CRYPTO_X25519_INVERSE(3MONOCYPHER) | 3MONOCYPHER | CRYPTO_X25519_INVERSE(3MONOCYPHER) |

# NAME

X25519 scalar multiplication with the multiplicative inverse of a scalar**#include <monocypher.h>**

`void`

**crypto_x25519_inverse**(

`uint8_t blind_salt[32]`,

`const uint8_t private_key[32]`,

`const uint8_t curve_point`);

# DESCRIPTION

The**crypto_x25519_inverse**() function performs scalar multiplication of the multiplicative inverse of a scalar for X25519.

**This is a highly advanced, specialized feature**; unless you are implementing a protocol that requires this specifically,

**you likely have no reason to be using these functions**and are probably looking for crypto_key_exchange() or crypto_x25519() instead. Expect elliptic curve jargon on this page. This function is used, for example, with exponential blinding in oblivious pseudo-random functions (OPRFs). The arguments are:

`blind_salt`- The output point.
`private_key`- The private key (scalar) to use. First, the value is clamped; then the clamped value's multiplicative inverse (modulo the curve order) is determined; the clamped value's multiplicative inverse then has its cofactor cleared, and that final value is then used for scalar multiplication.
`curve_point`- The curve point on X25519 to multiply with the
multiplicative inverse (modulo the curve order) of
`private_key`.

# SEE ALSO

crypto_x25519(), intro()# HISTORY

The**crypto_x25519_inverse**() function first appeared in Monocypher 3.1.0.

March 24, 2020 | Linux 4.15.0-109-generic |