c# - 无法获得签名

Question

我需要带有证书的签名数据。我的签名方法：

private static string CreateSignature(string signatureString)
    {
        SHA1CryptoServiceProvider sha1 = new SHA1CryptoServiceProvider();
        X509Certificate2 cert = new X509Certificate2(HttpContext.Current.Server.MapPath("~/eCommerceConnectCrfs/test-server.cert"), "password");
        RSACryptoServiceProvider rsaCryptoIPT = (RSACryptoServiceProvider)cert.PrivateKey;
        ASCIIEncoding encoder = new ASCIIEncoding();
        byte[] binData = encoder.GetBytes(signatureString);
        byte[] binSignature = rsaCryptoIPT.SignData(binData, sha1);
        return Convert.ToBase64String(binSignature);
    }

但字段 cert.PrivateKey 为 NULL。我有带有私钥的 *.pem 文件，但我不知道如何使用它。请帮助获得签名！