Writing RSA Private Key to PEM File with Passphrase - c++

Question

I am writing a C++ function to create an RSA key pair and store the private key in a PEM file. I use code as in the following example:
static unsigned char passphrase[] = "0123456789";
int keySize = 2048;
int keyExponent = 65537;
BIGNUM *bn;
. . .
bn = BN_new();
BN_set_word(bn, keyExponent);
rsa = RSA_new();
RSA_generate_key_ex(rsa, keySize, bn, nullptr);
FILE* fp = fopen(fileName, "w");
PEM_write_RSAPrivateKey(fp, rsa, EVP_des_ede3_cbc(), passphrase, 10, nullptr, nullptr);
This code works as expected but now I have been asked to store the private key without passphrase protection. Is there a way to get function PEM_write_RSAPrivateKey not to use a passphrase? One option I tried is to change the function call as follows:
outcome = PEM_write_RSAPrivateKey(fp, rsa, EVP_des_ede3_cbc(), nullptr, 0, nullptr, nullptr);
But this results in a prompt to the user to enter a passphrase (which is exactly as stated in the documentation of function PEM_write_RSAPrivateKey). I also tried using a passphrase call-back which returns zero (to signify that there is no passphrase) but this results in PEM_write_RSAPrivateKey returning an error.
What else can I do to have a private key stored in a PEM file without passphrase? I assume that this must somehow be possible because I can do it using openssl commands such as: openssl genrsa -out tmsPrivKey.pem 2048.

Answer 1

The third parameter is the encryption cipher to use to encrypt the private key with. So all you need to do is pass a nullptr to the function.
e.g.
outcome = PEM_write_RSAPrivateKey(fp, rsa, nullptr, nullptr, 0, nullptr, nullptr);
From the documentation:
The PEM functions which write private keys take an enc parameter which
specifies the encryption algorithm to use, encryption is done at the
PEM level. If this parameter is set to NULL then the private key is
written in unencrypted form.