How to Read and Use a PEM RSA Private Key in .NET?

Reading a PEM RSA Private Key in .NET

In .NET, there are several approaches to read a PEM RSA private key and instantiate an RSACryptoServiceProvider for decryption purposes.

Using .NET 5 and Above

For .NET 5 and later versions, support for reading PEM RSA private keys is built-in. The following code sample demonstrates how to do this:

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));

Using Bouncy Castle Library

For older versions of .NET, you can utilize the Bouncy Castle library to read PEM RSA private keys. Here's an example:

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)); 

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