How to Convert RSA Keys from PKCS#1 to X.509 Format in OpenSSL?

Converting RSA Keys from PKCS#1 to X.509 Format

In OpenSSL, RSA keys can be saved in two formats:

• PKCS#1: Only contains the public key.
• X.509: Includes the SubjectPublicKeyInfo structure, which contains the algorithm OID and the public key.

.NET can handle both ASN.1/DER-encoded keys (PKCS#1) and PEM-encoded keys (X.509). To convert a PKCS#1 key to an X.509 key:

1. Use PEM_write_bio_PUBKEY instead of PEM_write_bio_RSAPublicKey:

• PEM_write_bio_RSAPublicKey writes only the public key (PKCS#1).
• PEM_write_bio_PUBKEY writes the SubjectPublicKeyInfo (X.509).

2. Convert RSA key to EVP_PKEY using EVP_PKEY_set1_RSA:

• This function converts the RSA key to an EVP_PKEY, which includes the algorithm OID.

Example Code:

<code class="c++">#include <openssl/pem.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>

int main() {
    RSA *rsa = RSA_new();
    BN_set_word(BN_new(), RSA_F4);
    RSA_generate_key_ex(rsa, 2048, NULL, NULL);

    EVP_PKEY *pkey = EVP_PKEY_new();
    EVP_PKEY_set1_RSA(pkey, rsa);

    BIO *pem = BIO_new_file("rsa-public.pem", "w");
    PEM_write_bio_PUBKEY(pem, pkey);

    RSA_free(rsa);
    EVP_PKEY_free(pkey);
    return 0;
}</code>

This program generates an RSA key pair, converts the public key to an X.509 format, and saves it in the "rsa-public.pem" file.

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