How to Convert OpenSSL RSA Keys for Use with .Net?

Using OpenSSL RSA Key with .Net

OpenSSL offers varying key formats, causing compatibility issues with different modules.

Issue: .Net requires a key in format "-----BEGIN PUBLIC KEY-----", rather than the "-----BEGIN RSA PUBLIC KEY-----" format generated by OpenSSL.

Solution:

Use PEM_write_bio_PUBKEY instead of PEM_write_bio_RSAPublicKey to write the public key. PEM_write_bio_PUBKEY includes the required "SubjectPublicKeyInfo" structure, while PEM_write_bio_RSAPublicKey only outputs the public key.

Converting Keys to the Required Format:

To convert an OpenSSL RSA key to the format required by .Net, you can use the following code:

<code class="cpp">BIO_FILE_ptr pem1(BIO_new_file("rsa-public-1.pem", "w"), ::BIO_free);
EVP_KEY_ptr pkey(EVP_PKEY_new(), ::EVP_PKEY_free);

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

rc = EVP_PKEY_set1_RSA(pkey.get(), rsa.get());
ASSERT(rc == 1);

rc = PEM_write_bio_PUBKEY(pem1.get(), pkey.get());
ASSERT(rc == 1);</code>

This will create a PEM file named "rsa-public-1.pem" containing the public key in the required format.

Example:

Consider the following example, where the "publicKey" variable holds a key in the format not recognized by .Net:

<code class="cpp">string publicKey = @"-----BEGIN RSA PUBLIC KEY-----
...
-----END RSA PUBLIC KEY-----";

CryptoKey key = CryptoKey.FromPublicKey(publicKey, "");
RSA rsa = key.GetRSA();</code>

Replacing "publicKey" with the key generated using the provided code will resolve the exception encountered while trying to create a CryptoKey object.

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