# Which Problems Are Solved
TOTP codes could be reused, violating [RFC 6238, section
5.2](https://datatracker.ietf.org/doc/html/rfc6238#section-5.2).
# How the Problems Are Solved
Transmitted codes are now recorded in each success event. As the codes
amount to secrets, they are not stored verbatim but as a randomly salted
HMAC-SHA256 (`crypto.HMACValue`). Each value gets a fresh random salt,
so an equal code always produces a different stored value and two
entries can never be recognized as being derived from the same code.
This is deliberately not a replacement for proper secret storage: with
the salt stored alongside the hash and a 6 digit payload, anyone with
event access can brute-force the code in milliseconds. That is
acceptable here because a code is worthless once its window has passed.
The `HMACValue` doc comment states the same.
TOTP codes are low-entropy (6 digits), pseudo random codes. Therefore it
is possible for the same code to be generated more than once over the
lifetime of a user. In order to prevent the (unlikely) case where users
login with large time intervalls and a generated code may be equal to
the one last used a long time ago, we only check for codes used within
the recent period window.
A simpler implementation would be simply storing the latest consumed
step, but the TOTP library does not return this value. Because of the
allowed skew there are always 3 steps within the validation window, so
we can't calculate the current step based on time alone. The retained
history spans one period more than the validation window: validity is
decided by the app server's clock, while the stored events carry the
database clock, and the extra period absorbs the drift between them.
**Note:** a reused code pushes a distinct
`user.human.mfa.otp.check.reused` event, not a failed check event. A
replay therefore stays observable through the Events API without
counting towards the lockout policy. Otherwise anyone who captured a
code could replay it to lock the user out.
# Additional Changes
- Read only the latest events in recheck query
# Additional Context
- Closes https://github.com/zitadel/zitadel/issues/6452
---------
Co-authored-by: Livio Spring <[email protected]>
Co-authored-by: Copilot Autofix powered by AI <[email protected]>
Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: muhlemmer <[email protected]>
* fix: import totp in add human user with secret
* fix: import totp in add human user with secret
* fix: import totp in add human user with secret
* fix: review comment changes
This change renames the v2 user OTP registration endpoints and objects
to TOTP.
Also the v2 related code paths have been renamed to TOTP.
This change was discussed during the sprint review.
Stefan BenzandGitHub
Livio SpringandGitHub
FabiandGitHub