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

Reading a PEM RSA Private Key in .NET

In .NET, you can conveniently read a PEM RSA private key and instantiate an RSACryptoServiceProvider to decrypt data encrypted with the corresponding public key.

.NET 5 Support

.NET 5 now offers built-in support for this task. To utilize it, proceed as follows:

1. Generate a key pair and encrypt some text using an online tool like http://travistidwell.com/jsencrypt/demo/.
2. Use the following code snippet to import the PEM private key and decrypt the ciphertext:

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 BouncyCastle Solution

Before .NET 5, the BouncyCastle library provided 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)); 

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