customer support willing to or capable of resolving account access issues.

[lmz]

This is how we get sites that block software tokens and only allow a whitelist of hardware based tokens.

[otabdeveloper4]

You mean "hardware based token", because of course we only know of only one brand and specific make.

[lazide]

There is no system which cannot be used wrongly in a way which undermines it’s security.

[alt227]

OP:

> the fact that people often use it wrong undermines its security

[lazide]

Yes, that is what I am replying too.

That applies to everything.

[Jean-Papoulos]

I can chuck a brick at your head. Clearly the brick is broken

[alt227]

Breaks are meant to be built with, not thrown at heads.

If you build with the brick properly you will have a great wall, if you dont then it will fall down. Pretty simple.

[dpoloncsak]

Pass-The-Hash attacks exist and the only real countermeasure is to never log into user machines with privileged credentials

[aja12]

Actually, the real countermeasure to PTH is to disable NTLM auth and rely only on Kerberos (and then monitor NTLM as a very strong indicator that someone or something is attempting PTH)

Of course kerberos tickets can be abused too in a lot of fun ways, but on a modern network PTH is pretty much dead and a surefire way to raise a lot of alerts

(You are absolutely right that privileged accounts must never login on less privileged assets, however!)

[dpoloncsak]

Yeah...we just went through this process over here. I was more just making the point that "If its possible to use a system wrongly which undermines its security, it is already broken" isn't always true. I guess you could argue its NTLM there thats 'already broken', but the idea was more "SysAdmins are sometimes given red buttons to never press under any circumstances."

[justincormack]

I mean, TOTP is one of the earliest 2 factor systems, and works least well.

[Ferret7446]

Exactly, which is why TOTP is "weak". "Real" 2FA like FIDO on a security key makes it much harder.

[ACCount37]

TOTP is the "good enough" 2FA.

If I managed to intercept a login, a password and a TOTP key from a login session, I can't use them to log in. Simply because TOTP expires too quickly.

That's the attack surface TOTP covers - it makes stealing credentials slightly less trivial by making one of the credentials ephemeral.

[alphager]

The 30 seconds (+30-60 seconds to account for clock drift) are long enough to exploit.

TOTP is primarily a defense against password reuse (3rd party site gets popped and leaks passwords, thanks to TOTP my site isn't overrun by adversaries) and password stuffing attacks.

[vel0city]

In every system I've worked on recent successful TOTPs have been cached as well to validate they're not used more than once.

[vel0city]

In fact, re-reading RFC 6238 it states:

   Note that a prover may send the same OTP inside a given time-step
   window multiple times to a verifier.  The verifier MUST NOT accept
   the second attempt of the OTP after the successful validation has
   been issued for the first OTP, which ensures one-time only use of an
   OTP.

https://datatracker.ietf.org/doc/html/rfc6238

Assuming your adversary isn't actually directly impersonating you but simply gets the result from the successful attempt a few seconds later, the OTP should be invalid, being a one time password and all.

[susam]

Original source of the 8 liner Python code: https://github.com/susam/mintotp/blob/main/mintotp.py

[az09mugen]

Thanks for the link on TOTP and the associated code !

[ulrikrasmussen]

In practice most TOTP implementation also do not prove that you have a device which is the sole owner of the secret. Except for proprietary app-based solutions the usual protocol is to display a QR code which just encodes the secret in plain text.

As long as you never enter the secret anywhere but only do the computation is your head, this is just using your brain as the second factor. I would not call this a password since it is not used in the same way. Passwords are entered in plain text into fields that you trust, but that also means that passwords can be stolen. This proves that you are in possession of your brain.

[swiftcoder]

> Passwords are entered in plain text into fields that you trust, but that also means that passwords can be stolen

The only difference here is that you are hashing the password in your head, instead of trusting the client to hash it for you before submitting it to the server.

Which makes the threat model here what, exactly? Keyloggers, or login pages that use outdated/insecure methods to authenticate with the server?

[ulrikrasmussen]

Yes, but also plain guessing since passwords are usually chosen by the user and not generated by the server like TOTP secrets. Also phishing attacks tricking users into entering their passwords in fake login pages, and stolen password databases.

