ECDH_COMPUTE_KEY(3)        Library Functions Manual        ECDH_COMPUTE_KEY(3)

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.

Linux 6.8.2-arch2-1             August 29, 2023            Linux 6.8.2-arch2-1