pq一方共有の鍵2つから秘密鍵をつくる。

using System;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Threading.Tasks;

namespace decrypt
{
    class Program
    {
        private static BigInteger[] Extended_GCD(BigInteger A, BigInteger B)
        {
            if (A < B) //if A less than B, switch them 
            {
                var temp = A;
                A = B;
                B = temp;
            }
            BigInteger r = B;
            BigInteger x0 = 1, y0 = 0;
            BigInteger x1 = 0, y1 = 1;
            BigInteger x = 0, y = 0;

            while (r > 1)
            {
                r = A%B;
                var q = A/B;
                x = x0 - q*x1;
                y = y0 - q*y1;
                x0 = x1;
                y0 = y1;
                x1 = x;
                y1 = y;
                A = B;
                B = r;
            }
            return new BigInteger[] {r, x, y};
        }


        private static BigInteger[] KeyExtract(string first,string second)
        {
            var c1 =
                new X509Certificate2(X509Certificate.CreateFromCertFile(first)).PublicKey.Key as
                    RSACryptoServiceProvider;
            var c2 =
                new X509Certificate2(X509Certificate.CreateFromCertFile(second)).PublicKey.Key as
                    RSACryptoServiceProvider;

            var cred1Mod = new BigInteger(c1.ExportParameters(false).Modulus.Reverse().Concat(new byte[] {0}).ToArray());
            var cred2Mod = new BigInteger(c2.ExportParameters(false).Modulus.Reverse().Concat(new byte[] {0}).ToArray());

            var surplus = BigInteger.One;

            var i = BigInteger.Max(cred1Mod, cred2Mod);
            var j = BigInteger.Min(cred1Mod, cred2Mod);

            while (surplus != 0)
            {
                surplus = i%j;
                i = j;
                j = surplus;
            }

            return new BigInteger[] {i, (cred1Mod/i), (cred2Mod/i)};
        }

        private static RSAParameters makeParameters(BigInteger p, BigInteger q)
        {
            var e = 65537;

            if (p < q)
            {
                var r = q;
                q = p;
                p = r;
            }

            BigInteger u;
            var d = (Extended_GCD(e, (p - 1) * (q - 1))[2] + (p - 1) * (q - 1)) % ((p - 1) * (q - 1));

            var iq = (Extended_GCD(q, p)[2] + p) % p;
            if ((iq * q) % p != 1)
                throw new Exception("FAILED!");

            RSAParameters param;

            var tmp = new RSACryptoServiceProvider(2048).ExportParameters(true);

            param.P = p.ToByteArray().Take(tmp.P.Length).Reverse().ToArray();
            param.Q = q.ToByteArray().Take(tmp.Q.Length).Reverse().ToArray();
            param.D = d.ToByteArray().Take(tmp.D.Length).Reverse().ToArray();
            param.DP = (d % (p - 1)).ToByteArray().Take(tmp.DP.Length).Reverse().ToArray();
            param.DQ = (d % (q - 1)).ToByteArray().Take(tmp.DQ.Length).Reverse().ToArray();
            param.InverseQ = iq.ToByteArray().Take(tmp.InverseQ.Length).Reverse().ToArray();
            param.Exponent = new BigInteger(e).ToByteArray().Take(tmp.Exponent.Length).Reverse().ToArray();
            param.Modulus = (p * q).ToByteArray().Take(tmp.Modulus.Length).Reverse().ToArray();

            new RSACryptoServiceProvider().ImportParameters(param);

            return param;
        }

        private static void Main(string[] args)
        {
            var keys = KeyExtract("01.der", "02.der");

            using (var w = new StreamWriter(File.Create("01.pem")))
            {
                PEM.ExportPrivateKey(makeParameters(keys[0],keys[1]), w);
                w.Flush();
                w.Close();
            }

            using (var w = new StreamWriter(File.Create("02.pem")))
            {
                PEM.ExportPrivateKey(makeParameters(keys[0], keys[2]), w);
                w.Flush();
                w.Close();
            }

        }
    }
}