[SOLVED] How to calculate public key for TLS key exchange using OpenSSL functions?

How to calculate public key for TLS key exchange using OpenSSL functions?

I'm learning TLS 1.3 scheme and trying to implement basic handshake steps in C++ without using ready wrappers from OpenSSL such as SSL_accept or SSL_do_handshake. The project already uses OpenSSL for common crypto operations, so I would like not to link it with any other crypto libraries where the x25519 calculation is more obvious than in OpenSSL. So...

there are two buffers:

unsigned char private_key[32];
RAND_bytes(private_key, 32);

unsigned char public_key[32];
memset(public_key, 0, 32);

Which method(s) of OpenSSL should I use to fill public_key buffer with bytes computed from private_key using x25519 algo (copying and allocating as less as possible)?

Solution

Create an EVP_PKEY object using the function EVP_PKEY_new_raw_private_key as described here:

https://www.openssl.org/docs/man1.1.1/man3/EVP_PKEY_new_raw_private_key.html

    EVP_PKEY key = EVP_PKEY_new_raw_private_key(EVP_PKEY_X25519, NULL, private_key, sizeof(private_key));
    if (key == NULL)
        /* Error */;

You can then obtain the public key bytes using the function EVP_PKEY_get_raw_public_key as described on the same page as above:

    size_t publen = sizeof(public_key);
    if (!EVP_PKEY_get_raw_public_key(key, public_key, &publen))
        /* Error */;