There are two different algorithms to choose from for your OTP generators. Time Based (TOTP) and Counter Based (HOTP). For TOTP, your token generator will hash the current time and a shared secret. The server validates the OTP by comparing the all hashes within a certain window of time to the submitted value. So, TOTPs are valid only for a short window of time (usually 30 seconds). For HOTP a shared counter is used instead of the current time. The server increments the counter with each successful OTP login. So, valid OTPs only change after a successful login.

TOTP is considered a little more secure because the matchable OTP is only valid for a short window of time while the OTP for HOTP can be valid for an indeterminate amount of time. HOTP is much more user friendly as the user won’t have to hurry to enter in their OTP before the time interval is up. With the way Keycloak has implemented TOTP this distinction becomes a little more blurry. HOTP requires a database update every time the server wants to increment the counter. This can be a performance drain on the authentication server when there is heavy load. So, to provide a more efficient alternative, TOTP does not remember passwords used. This bypasses the need to do any DB updates, but the downside is that TOTPs can be re-used in the valid time interval. For future versions of Keycloak it is planned that you will be able to configure whether TOTP checks older OTPs in the time interval.