using Org.BouncyCastle.Crypto;

using Org.BouncyCastle.Crypto.Digests;

using Org.BouncyCastle.Crypto.Encodings;

using Org.BouncyCastle.Crypto.Engines;

using Org.BouncyCastle.OpenSsl;

using System;

using System.IO;

public class Program

{

        public static void Main()

        {

            // Ciphertext generated with the NodeJS code using the public key associated with this private key. 

            string encryptedKey = "QHfNSZ7pY/kd8bLDxIM/V1rGKj+5hIGZU2PwYIBf28IU8mtUnpicb7ipi9y/RuGKuCQKnOuDDYneO6i1mp/RsFaaoOsWFpHMllSGBElNKJkK/WqivlSCxpyp7Dpbx7ElPay2kEwz/bSIvZZEfOW4/9CEVpNWuL3JhcnJKBBiHLX62M6U4eOKeV0/y21q2pu/AyrAJ9kIpNkRf9L13h8JHKN5wAdO5toCBWI8+ePCHt/ApCTYK41ztBPtSZvXewNFMA/lV1Ioi9qycaycnkrYFVwTLpAeeMfKz2YtdNzsAw7Tft8uO5QwxoSocwJSw/F4gIEBpdjMGqJGKrvJKtl/9w==";

            Console.WriteLine(DecryptKey(encryptedKey));

        }

​

        public static string DecryptKey(string encryptedKey)

        {

            var privateKey = @"-----BEGIN RSA PRIVATE KEY-----

MIIEpAIBAAKCAQEAunF5aDa6HCfLMMI/MZLT5hDk304CU+ypFMFiBjowQdUMQKYH

Z+fklB7GpLxCatxYJ/hZ7rjfHH3Klq20/Y1EbYDRopyTSfkrTzPzwsX4Ur/l25Ct

dQldhHCTMgwf/Ev/buBNobfzdZE+Dhdv5lQwKtjI43lDKvAi5kEet2TFwfJcJrBi

RJeEcLfVgWTXGRQn7gngWKykUu5rS83eAU1xH9FLojQfyia89/EykiOO7/3UWwd+

MATZ9HLjSx2/Lf3g2jr81eifEmYDlri/OZp4OhZu+0Bo1LXloCTe+vmIQ2YCX7Ea

tUOuyQMt2Vwx4uV+d/A3DP6PtMGBKpF8St4iGwIDAQABAoIBAEpSY9BMSKJippgY

RvKvEjNbNrFhuoHUDI7OjBHpbkU/9XNr+/XKtg7pucv/lAZfMfE1Cjyki6Qi7Phl

5IlSoB16xZaqDfjmEAKxthFFs7jg8HM3WD4YbqQV8Ed6F+IONZPwbGH0H2QVcwRR

CXVqyAy8gFBVjZX3RiI9gU4gKMkn8qpmpr5lvG7L3PHKIhpWdvFAvOf16sfcDs43

hqEZAh+z3EzJXPRe0sUrYcF/5NVQV5jTh2vh8xjoDv1t5NSw6iW6tsCo2U1t4lRI

NlaHY8YASzJtDLnkhqCOiLmmUjzMhRDN4BrwtvnAwSNki/2poYk1Gv1Q5U1cPyCm

0rmq/bkCgYEA3e+jND6OS6ofGYUN6G4RapHzuRAV8ux1C9eXMOdZFbcBehn/ydhz

R48LIPTW9HiRE00um27lXfW5/POCaEUvfOp1UxTWeHZ4xICo40PBo383ZKW1MbES

1oiMbjkEqSFGRnTItnLU07bKbzLA7I0UWHWCEAnv0g7HRxk973FAsm8CgYEA1w8+

olZ2POBYeYgw1a0DkeJWKMQi/4pAgyYwustZo0dHlRXQT0OI9XQ0j1PZWoQS28tF

cmoEAg6f5MUDpdM9swS0SOCPI1Lc/f/Slus3u1O3UCezk37pneSPezskDhvV2cCl

JEYH8m/zwDAUlEi4KLIt/H/jgtyDd6pbxxc78RUCgYEAiE6VAxJknM4oeakBiL6J

TdXEReY+RMu7e4F2518/lJmoe5CaTCL3cnzFTgFyQAYIvD0MIgSzNMkl6Ni6QEY1

y1fIpTVIIAZLWAzZLXPA6yTIJbWsmo9xzXdiIJQ+a433NnClkYDne/xpSnB2kxJ2

63mIX0drFq1i8STsqDH7lVsCgYBWpQmzFeqlNC3xKPGj5QTfLbVQz1TaZ7T+IdDY

MT14DyvD4PoANVFksuDKLhoP4c5HR2o/Yn8i5Ql+ffGuSaE+EtMf2XlR3fyzSPJr

Y3Ecw+nDHXu4HRW6S2+TUoyAMq7CAF+Icb6Z6ojmEXj5FSM9Iixs4omjG3zMJZK8

b5vy0QKBgQCz0W7UmxNqT4Lzhyh0xj6+4Cm5rFlWznEAPKxvNNU9/Fv1D3tV7cpz

BI880NvTmlXkML1Dffg8RVyemL8I+ooeFnatdPYuzqNXw5zHMzkDfQynIdnjQB6z

UjjMiwwgZvvsG8hwLEtk9FTvPoMywAb4mZuQBQkFn5HuC2aOZjktdA==

-----END RSA PRIVATE KEY-----";

​

            var bytesToDecrypt = Convert.FromBase64String(encryptedKey);

            //var decryptEngine = new Pkcs1Encoding(new RsaEngine());

            var decryptEngine = new OaepEncoding(new RsaEngine(), new Sha256Digest(), new Sha256Digest(), null); // Fix 1: Apply OAEP/SHA256

            using (var txtreader = new StringReader(privateKey))

            {

                AsymmetricCipherKeyPair keyPair = (AsymmetricCipherKeyPair)new PemReader(txtreader).ReadObject();

                decryptEngine.Init(false, keyPair.Private);

            }

            var decrypted = Convert.ToHexString(decryptEngine.ProcessBlock(bytesToDecrypt, 0, bytesToDecrypt.Length)); // Fix 2: Apply hex encoding

            return decrypted;

        }

}