// verify EC signature
//
// Verify UserID input
//   will read files: UserID.public, input, input.sig
//
import java.math.BigInteger;
import java.security.*;
import java.security.spec.*;
import java.security.interfaces.*;
import java.io.*;

public class Verify {

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

   // input file names
   //
    String UserID = args[0];
    String input = args[1];
    String sig = input + ".sig";
      
    BigInteger WX, WY;

   // get signer EC public key
   //
    String fname = UserID + ".public";
    File file = new File( fname);
    int len = (int) file.length();
    FileInputStream fis = new FileInputStream( file);
    byte b[] = new byte[len];
    int nbytes = fis.read( b);
    fis.close();
    if( nbytes != len) throw new Exception( "bad read: " + fname); 

    X509EncodedKeySpec x509 = new X509EncodedKeySpec( b);
    KeyFactory kf = KeyFactory.getInstance( "EC");
    PublicKey pub = kf.generatePublic( x509);

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

    s.initVerify( pub);

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

    FileInputStream in = new FileInputStream( input);

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

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

    in.close();

   // check signature
   //
    file = new File( sig);
    len = (int) file.length();
    fis = new FileInputStream( file);
    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.");
  }
}