小编dpi*_*wal的帖子

我们是postgres的新手,我们有以下查询,我们可以从中选择每个类别的前N个记录.

 create table temp (
     gp char,
     val int
 );

 insert into temp values ('A',10);
 insert into temp values ('A',8);
 insert into temp values ('A',6);
 insert into temp values ('A',4);
 insert into temp values ('B',3);
 insert into temp values ('B',2);
 insert into temp values ('B',1);

 select a.gp,a.val
 from   temp a
 where  a.val in (
              select b.val
              from   temp b
              where  a.gp=b.gp
              order by b.val desc
             limit 2);

上述查询的输出是这样的

 gp   val
 ----------
 A    10
 A    8
 B    3
 B    2

但是我们的要求是不同的,我们希望从n类未固定的每个类别中选择前n%记录,n基于每个组中元素的百分比.

我有一个数字签名的 XML 文件和签名者的公共证书，我想验证签名。响应 xml 的原始内容返回 false，但是当我修改 xml 时它返回 true。我的java代码如下：-

import java.io.FileInputStream;
import java.io.IOException;
import java.security.GeneralSecurityException;
import java.security.Security;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import javax.xml.crypto.dsig.XMLSignature;
import javax.xml.crypto.dsig.XMLSignatureFactory;
import javax.xml.crypto.dsig.dom.DOMValidateContext;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.w3c.dom.Document;
import org.w3c.dom.NodeList;
public class SignatureVerifierOneFile {
  public static void main(String[] args){       
        Security.addProvider(new BouncyCastleProvider());       
        //Signed xml path
        String signedXmlPath = "C:/signedXML.xml";      
        SignatureVerifierOneFile signatureVerifier = new SignatureVerifierOneFile();
        boolean signatureStatus = 
signatureVerifier.verify(signedXmlPath,"C:/Cert.cer");
        System.out.println("xml signature validateionis " + signatureStatus);

    }
    public boolean verify(String signedXml,String publicKeyFile) {

        boolean verificationResult = …

我正在使用itext库验证数字签名的pdf,以下示例可在https://developers.itextpdf.com/examples/security/digital-signatures-white-paper/digital-signatures-chapter-5站点上找到.我在验证数字签名文档时遇到以下错误.任何人都可以帮助解决这个问题.

Exception in thread "main" ExceptionConverter: java.security.NoSuchAlgorithmException: SHA256with1.2.840.10045.4.3.2 Signature not available
at java.security.Signature.getInstance(Signature.java:229)
at com.itextpdf.text.pdf.security.PdfPKCS7.initSignature(PdfPKCS7.java:697)
at com.itextpdf.text.pdf.security.PdfPKCS7.<init>(PdfPKCS7.java:459)
at com.itextpdf.text.pdf.AcroFields.verifySignature(AcroFields.java:2420)
at com.itextpdf.text.pdf.AcroFields.verifySignature(AcroFields.java:2373)
at nic.test.C5_01_SignatureIntegrity.verifySignature(C5_01_SignatureIntegrity.java:24)
at test.ExtractSignInfor.inspectSignature(ExtractSignInfor.java:95)
at test.ExtractSignInfor.inspectSignatures(ExtractSignInfor.java:135)
at test.ExtractSignInfor.main(ExtractSignInfor.java:63)