jsapi支付接口回调验签结果总是false 是什么原因呢

//certPublickey 解密后平台证书的公钥

// sign 接口应答签名

//s 接口应答字符串（应答时间戳\n应答随机串\n应答主体\n）

  public bool VerifySign(string certPublickey, string sign, string s)

    {

        writeLog(certPublickey+",,"+sign+",,"+s);

        X509Certificate2 publicCert = new X509Certificate2(Encoding.UTF8.GetBytes(certPublickey));

        RSACryptoServiceProvider publicKey = (RSACryptoServiceProvider)publicCert.PublicKey.Key;

        SHA256CryptoServiceProvider sHA = new SHA256CryptoServiceProvider();

        return publicKey.VerifyData(Encoding.UTF8.GetBytes(s), sHA, Convert.FromBase64String(sign));

    }