How to generate private/public key pairs in c++ using openssl

Question

I have 2 questions:

1. How can I generate private/public key pairs with c++ and openssl? I don't want them saved to a file. I want the values to be available to my c++ program.
2. Given a private key, how can I regenerate its public key with c++ and openssl?

For the first question, I need to do something like this, except in c++.

$ openssl ecparam -name secp256k1 -genkey | openssl ec -text
read EC key
Private-Key: (256 bit)
priv:
    0c:25:3a:f2:dc:5b:07:b0:45:9c:ad:ac:13:e1:ef:
    14:50:68:cc:cc:6b:8b:09:66:1b:8d:c1:a4:f8:e8:
    ab:df
pub:
    04:8c:a7:6e:b0:23:8f:ad:c9:b0:4b:35:b7:65:1d:
    4d:67:1b:6a:5d:be:9e:84:14:99:97:4f:bf:d2:46:
    16:0f:14:e3:71:08:13:2c:c1:3b:95:1a:38:92:1c:
    cf:79:5d:9b:2e:18:64:47:8e:1f:4c:b2:5e:42:f4:
    47:1d:e4:81:d5
ASN1 OID: secp256k1
writing EC key
-----BEGIN EC PRIVATE KEY-----
MHQCAQEEIAwlOvLcWwewRZytrBPh7xRQaMzMa4sJZhuNwaT46KvfoAcGBSuBBAAK
oUQDQgAEjKdusCOPrcmwSzW3ZR1NZxtqXb6ehBSZl0+/0kYWDxTjcQgTLME7lRo4
khzPeV2bLhhkR44fTLJeQvRHHeSB1Q==
-----END EC PRIVATE KEY-----

The only thing I've been able to find are limited examples which got me nowhere. Here is what I have (with error checking removed for readability).

EVP_PKEY_CTX* pkey_context = EVP_PKEY_CTX_new_id (EVP_PKEY_EC, NULL);
EVP_PKEY_keygen_init (pkey_context);
EVP_PKEY_CTX_set_ec_paramgen_curve_nid (pkey_context, NID_secp256k1);
EVP_PKEY *pkey = NULL;
EVP_PKEY_keygen (pkey_context, &pkey);
cout << "pkey = " << pkey << "\n";
EVP_PKEY_CTX_free (pkey_context);

The result of this code is:

pkey = 0x55f94ba2ca10

It prints the pointer to the variable, but I can't even find a definition of EVP_PKEY, so I have no way of knowing what is in that object.

Any help in figuring this out, or pointing me to better documentation, would be greatly appreciated.

Answer 1

Usually you'd just pass around the EVP_PKEY structure to other OpenSSL calls.

To put the secret into memory, you can use

char* secret_key;
BIO* bio_priv = BIO_new(BIO_s_mem());
PEM_write_bio_PrivateKey(bio_priv, pkey, NULL, NULL, 0, NULL, NULL);
BIO_get_mem_data(bio_priv, &secret_key);
...
BIO_free(bio_priv);

This will set secret_key to point at the data inside bio_priv (before it is freed) containing the secret key. You can similarly use PEM_write_bio_PUBKEY to get the public key.

Also note there is a cryptography library designed specifically for the secp256k1 curve and Bitcoin, which is used by Bitcoin Core, so you could use that instead of OpenSSL. It can be found here.