NAME

oath_hotp_validate_callback - API function

SYNOPSIS

#include <oath.h>

int oath_hotp_validate_callback(const char * secret, size_t secret_length, uint64_t start_moving_factor, size_t window, unsigned digits, oath_validate_strcmp_function strcmp_otp, void * strcmp_handle);

ARGUMENTS

const char * secret

the shared secret string

size_t secret_length

length of secret

uint64_t start_moving_factor

start counter in OTP stream

size_t window

how many OTPs after start counter to test

unsigned digits

number of requested digits in the OTP

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 HOTP algorithm per RFC 4226.

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.

RETURNS

Returns 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

1.4.0

REPORTING BUGS

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

COPYRIGHT

Copyright © 2009-2020 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.