EVP_KEY get raw private key in C

Question

Good day,

I have been trying to do a simple exercise where I could generate the public and the private key using RSA with Openssl and print them both. My code looks something like this:

size_t private_key_len = KEY_LENGTH;
EVP_PKEY *pkey = EVP_RSA_gen(KEY_LENGTH);
if (pkey == NULL)
{
    fprintf(stderr, "error: rsa gen\n");
    ERR_print_errors_fp(stderr);
    return NULL;
}
unsigned char *private_key = calloc((KEY_LENGTH + 1),sizeof(unsigned char));
EVP_PKEY_get_raw_private_key(pkey, private_key, &private_key_len);
printf("%s\n",private_key);

So normally it should print the private key, given that KEY_LENGTH is 1024, however it just prints nothing (The zeros initialized by calloc). I have tried with malloc too, the result is similar the only difference being that it prints 0xBE.

So basically the array private_key is never filled, and I have no idea why.

What am I missing to make this work? Thanks in advance!

Answer 1

Quoting the man page with emphasis changed:

EVP_PKEY_get_raw_private_key() fills the buffer provided by priv with raw private key data. The size of the priv buffer should be in *len on entry to the function, and on exit *len is updated with the number of bytes actually written. If the buffer priv is NULL then *len is populated with the number of bytes required to hold the key. The calling application is responsible for ensuring that the buffer is large enough to receive the private key data. This function only works for algorithms that support raw private keys. Currently this is: EVP_PKEY_HMAC, EVP_PKEY_POLY1305, EVP_PKEY_SIPHASH, EVP_PKEY_X25519, EVP_PKEY_ED25519, EVP_PKEY_X448 or EVP_PKEY_ED448.

Notice that RSA is not one of the supported algorithms.

You can "print" an RSA key either by converting each of its components (n,e,d,p,q,dp,dp,qinv) to printable form, which EVP_PKEY_print_private does for you, or getting the encoding of the whole key in PEM which is 'printable' in the sense of being printable and typable characters, but not the sense of being easily understood (or copied or created) by people, with PEM_write_PrivateKey or PEM_write_RSAPrivateKey.

Also, the value you pass to EVP_RSA_gen is in bits, but the size of displayed components of an RSA key (other than e, which is small) will be in hex or decimal digits or (mostly) base64 characters.