[swiftcoder]

> Yes, but also plain guessing since passwords are usually chosen by the user and not generated by the server like TOTP secrets.

If we were talking a >256-bit secret, I'd buy this, but in the human-calculated case I don't see how it actually helps with this, because you've substituted a ~8 character password for a 6 digit number, which is significantly less search space to brute-force.

> Also phishing attacks tricking users into entering their passwords in fake login pages

yes, this is more-or-less a subset of the "keylogger/insecure login page" case

> and stolen password databases

There's still a server-side TOTP secret database to be stolen, no? And normally that would be hard to reverse-engineer the actual secret from, but again, you've shrunk the search space down to 1,000,000 entries, which is trivial to brute force.

[josephg]

> 2FA is "something you have" (or ".. you are", for biometrics): it is supposed to prove that you currently physically posses the single copy of a token. The textbook example is a TOTP stored in a Yubikey.

No, 2FA means authentication using 2 factors of the following 3 factors:

- What you know (eg password)

- What you have (eg physical token)

- What you are (eg biometrics)

You can "be the 2FA" without a token by combining a password (what you know) and biometrics (what you are). Eg, fingerprint reader + password, where you need both to login.

[crote]

Of course, but in most applications the use of a password is a given, so in day-to-day use "2FA" had come to mean "the other auth method, besides your password".

Combine that with the practical problems with biometrics when trying to auth to a remote system, and in practice that second factor is more often than not "something you have". And biometrics is usually more of a three-factor system, with the device you enrolled your fingerprints on being an essential part of the equation.

[moralestapia]

This.

GP ignores the conventions of the field.

[newpavlov]

>The obvious next step is to do all the math in client-side code and just have the user enter the secret

https://en.wikipedia.org/wiki/Password-authenticated_key_agr...

[chias]

> this has been watered down a lot by the way-too-common practice of storing TOTP secrets in password managers

I'm open to discovering I'm wrong here, but I have never understood this line of thinking. Assuming you 2fa into your password manager when you first sign in on your device, it's still 2 factors all the way down.

As you sign into your password manager, the "something you have" is your 2fa device that you use to sign into your password manager (which is obviously not being filled in by your password manager). Subsequent password manager unlocks which don't prompt for your token are still 2fa because the "something you have" is your computer with which you signed into your password manager.

Why is this a problem?

[theamk]

What if your computer, which runs your password manager, is compromised? If the malware has system access, it can often export all the passwords. Depending on level of protection and OS, this could require kernel access, root access, a regular user access or maybe just a hijacked browser extension.

This leaks every single password in the vault, including any TOTP keys - so if you were storing your TOTP password here, you are now screwed, and attacker has a full access. On the other hand, if your TOTP was a separate device, your TOTP-protected accounts are fine. And even if it's just an app on your phone, you are likely still fine, as phones have much stronger isolation, and people don't usually "npm install" random stuff on them.

(And that's Google Authenticator adding cloud backup functionality is such a bad idea.. If you enable it, then all your 2FAs are leaked once Google password is leaked)

(You could argue that your password manager stores TOTP secrets in secure enclave and it's impossible to extract from there... but those same secrets have to be stored in your account as well, and they could be extracted from there)

[chias]

Isn't this the same chicken-and-egg problem?

> If you enable it, then all your 2FAs are leaked once Google password is leaked

Nope, you'd also need my Google 2fa.

[brna-2]

Time based skew makes it a changeable second factor, additional changeable pass makes it the second factor, Also - if the first factor is a password manager or ssh key - this is the second factor.

The idea of it was so neat to me, I just had to thinker with it.

[labcomputer]

I tend to agree that this doesn't really add a second factor. After all, both the mTOTP secret and the password are something you know, and something you could tell someone else.

However, I do think there is added value here, at least in principle: It increases the difficulty of credential stuffing attacks, which, IMHO is the main value of having a TOTP secret stored in one or more password managers.

"Regular" TOTP stored in a password manager also helps with password reuse when a site is compromised because each site has a different TOTP secret. It seems implausible that a user could remember a different mTOTP secret for each site.

