oath_totp_validate4_callback(3)

NAME
oath_totp_validate4_callback - API function

SYNOPSIS
#include <oath.h>

int oath_totp_validate4_callback(const char * secret, size_t secret_length, time_t now, unsigned time_step_size, time_t start_offset, unsigned digits, size_t window, int * otp_pos, uint64_t * otp_counter, int flags, oath_validate_strcmp_function strcmp_otp, void * strcmp_handle);

ARGUMENTS

const char * secret
    the shared secret string

size_t secret_length
    length of secret

time_t now
    Unix time value to compute TOTP for

unsigned time_step_size
    time step system parameter (typically 30)

time_t start_offset
    Unix time of when to start counting time steps (typically 0)

unsigned digits
    number of requested digits in the OTP

size_t window
    how many OTPs after start counter to test

int * otp_pos
    output search position in search window (may be NULL).

uint64_t * otp_counter
    counter value used to calculate OTP value (may be NULL).

int flags
    flags indicating mode, one of oath_totp_flags

oath_validate_strcmp_function strcmp_otp
    function pointer to a strcmp-like function.

void * strcmp_handle
    caller handle to be passed on to strcmp_otp.

DESCRIPTION
Validate an OTP according to OATH TOTP algorithm per RFC 6238.

Validation is implemented by generating a number of potential OTPs and performing a call to the strcmp_otp function, to compare the potential OTP against the given otp. It has the following prototype:

int (*oath_validate_strcmp_function) (void *handle, const char *test_otp);

The function should be similar to strcmp in that it return 0 only on matches. It differs by permitting use of negative return codes as indication of internal failures in the callback. Positive values indicate OTP mismatch.

This callback interface is useful when you cannot compare OTPs directly using normal strcmp, but instead for example only have a hashed OTP. You would then typically pass in the hashed OTP in the strcmp_handle and let your implementation of strcmp_otp hash the test_otp OTP using the same hash, and then compare the results.

Currently only OTP lengths of 6, 7 or 8 digits are supported. This restrictions may be lifted in future versions, although some limitations are inherent in the protocol.

The flags parameter may be used to change the MAC function, for example OATH_TOTP_HMAC_SHA256 or OATH_TOTP_HMAC_SHA512.

RETURNS
Returns absolute value of position in OTP window (zero is first position), or OATH_INVALID_OTP if no OTP was found in OTP window, or an error code.

SINCE
2.6.0

REPORTING BUGS
Report bugs to <oath-toolkit-help@nongnu.org>. liboath home page: https://oath-toolkit.codeberg.page/ General help using GNU software: http://www.gnu.org/gethelp/

COPYRIGHT
Copyright © 2009-2026 Simon Josefsson.
Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved.