- hotp-verify SECRET COUNTER CODE #!key (digits 6) (algorithm 'sha1) (look-ahead 0)procedure
Verifies CODE against COUNTER and, if look-ahead is greater than zero, against each of the look-ahead counters following it (RFC 4226 section 7.4 resynchronization). Returns the matched offset (an exact integer between 0 and look-ahead, inclusive) on success, or #f if no counter in that range produces CODE.
On a successful verification, the caller should persist (+ COUNTER offset 1) as the new stored counter for this secret, so that a code cannot be verified a second time.