DKIM.OpenSslKey.DecodeOpenSSLPrivateKey C# (CSharp) Method

        public static byte[] DecodeOpenSSLPrivateKey([NotNull]string instr)
        {
            if (instr == null)
            {
                throw new ArgumentNullException("instr");
            }

            const string pemprivheader = "-----BEGIN RSA PRIVATE KEY-----";
            const string pemprivfooter = "-----END RSA PRIVATE KEY-----";
            string pemstr = instr.Trim();
            if (!pemstr.StartsWith(pemprivheader) || !pemstr.EndsWith(pemprivfooter))
                return null;

            var sb = new StringBuilder(pemstr);
            sb.Replace(pemprivheader, "");  //remove headers/footers, if present
            sb.Replace(pemprivfooter, "");

            string pvkstr = sb.ToString().Trim();	//get string after removing leading/trailing whitespace

            try
            {
                // if there are no PEM encryption info lines, this is an UNencrypted PEM private key
                return Convert.FromBase64String(pvkstr);
            }
            catch (FormatException e)
            {		//if can't b64 decode, it must be an encrypted private key
                throw new FormatException("Not an unencrypted OpenSSL PEM private key", e);
            }


        }
    }

        private PrivateKeySigner([NotNull] string privateKey)
        {
            if (privateKey == null)
            {
                throw new ArgumentNullException("privateKey");
            }

            _key = OpenSslKey.DecodeOpenSSLPrivateKey(privateKey);
        }