OpenSSL PEM_write_bio_PrivateKey Function: Writing Private Keys in PEM Format

PEM_write_bio_PrivateKey is a function within the OpenSSL library that writes a private key to a memory BIO (a type of I/O abstraction used in OpenSSL) in PEM format. The function takes three arguments:

1. bio: A pointer to a BIO object where the private key will be written.
2. pkey: A pointer to the private key structure.
3. enc: The encryption type that should be used for the output. This can be set to 'NULL' for no encryption.

The function returns 1 on success and 0 on failure. Here is an example usage of PEM_write_bio_PrivateKey:

#include <openssl/bio.h>
#include <openssl/evp.h>
#include <openssl/pem.h>

void write_private_key_to_bio(EVP_PKEY *pkey, BIO *bio) {
    // Set up PEM encryption
    const EVP_CIPHER *cipher = EVP_get_cipherbyname('aes-256-cbc');
    const char *passphrase = 'mysecretpassphrase';
    EVP_PKEY *enc_key = EVP_PKEY_new();
    EVP_PKEY_assign(enc_key, EVP_PKEY_RSA, cipher, (unsigned char *)passphrase, strlen(passphrase));

    // Write private key to BIO
    PEM_write_bio_PrivateKey(bio, pkey, cipher, NULL, 0, NULL, enc_key);
    EVP_PKEY_free(enc_key);
}

In this example, we first set up the encryption for the PEM output by creating an EVP_PKEY structure with an RSA key and a passphrase. We then call PEM_write_bio_PrivateKey with the output BIO, the private key, the encryption cipher, and the encryption key. Finally, we free the encryption key structure.