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 places the result in r ((a*r)%n==1). If r is NULL, a new BIGNUM is created.
ctx is a previously allocated BN_CTX used for temporary variables. r may be the same BIGNUM as a.
It is an error to use the same BIGNUM as n.
BN_mod_inverse() returns the BIGNUM containing the inverse, and NULL on error. The error codes can be obtained by ERR_get_error(3).
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