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oath_totp_validate4_callback(3)     liboath    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:
       http://www.gnu.org/software/liboath/ General help using GNU software:
       http://www.gnu.org/gethelp/

COPYRIGHT
       Copyright (C) 2009-2015 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.

liboath                              2.6.1     oath_totp_validate4_callback(3)