i have a .PEM file that includes public key and a private key for SSL data transfer like this:
-----BEGIN RSA PRIVATE KEY-----
private key data
-----EN
I encountered the same issue and found a solution as below:
First convert prkey.pem into prkey.xml with this tool //https://superdry.apphb.com/tools/online-rsa-key-converter
var dataString = "test";
byte[] dataToEncrypt = Encoding.UTF8.GetBytes(dataString);
RSACryptoServiceProvider provider = new RSACryptoServiceProvider();
provider.FromXmlString(File.ReadAllText("C:\prkey.xml"));
byte[] signedBytes = provider.SignData(dataToEncrypt, new SHA256CryptoServiceProvider());
textBox3.Text = BitConverter.ToString(signedBytes);