How to Convert OpenSSL RSA Keys to X.509 Format for .NET?

Using OpenSSL RSA Keys in .NET

When creating an RSA key pair using OpenSSL RSA_generate_key(), it generates public and private keys in the pkcs#1 format by default. However, .NET expects them to be in the x509 format.

PEM Encoding Formats

A PEM-encoded key can be written using two formats:

• Traditional (SubjectPublicKeyInfo): Includes the OID for the algorithm and the public key.
• PKCS (just public key): Contains only the public key.

.NET Support for Key Formats

.NET supports both ASN.1/DER encoded keys and PEM encoded keys that write the SubjectPublicKeyInfo structure, not just the public key.

Converting to X509 Format

To convert the pkcs#1 public key to x509 format, use PEM_write_bio_PUBKEY instead of PEM_write_bio_RSAPublicKey in your C code. This writes the SubjectPublicKeyInfo structure.

Code Example

<code class="cpp">...
// 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 in PEM
// Load with PEM_read_bio_PUBKEY
rc = PEM_write_bio_PUBKEY(pem2.get(), pkey.get());
ASSERT(rc == 1);
...</code>

Additional Notes

• Save the private key securely as it will be needed to decrypt messages.
• The set1 function in EVP_PKEY_set1_RSA increments the reference count to prevent a double free error.

Related Links

• Generating RSA private keys with OpenSSL: [How to generate RSA private key using openssl?](https://stackoverflow.com/questions/26154252/how-to-generate-rsa-private-key-using-openssl)

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