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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