How to Use OpenSSL to Generate RSA Keys in C/C++

It is known that RSA is a cryptosystem which is used for the security of data transmission. This tutorial introduces how to use RSA to generate a pair of public and private keys on Windows.

  1. Download and install OpenSSL https://www.openssl.org/community/binaries.html.
  2. Find libeay32.lib, ssleay32.lib and libeay32.dll.
  3. The following sample code will generate a public key “public.pem” and a private key “private.pem”.
#include <stdio.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>

bool generate_key()
{
	int				ret = 0;
	RSA				*r = NULL;
	BIGNUM			*bne = NULL;
	BIO				*bp_public = NULL, *bp_private = NULL;

	int				bits = 2048;
	unsigned long	e = RSA_F4;

	// 1. generate rsa key
	bne = BN_new();
	ret = BN_set_word(bne,e);
	if(ret != 1){
		goto free_all;
	}

	r = RSA_new();
	ret = RSA_generate_key_ex(r, bits, bne, NULL);
	if(ret != 1){
		goto free_all;
	}

	// 2. save public key
	bp_public = BIO_new_file("public.pem", "w+");
	ret = PEM_write_bio_RSAPublicKey(bp_public, r);
	if(ret != 1){
		goto free_all;
	}

	// 3. save private key
	bp_private = BIO_new_file("private.pem", "w+");
	ret = PEM_write_bio_RSAPrivateKey(bp_private, r, NULL, NULL, 0, NULL, NULL);

	// 4. free
free_all:

	BIO_free_all(bp_public);
	BIO_free_all(bp_private);
	RSA_free(r);
	BN_free(bne);

	return (ret == 1);
}

int main(int argc, char* argv[]) 
{
	generate_key();
        return 0;
}

You can feel free to download the sample code, and run it in Visual Studio.

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