// verify EC signature using public key from certificate
//
// CertVerify UserID input
//   will read files: UserID.cert, UserID.CAcert (if it exists), input, input.sig
// 
import java.security.*;
import java.security.spec.*;
import java.security.cert.*;
import java.security.interfaces.*;
import java.io.*;

public class CertVerify
{
  public static void usage()
  {
    System.err.println( "Usage: CertVerify UserID input");
    System.exit(1);
  }
  
  public static void main( String args[]) throws Exception
  {
    if( args.length != 2)
      usage();

    String UserID = args[0];
    String input = args[1];
    String sig = input + ".sig";

    String certfile = UserID + ".cert";
    String CAcertfile = UserID + ".CAcert";

   // read certificate and extract public key
   //
    FileInputStream fis = new FileInputStream( certfile);
    CertificateFactory cf = CertificateFactory.getInstance( "X509");
    X509Certificate cert = (X509Certificate) cf.generateCertificate( fis);
    fis.close();
    PublicKey pub = cert.getPublicKey();
    String alg = pub.getAlgorithm();

   // print some info and check validity (e.g. not expired)
   //
    System.out.println( "Certificate Subject:\n" + cert.getSubjectX500Principal());
    System.out.println( "Checking certificate validity...");
    cert.checkValidity(); // will throw an exception if not valid
    System.out.println( "Certificate is valid.");
    System.out.println( "Key type is " + alg);

   // if CAcert file exists, check the cert signature
   //
    fis = null;
    try { fis = new FileInputStream( CAcertfile); } catch( Exception ex) {}
    if( fis == null)
      System.out.println( CAcertfile +
	" file not found, skipping certificate signature check.");
    else
    {
      X509Certificate CAcert = (X509Certificate) cf.generateCertificate( fis);
      fis.close();
      PublicKey CApub = CAcert.getPublicKey();
      System.out.println( "Checking certificate signature...");
      cert.verify( CApub); // will throw an exception if not verified
      System.out.println( "Certificate signature is verified.");
    }

   // initialize signature
   //
    Signature s = Signature.getInstance( "SHA256withECDSA");

    s.initVerify( pub);

   // read message
   //
    byte msg[] = new byte[4096]; int nbytes;

    fis = new FileInputStream( input);

    System.err.println( "Verifying " + input + " using " + certfile);

    while( (nbytes = fis.read( msg)) > 0)
      s.update( msg, 0, nbytes);

    fis.close();

   // check signature
   //
    File file = new File( sig);
    int len = (int) file.length();
    fis = new FileInputStream( file);
    byte b[] = new byte[len];
    nbytes = fis.read( b);
    fis.close();
    if( nbytes != len) throw new Exception( "bad read: " + sig); 

    if( s.verify( b))
      System.err.println( "Signature is valid.");
    else
      System.err.println( "INVALID signature.");
  }
}