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X25519(3) FreeBSD Library Functions Manual X25519(3)

X25519, X25519_keypair
Elliptic Curve Diffie-Hellman primitive based on Curve25519

#include <openssl/curve25519.h>
X25519(uint8_t out_shared_key[X25519_KEY_LENGTH], const uint8_t private_key[X25519_KEY_LENGTH], const uint8_t peer_public_value[X25519_KEY_LENGTH]);
X25519_keypair(uint8_t out_public_value[X25519_KEY_LENGTH], uint8_t out_private_key[X25519_KEY_LENGTH]);

Curve25519 is an elliptic curve over a prime field specified in RFC 7748. The prime field is defined by the prime number 2^255 - 19.
X25519() is the Diffie-Hellman primitive built from Curve25519 as described in RFC 7748 section 5. Section 6.1 describes the intended use in an Elliptic Curve Diffie-Hellman (ECDH) protocol.
X25519() writes a shared key to out_shared_key that is calculated from the given private_key and the peer_public_value by scalar multiplication. Do not use the shared key directly, rather use a key derivation function and also include the two public values as inputs.
X25519_keypair() sets out_public_value and out_private_key to a freshly generated public/private key pair. First, the out_private_key is generated with arc4random_buf(3). Then, the opposite of the masking described in RFC 7748 section 5 is applied to it to make sure that the generated private key is never correctly masked. The purpose is to cause incorrect implementations on the peer side to consistently fail. Correct implementations will decode the key correctly even when it is not correctly masked. Finally, the out_public_value is calculated from the out_private_key by multiplying it with the Montgomery base point uint8_t u[32] = {9}.
The size of a public and private key is X25519_KEY_LENGTH = 32 bytes each.

X25519() returns 1 on success or 0 on error. Failure can occur when the input is a point of small order.

D. J. Bernstein, A state-of-the-art Diffie-Hellman function: How do I use Curve25519 in my own software?,

RFC 7748: Elliptic Curves for Security
August 10, 2018 FreeBSD 12.0-RELEASE

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