How to read a PEM RSA private key from .NET

Question

I've got an RSA private key in PEM format, is there a straight forward way to read that from .NET and instantiate an RSACryptoServiceProvider to decrypt data encrypted with the corresponding public key?

Answer 1

Update 03/03/2021

.NET 5 now supports this out of the box.

To try the code snippet below, generate a keypair and encrypt some text at http://travistidwell.com/jsencrypt/demo/

var privateKey = @"-----BEGIN RSA PRIVATE KEY----- { the full PEM private key }  -----END RSA PRIVATE KEY-----";  var rsa = RSA.Create(); rsa.ImportFromPem(privateKey.ToCharArray());  var decryptedBytes = rsa.Decrypt(     Convert.FromBase64String("{ base64-encoded encrypted string }"),      RSAEncryptionPadding.Pkcs1 );  // this will print the original unencrypted string Console.WriteLine(Encoding.UTF8.GetString(decryptedBytes)); 

Original answer

I solved, thanks. In case anyone's interested, bouncycastle did the trick, just took me some time due to lack of knowledge from on my side and documentation. This is the code:

var bytesToDecrypt = Convert.FromBase64String("la0Cz.....D43g=="); // string to decrypt, base64 encoded   AsymmetricCipherKeyPair keyPair;    using (var reader = File.OpenText(@"c:\myprivatekey.pem")) // file containing RSA PKCS1 private key     keyPair = (AsymmetricCipherKeyPair) new PemReader(reader).ReadObject();    var decryptEngine = new Pkcs1Encoding(new RsaEngine()); decryptEngine.Init(false, keyPair.Private);    var decrypted = Encoding.UTF8.GetString(decryptEngine.ProcessBlock(bytesToDecrypt, 0, bytesToDecrypt.Length));  

Answer 2

With respect to easily importing the RSA private key, without using 3rd party code such as BouncyCastle, I think the answer is "No, not with a PEM of the private key alone."

However, as alluded to above by Simone, you can simply combine the PEM of the private key (*.key) and the certificate file using that key (*.crt) into a *.pfx file which can then be easily imported.

To generate the PFX file from the command line:

openssl pkcs12 -in a.crt -inkey a.key -export -out a.pfx 

Then use normally with the .NET certificate class such as:

using System.Security.Cryptography.X509Certificates;  X509Certificate2 combinedCertificate = new X509Certificate2(@"C:\path\to\file.pfx"); 

Now you can follow the example from MSDN for encrypting and decrypting via RSACryptoServiceProvider:

I left out that for decrypting you would need to import using the PFX password and the Exportable flag. (see: BouncyCastle RSAPrivateKey to .NET RSAPrivateKey)

X509KeyStorageFlags flags = X509KeyStorageFlags.Exportable; X509Certificate2 cert = new X509Certificate2("my.pfx", "somepass", flags);  RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)cert.PrivateKey; RSAParameters rsaParam = rsa.ExportParameters(true);