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In the HOTP (HMAC-based One-time Password algorithm ) algorithm the client and the server can have different counter values. Es. if an attacker tries to crack the password with no success. How HOPT deal with this ?

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  • $\begingroup$ The usual recovery would probably be to require multiple consecutive correct entries. $\endgroup$ – SEJPM Nov 24 '19 at 19:37
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IETF RFC 4226 defines HOTP: An HMAC-Based One-Time Password Algorithm. The parties - authenticated (client) and authenticator (server) - establish some parameters

  • A cryptographic hash function, $H$, and the default is SHA-1.
  • A secret key, $K$, which is an arbitrary byte string, and must remain private
  • A HOTP value length, $d$ (6–10, default is 6, and 6–8 is recommended)

Now, they use different counter but not far from each other. The reason is that the authenticator only increments after a successful authentication where the authenticated counter increased whenever a new HTOP is requested.

Let say HOTP has look-ahead synchronization window size $s$ - usually determined by the authenticator. The authenticator than try with the current counter and look forward until an $s$ amount.

Now an attacker can try $s$ amount to execute a brute-force attack. In this case, the $d$ is important the reduce the attack probability. The suggestion and common method is the lock of the system after a small number of failed attempts. This is common with the bank account authenticators. In case of locking, you contact the bank to resync are even re-keying.

One can also, put an increased delay after each failure like in Linux/Windows login failures. In the case of banking, the previous is better.

RFC 4226 for Resynchronization of the Counter

Optionally, the system MAY require the user to send a sequence of (say, 2, 3) HOTP values for resynchronization purpose, since forging a sequence of consecutive HOTP values is even more difficult than guessing a single HOTP value.

The probability of success of the adversary $Sec$ is given by

$$Sec = \frac{s v}{10^\text{d}}$$ where $v$ is the number of verification attempts.

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