NAME

ECDH_compute_key, ECDH_size — Elliptic Curve Diffie-Hellman key exchange

SYNOPSIS

#include <openssl/ec.h> int
ECDH_compute_key(void *out, size_t outlen, const EC_POINT *public_key, EC_KEY *ecdh, void *(*KDF)(const void *in, size_t inlen, void *out, size_t *outlen)); int
ECDH_size(const EC_KEY *ecdh);

DESCRIPTION

ECDH_compute_key() performs Elliptic Curve Diffie-Hellman key agreement. It combines the private key contained in ecdh with the other party's public_key, takes the x component of the affine coordinates, and optionally applies the key derivation function KDF. It stores the resulting symmetric key in the buffer out, which is outlen bytes long. If KDF is NULL, outlen must be at least ECDH_size(ecdh). ECDH_size() returns the number of bytes needed to store an affine coordinate of a point on the elliptic curve used by ecdh, which is one eighth of the degree of the finite field underlying that elliptic curve, rounded up to the next integer number.

RETURN VALUES

ECDH_compute_key() returns the length of the computed key in bytes or -1 if an error occurs. ECDH_size() returns the number of bytes needed to store an affine coordinate.

SEE ALSO

DH_generate_key(3), DH_size(3), EC_GROUP_new(3), EC_KEY_new(3), EC_POINT_new(3), X25519(3)

HISTORY

ECDH_compute_key() first appeared in OpenSSL 0.9.8 and has been available since OpenBSD 4.5. ECDH_size() first appeared in OpenBSD 6.1.