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));