[rcxdude]

The single copy part would be a lot more common if it was widely supported to have multiple tokens registered to an account.

And the main point (though I agree that it doesn't make it 2FA), is to not have the secret be disclosed when you prove that you have it, which is what TOTP also achieves, which makes phishing or sniffing it significantly less valuable.

[crote]

Are there any mainstream websites which only allow a single TOTP token to be enrolled? I can't remember having ever run into that issue. I do recall it occasionally being an issue with Passkeys, though.

The non-disclosure is indeed neat, but the same can be achieved with a password. For example: generate public/private keypair on account creation. Encrypt private key with user password. Store both on server. On auth, client downloads encrypted priv key, decrypts it with user-entered password, then signs nonce and provides it to server as proof of knowledge of user password.

[labcomputer]

> Are there any mainstream websites which only allow a single TOTP token to be enrolled?

I have definitely run into this a more often than not. It seems like only the largest and/or most security-focused tech companies allow multiple TOTP secrets. At this point I have made it a habit to ensure the TOTP secret has been stored in more than one place before dismissing the QR code.

Some even restrict you to one of: one hardware security token, one PassKey or one TOTP token.

> The non-disclosure is indeed neat, but the same can be achieved with a password. For example: generate public/private keypair on account creation. Encrypt private key with user password. Store both on server. On auth, client downloads encrypted priv key, decrypts it with user-entered password, then signs nonce and provides it to server as proof of knowledge of user password.

That's basically how PassKeys work, except that the security token's private key is used to encrypt/decrypt the login private key and sign the nonce.

[fc417fc802]

You don't need to involve a private key there. Modern password authentication algorithms never reveal the bare secret (outside of initial registration ofc). For example, PAKE uses Diffie-Hellman coupled with the (salted) password hash to independently derive the same session key on both sides of the connection.

AFAIK the primary technical concerns are insecure storage by the server (bad hash or salt) or keylogging of the client device. But the real issue is the human factor - ie phishing. As long as the shared secret can't be phished it solves the vast majority of real world problems.

Point being, TOTP on a rooted phone handled by a FOSS password manager app whose secret store the end user retains full access to will successfully prevent the vast majority of real world attacks. You probably shouldn't use a FOSS password manager on a rooted device for your self hosted crypto wallet though.

[crote]

Ah, of course! I did initially consider DH as example, but discounted it because of the need for the server to store the plaintext password - the fact that you can just hash it first completely slipped my mind.

I completely agree about phishing being the main attack vector. However, I do think malware is a not-too-distant second - which makes having a single device contain both your password and TOTP secret a Really Bad Idea. Having not-perfectly-secure TOTP codes only your phone and a password manager DB only on your desktop is a pretty decent solution for that.

[rcxdude]

I would say the majority of services I have TOTP set up for only support one token at a time. It's only the bigger, techier services that have support for multiple.

[fc417fc802]

I guess it's a spectrum. At one extreme is the most physically resistant hardware token in existence. On the other end is a password transmitted in plaintext.

An ssh keyfile requires an attacker to break into the device but is likely fairly easy to snag with only user level access.

Bypassing a password manager that handles TOTP calculations or your ssh key or similar likely requires gaining root and even then could be fairly tricky depending on the precise configuration and implementation. That should generally be sufficient to necessitate knowledge of the master password plus device theft by an insufficiently sophisticated attacker.

Given TOTP or an ssh key managed exclusively by a hardware token it will be all but impossible for anyone to avoid device theft. Still, even TPMs have occasionally had zero day vulnerabilities exposed.

[madeofpalk]

I don't think OP claimed it adds value.

> It explores the limits of time-based authentication under strict human constraints and makes no claims of cryptographic equivalence to standard TOTP.

I think they're just having fun.

[PunchyHamster]

misunderstanding of 2FA annoys me.

Like, banking site requiring phone's 2FA (whether actual or SMS), okay, you have to know password and access to the device or at least a SIM card so 2 things need to be compromised. Computer vulnerable, no problem, phone vulerable, no problem, both need to be vulnerable to defeat it

...then someone decided to put banking on the second factor and now phone has both password and token (or access to SMS) to make a transaction, so whole system is one exploit away from defeat.