using System;

using System.Text;

using Org.BouncyCastle.Asn1;

using Org.BouncyCastle.Asn1.Pkcs;

using Org.BouncyCastle.Asn1.X509;

using Org.BouncyCastle.Crypto;

using Org.BouncyCastle.Crypto.Generators;

using Org.BouncyCastle.Crypto.Parameters;

using Org.BouncyCastle.Crypto.Prng;

using Org.BouncyCastle.Math;

using Org.BouncyCastle.OpenSsl;

using Org.BouncyCastle.Pkcs;

using Org.BouncyCastle.Security;

using Org.BouncyCastle.X509;

public class Program

{

    public static void Main()

    {

        var serializedPublicKey = "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAw8aoKG/l1bmCJehOhgmz1KVF27NRUwy8uc5l7/nOqLsoWjyprZCb0MlOA2VBH0okvIamXu8xXkup7tvBRUXUEKQBRrVR2shzwl2NAgdNkgNU50Olm1ruYiHu58ruwZdRnWvckXW5vJlCjeg/X/XJ0R8VtP91r2xli0tn4W/l5l4CNR7e6QbizoKb+/OwpHmL4uIQzlvKy7Yurjk5f8qTSarMns5Yb/HSiZl2JJgu6eWwqSt/ErxqwCknzHYFw91eCRoHmQnAXYS8nMuypLM74mOZJUGdByYOR5f3fTve8UrxuteoVGHdZwEbE+hhx+THFMv68Yy6B5niGQ8i1rBBCwIDAQAB";   

        var publicKey = (Org.BouncyCastle.Crypto.Parameters.RsaKeyParameters)Org.BouncyCastle.Security.PublicKeyFactory.CreateKey(Convert.FromBase64String(serializedPublicKey));

        SubjectPublicKeyInfo publicKeyInfo = SubjectPublicKeyInfoFactory.CreateSubjectPublicKeyInfo(publicKey);

        Console.WriteLine(FormatPem(Convert.ToBase64String(publicKeyInfo.GetEncoded()), "PUBLIC KEY"));

    }

    /// <summary>

    /// Decodes the specified string.

    /// </summary>

    /// <param name="arg">The argument.</param>

    /// <returns></returns>

    /// <exception cref="System.Exception">Illegal base64url string!</exception>

    public static byte[] Decode(string arg)

    {

        var s = arg;

        s = s.Replace('-', '+'); // 62nd char of encoding

        s = s.Replace('_', '/'); // 63rd char of encoding

        switch (s.Length % 4) // Pad with trailing '='s

        {

            case 0: break; // No pad chars in this case

            case 2: s += "=="; break; // Two pad chars

            case 3: s += "="; break; // One pad char

            default: throw new Exception("Illegal base64url string!");

        }

        return Convert.FromBase64String(s); // Standard base64 decoder

    }

    private static string FormatPem(string pem, string keyType)

    {

        var sb = new StringBuilder();

        sb.AppendFormat("-----BEGIN {0}-----\n", keyType);

        int line = 1, width = 64;

        while ((line - 1) * width < pem.Length)

        {

            int startIndex = (line - 1) * width;

            int len = line * width > pem.Length

                ? pem.Length - startIndex

                : width;

            sb.AppendFormat("{0}\n", pem.Substring(startIndex, len));

            line++;

        }

        sb.AppendFormat("-----END {0}-----\n", keyType);

        return sb.ToString();

    }

}