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BN_MOD_INVERSE(3) Library Functions Manual BN_MOD_INVERSE(3)

BN_mod_inverse
compute inverse modulo n

#include <openssl/bn.h>
BIGNUM *
BN_mod_inverse(BIGNUM *r, BIGNUM *a, const BIGNUM *n, BN_CTX *ctx);

BN_mod_inverse() computes the inverse of a modulo n and places the result in r ((a*r)%n==1). If r is NULL, a new BIGNUM is created.
If the flag BN_FLG_CONSTTIME is set on a or n, it operates in constant time.
ctx is a previously allocated BN_CTX used for temporary variables. r may be the same BIGNUM as a or n.

BN_mod_inverse() returns the BIGNUM containing the inverse, or NULL on error. The error codes can be obtained by ERR_get_error(3).

BN_add(3), BN_new(3), BN_set_flags(3)

BN_mod_inverse() first appeared in SSLeay 0.5.1 and has been available since OpenBSD 2.4.
The r argument was added in SSLeay 0.9.1 and OpenBSD 2.6.
April 29, 2018 OpenBSD-current