How to perform Public Key Encryption with RSA using C++ - c++

Question

I am trying to implement asymmetrc encryption into a project. My end goal is to implement EVP Asymmetric Encryption and Decryption of an Envelope but for now I am going to stick with the basics. I am rather new to openssl so please forgive me if there is an obvious answer to this question.
Currently I am following this tutorial here: Simple Public Key Encryption with RSA and OpenSSL
I plan to transmit the RSA public key over a network to a seperate client which will encrypt a message with the generated public key. The encrypted message will then be sent back to the server for decryption.
I am having trouble with encrypting only using the public key instead of using the keypair. For obvious reasons, I don't want to transmit the keypair over the network.
Here is the code that I have so far:
#include <openssl/rsa.h>
#include <openssl/pem.h>
#include <openssl/err.h>
#include <stdio.h>
#include <string.h>
#define KEY_LENGTH 2048
#define PUB_EXP 3
#define PRINT_KEYS
#define WRITE_TO_FILE
int main(void) {
size_t pri_len; // Length of private key
size_t pub_len; // Length of public key
char *pri_key; // Private key
char *pub_key; // Public key
char msg[KEY_LENGTH/8]; // Message to encrypt
char *encrypt = NULL; // Encrypted message
char *decrypt = NULL; // Decrypted message
char *err; // Buffer for any error messages
// Generate key pair
printf("Generating RSA (%d bits) keypair...", KEY_LENGTH);
fflush(stdout);
RSA *keypair = RSA_generate_key(KEY_LENGTH, PUB_EXP, NULL, NULL);
// To get the C-string PEM form:
BIO *pri = BIO_new(BIO_s_mem());
BIO *pub = BIO_new(BIO_s_mem());
PEM_write_bio_RSAPrivateKey(pri, keypair, NULL, NULL, 0, NULL, NULL);
PEM_write_bio_RSAPublicKey(pub, keypair);
//SEND KEY TO CLIENT HERE
//FAKE CLIENT RECIEVE
RSA *keypair2 = NULL;
PEM_read_bio_RSAPublicKey( pub, &keypair2, NULL, NULL);
// Get the message to encrypt
printf("Message to encrypt: ");
fgets(msg, KEY_LENGTH-1, stdin);
msg[strlen(msg)-1] = '\0';
// Encrypt the message using public key only
encrypt = malloc(RSA_size(keypair2));
int encrypt_len;
err = malloc(130);
if((encrypt_len = RSA_public_encrypt(strlen(msg)+1, (unsigned char*)msg, (unsigned char*)encrypt,
keypair2, RSA_PKCS1_OAEP_PADDING)) == -1) {
ERR_load_crypto_strings();
ERR_error_string(ERR_get_error(), err);
fprintf(stderr, "Error encrypting message: %s\n", err);
goto free_stuff;
}
#ifdef WRITE_TO_FILE
// Write the encrypted message to a file
FILE *out = fopen("out.bin", "w");
fwrite(encrypt, sizeof(*encrypt), RSA_size(keypair), out);
fclose(out);
printf("Encrypted message written to file.\n");
free(encrypt);
encrypt = NULL;
// Read it back
printf("Reading back encrypted message and attempting decryption...\n");
encrypt = malloc(RSA_size(keypair));
out = fopen("out.bin", "r");
fread(encrypt, sizeof(*encrypt), RSA_size(keypair), out);
fclose(out);
#endif
//SEND MESSAGE BACK TO CLIENT FOR DECRYPT
// Decrypt it
decrypt = malloc(encrypt_len);
if(RSA_private_decrypt(encrypt_len, (unsigned char*)encrypt, (unsigned char*)decrypt,
keypair, RSA_PKCS1_OAEP_PADDING) == -1) {
ERR_load_crypto_strings();
ERR_error_string(ERR_get_error(), err);
fprintf(stderr, "Error decrypting message: %s\n", err);
goto free_stuff;
}
printf("Decrypted message: %s\n", decrypt);
free_stuff:
RSA_free(keypair);
BIO_free_all(pub);
BIO_free_all(pri);
free(pri_key);
free(pub_key);
free(encrypt);
free(decrypt);
free(err);
return 0;
}
How could I achieve encrypting data only using the public key rather than the whole keypair?
Thanks