How to Use an OpenSSL RSA Key with .NET?

Use OpenSSL RSA key with .Net

Problem:
The .Net framework throws an exception when attempting to use an RSA public key generated with OpenSSL due to a format mismatch. The key is in PKCS#1 format, while .Net expects a X.509 format.

Solution:

Conversion from PKCS#1 to X.509 Format:

To convert the RSA public key from PKCS#1 to X.509 format, use the PEM_write_bio_PUBKEY function instead of PEM_write_bio_RSAPublicKey. This will output the key in the SubjectPublicKeyInfo format with an OID and the public key.

Additionally, you will need to use EVP_PKEY_set1_RSA to convert the RSA key to an EVP_PKEY.

Example Code:

<code class="c++">// Include necessary headers
#include <openssl/bn.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>
#include <openssl/bio.h>
#include <openssl/x509.h>

#include <cassert>
#define ASSERT assert

using BN_ptr = std::unique_ptr<BIGNUM, decltype(&amp;::BN_free)>;
using RSA_ptr = std::unique_ptr<RSA, decltype(&amp;::RSA_free)>;
using EVP_KEY_ptr = std::unique_ptr<EVP_PKEY, decltype(&amp;::EVP_PKEY_free)>;
using BIO_FILE_ptr = std::unique_ptr<BIO, decltype(&amp;::BIO_free)>;

int main(int argc, char* argv[])
{
    int rc;

    RSA_ptr rsa(RSA_new(), ::RSA_free);
    BN_ptr bn(BN_new(), ::BN_free);

    BIO_FILE_ptr pem1(BIO_new_file("rsa-public-1.pem", "w"), ::BIO_free);
    BIO_FILE_ptr der1(BIO_new_file("rsa-public-1.der", "w"), ::BIO_free);

    rc = BN_set_word(bn.get(), RSA_F4);
    ASSERT(rc == 1);

    // Generate key
    rc = RSA_generate_key_ex(rsa.get(), 2048, bn.get(), NULL);
    ASSERT(rc == 1);

    // Convert RSA key to PKEY
    EVP_KEY_ptr pkey(EVP_PKEY_new(), ::EVP_PKEY_free);
    rc = EVP_PKEY_set1_RSA(pkey.get(), rsa.get());
    ASSERT(rc == 1);

    // Write SubjectPublicKeyInfo with OID and public key in ASN.1/DER
    rc = i2d_RSA_PUBKEY_bio(der1.get(), rsa.get());
    ASSERT(rc == 1);

    // Write SubjectPublicKeyInfo with OID and public key in PEM
    rc = PEM_write_bio_PUBKEY(pem1.get(), pkey.get());
    ASSERT(rc == 1);

    return 0;
}</code>

This code generates an RSA key pair and writes the public key in both ASN.1/DER and PEM formats. You can then use the X.509 formatted public key with your .Net application.

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