RSA Private Key Retrieval in .NET from PEM Format

In .NET, reading a PEM-formatted RSA private key and initializing an RSACryptoServiceProvider instance for decrypting data encrypted using the corresponding public key requires specific steps. This article outlines two approaches to accomplish this task.

.NET 5 and Later

Starting with .NET 5, a built-in capability is available to read PEM private keys:

var privateKey = @"-----BEGIN RSA PRIVATE KEY-----
{ the full PEM private key } 
-----END RSA PRIVATE KEY-----";

var rsa = RSA.Create();
rsa.ImportFromPem(privateKey.ToCharArray());

Pre-Installed Libraries

For earlier versions of .NET or if the built-in functionality is not suitable, external libraries like BouncyCastle provide a solution:

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));