RSA public encryption using public key from X509 certificate c++ - c++

Question

The task is to encrypt some message with RSA public key which lies in .crt file - X509 certificate file.
With issue below I succesfully get sertificate information, but can not find any issue how to use public key from x to encrypt message using RSA algorythm.
When I trying to create BIO buffer with key data and to use PEM_read_bio_RSA_PUBKEY function to write key into RSA struct it returns an error "error:0906D06C:PEM routines:PEM_read_bio:no start line" which is clear why appear but not clear how to correctly solve. Are any issue how I can do encryption without creating another buffer and adding manualy lines with "-----BEGIN RSA PUBLIC KEY-----" and "-----END RSA PUBLIC KEY-----"?
Here are my code for this job:
unsigned char message[] = "Hello cryptographic world!";
unsigned char *encrypted_text;
int CyferWithHostKeyPub()
{
ERR_load_crypto_strings();
int res_length = 0;
RSA *public_key;
X509 *x;
BIO *fp = BIO_new_file("cert.crt", "r");
if (!fp)
{
printf("\nCan't open file.\n");
return -1;
}
x = PEM_read_bio_X509(fp, 0, 0, NULL);
if (!x) {
printf("%s\n", ERR_error_string(ERR_get_error(), NULL));
return 1;
}
BIO *membuf = BIO_new_mem_buf((void*)x->cert_info->key->public_key->data, x->cert_info->key->public_key->length);
public_key = PEM_read_bio_RSA_PUBKEY(membuf, &public_key, NULL, NULL);
if (!public_key) {
printf("%s\n", ERR_error_string(ERR_get_error(), NULL));
return 1;
}
int encrypted_length = RSA_size(public_key);
encrypted_text = new unsigned char[ encrypted_length ];
RSA_public_encrypt(sizeof(message), message, encrypted_text, public_key, RSA_PKCS1_PADDING);
RSA_free(public_key);
X509_free(x);
BIO_free(membuf);
BIO_free(fp);
return encrypted_length;
}
So, how can I do this correctly? Thanks!

Answer 1

Use the X509_get_pubkey() function to obtain a reference to the EVP_PKEY structure contained within the X509 structure. Then you can use EVP_PKEY_get1_RSA() to get a reference to the RSA structure.
See these man pages for further info:
https://www.openssl.org/docs/man1.1.0/crypto/X509_get_pubkey.html
https://www.openssl.org/docs/man1.1.0/crypto/EVP_PKEY_get1_RSA.html