Separating public and private keys of ECDSA keypair

Question

I'm using c++ to try to generate a ECDSA key-pair following Elliptic_Curve_Cryptography from the OpenSSL wiki.

#include <iostream>

#include <openssl/obj_mac.h>
#include <openssl/ec.h>

int main()
{
    EC_KEY *key;

    if(NULL == (key = EC_KEY_new_by_curve_name(NID_secp224r1)))
        std::cout << "error on new_curve_by_name" << std::endl;

    if(1 != EC_KEY_generate_key(key)) std::cout << "error in generating keys" << std::endl;
}

The key pair is generated without errors, but I don't know how to extract public and private key in two different objects (EC_POINT and BIGNUM), any idea?

Answer 1

I think the methods you want are:

EC_KEY_get0_private_key and EC_KEY_get0_public_key

This works for me:

EC_KEY* key = EC_KEY_new_by_curve_name(NID_secp224r1);

if(!key)
{
    std::cerr << "Error creating curve key" << '\n';
    return EXIT_FAILURE;
}

if(!EC_KEY_generate_key(key))
{
    std::cerr << "Error generating curve key" << '\n';
    EC_KEY_free(key);
    return EXIT_FAILURE;
}

BIGNUM const* prv = EC_KEY_get0_private_key(key);
if(!prv)
{
    std::cerr << "Error getting private key" << '\n';
    EC_KEY_free(key);
    return EXIT_FAILURE;
}

std::cout << "Private key: " << prv << '\n';

EC_POINT const* pub = EC_KEY_get0_public_key(key);
if(!pub)
{
    std::cerr << "Error getting public key" << '\n';
    EC_KEY_free(key);
    return EXIT_FAILURE;
}

std::cout << "Public key: " << pub << '\n';

// Use keys here ...

EC_KEY_free(key);

NOTE:

When using C libraries like this I often define a custom smart pointer to take care if the deletions. This makes the code less prone to memory leaks and "exception safe".

For example I would define something like this:

struct ec_key_dter{void operator()(EC_KEY* k)const{if(k) EC_KEY_free(k);}};
using  ec_key_uptr = std::unique_ptr<EC_KEY, ec_key_dter>;

And use it like this:

auto key = ec_key_uptr(EC_KEY_new_by_curve_name(NID_secp224r1));

if(!key)
    throw std::runtime_error("Error creating curve key");

if(!EC_KEY_generate_key(key.get()))
    throw std::runtime_error("Error generating curve key");

if(!EC_KEY_check_key(key.get()))
    throw std::runtime_error("Error checking curve key");

// ... etc ...

// Do not delete the key manually!!