---
name: "BN_mod_inverse"
section: "3ssl"
platform: "linux"
package: "openssl"
version: "3.6.5-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/BN_mod_inverse"
---

# BN\_mod\_inverse(3ssl)

compute inverse modulo n

## NAME

BN\_mod\_inverse - compute inverse modulo n

## SYNOPSIS

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

## DESCRIPTION

**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**.

## NOTES

It is an error to use the same **BIGNUM** as **n**.

## RETURN VALUES

**BN\_mod\_inverse()** returns the **BIGNUM** containing the inverse, and NULL on error. The error codes can be obtained by **ERR\_get\_error**(3).

## SEE ALSO

**ERR\_get\_error**(3), **BN\_add**(3)

## COPYRIGHT

Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use this file except in compliance with the License.  You can obtain a copy in the file LICENSE in the source distribution or at \<https://www.openssl.org/source/license.html\>.