Construct RSACryptoServiceProvider from public key (not certificate)

Question

I am working on a project where I need to use a "public key" to encrypt a message using RSA algorithm. I was provided with a certificate and my first thought was to use Public Key from that certificate and after investigation I learned I need to use RSACryptoServiceProvider for encryption.

I have checked msdn and only method I thought I should use is RSACryptoServiceProvider.ImportCspBlob(byte[] keyBlob). When I tried to use public key exported from certificate I was getting an error that the header data for certificate is invalid.

I know I can cast X509certificate2.PublicKey.Key to RSACryptoServiceProvider but from what I understood from my client is that going forward I will be given only a public key and not the certificate. This key will have to be saved in .xml configuration file.

So to summarize: Is there a way to generate an RSACryptoServiceProvider given only a certificate's public key?

Answer 1

You can try to look at this example: RSA public key encryption in C#

var publicKey = "<RSAKeyValue><Modulus>21wEnTU+mcD2w0Lfo1Gv4rtcSWsQJQTNa6gio05AOkV/Er9w3Y13Ddo5wGtjJ19402S71HUeN0vbKILLJdRSES5MHSdJPSVrOqdrll/vLXxDxWs/U0UT1c8u6k/Ogx9hTtZxYwoeYqdhDblof3E75d9n2F0Zvf6iTb4cI7j6fMs=</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>";

var testData = Encoding.UTF8.GetBytes("testing");

using ( var rsa = new RSACryptoServiceProvider(1024))
{
    try
    {
        // client encrypting data with public key issued by server
        //
        rsa.FromXmlString(publicKey);
        var encryptedData = rsa.Encrypt(testData, true);

        var base64Encrypted = Convert.ToBase64String(encryptedData);

    }
    finally
    {
        rsa.PersistKeyInCsp = false;
    }
}