How to Convert OpenSSL RSA Public Keys to .Net Compatible Format?

Use OpenSSL RSA Key with .Net

When generating a public-private key pair using RSA_generate_key(), OpenSSL outputs public keys in the following format:

-----BEGIN RSA PUBLIC KEY-----
...
-----END RSA PUBLIC KEY-----

However, some modules in .Net require keys in this format:

-----BEGIN PUBLIC KEY-----
...
-----END PUBLIC KEY-----

To convert the keys to the required format, use PEM_write_bio_PUBKEY instead of PEM_write_bio_RSAPublicKey. The former writes the SubjectPublicKeyInfo, including the OID and the public key.

To convert the key, follow these steps:

1. Create an EVP_PKEY using EVP_PKEY_set1_RSA.
2. Convert the RSA key to ASN.1/DER format using i2d_RSA_PUBKEY_bio or to PEM format using PEM_write_bio_PUBKEY.

C Example

The following C program demonstrates the conversion:

<code class="cpp">#include <memory>
#include <openssl>
#include <openssl>
#include <openssl>
#include <openssl>
#include <openssl>

#include <cassert>
#define ASSERT assert

using BN_ptr = std::unique_ptr<bignum decltype>;
using RSA_ptr = std::unique_ptr<rsa decltype>;
using EVP_KEY_ptr = std::unique_ptr<evp_pkey decltype>;
using BIO_FILE_ptr = std::unique_ptr<bio decltype>;

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 pem2(BIO_new_file("rsa-public-2.pem", "w"), ::BIO_free);
    BIO_FILE_ptr der1(BIO_new_file("rsa-public-1.der", "w"), ::BIO_free);
    BIO_FILE_ptr der2(BIO_new_file("rsa-public-2.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 just the public key in ASN.1/DER
    // Load with d2i_RSAPublicKey_bio
    rc = i2d_RSAPublicKey_bio(der1.get(), rsa.get());
    ASSERT(rc == 1);

    // Write just the public key in PEM
    // Load with PEM_read_bio_RSAPublicKey
    rc = PEM_write_bio_RSAPublicKey(pem1.get(), rsa.get());
    ASSERT(rc == 1);

    // Write SubjectPublicKeyInfo with OID and public key in ASN.1/DER
    // Load with d2i_RSA_PUBKEY_bio
    rc = i2d_RSA_PUBKEY_bio(der2.get(), rsa.get());
    ASSERT(rc == 1);

    // Write SubjectPublicKeyInfo with OID and public key in PEM
    // Load with PEM_read_bio_PUBKEY
    rc = PEM_write_bio_PUBKEY(pem2.get(), pkey.get());
    ASSERT(rc == 1);

    return 0;
}</bio></evp_pkey></rsa></bignum></cassert></openssl></openssl></openssl></openssl></openssl></memory></code>

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