Encrypting class files

Does anyone know if its possible to encrypt your compiled class files?
Purely for the reasons to stop people using tools to decompile your class files before they have paid for them etc..
Cheers

Yes. Just use any encryption program, like PGP.
The problem is of course that the JVM won't be able to decrypt it, so that is kind of pointless. :)
What you are looking for is not encryption, but obfuscation. Yes, there are programs out there that will change your bytecode in order to
a) Make it harder for decompilers to decompile it
and/or
b) Make it harder for humans to understand what is going on if someone still manages to decompile the bytecode.
Once upon a time I used a program called "SourceGuard" or something like that for this. I have later seen other programs mentioned in discussions about this, like "Zelix Klassmaster" http://www.zelix.com/klassmaster/
None of these tools should be expected to be absolutely safe, but they will make it so much harder that most people probably won't care to do the decompilation.
There is also a risk that the changes done with the bytecode will make the code unrunnable by some VMs.

Similar Messages

Building Encrypted Class Files

I would like to know if a Java file can be compiled in such a way that it can't be decompiled? Using a decompiler, any Java Class file can be decompiled, thus revealing the source code. But I have seen some Java Class files that when decompiled, gives encoded characters, instead of the text. So, the source code cannot be seen, but can be executed. How is this achieved? Is there some way to encrypt the class file, and still keep it executable?
Thanks,
TechnoSam

Use a code obfuscator. This makes it harder to read, but
it can still be decompilet. Since Java is compiled into
bytecodes, all you need to do is translate the bytecodes
back into source code in order to see the "source".
When you get right down to it, any program written
in any language can be decompiled. Microsoft used to
ship a program named 'debug' with DOS. This program
allowed you to decompile .exe files.
Mark

Advice on encrypting the class files so that it cannot be reverse engineer

Hi,
i will like to ask if there is any way where we can encrypt the class files so that people cannot use reverse engineering to modify the codes?
Thanks

tommickey wrote:
leeChaolan wrote:
Hi,
i will like to ask if there is any way where we can encrypt the class files so that people cannot use reverse engineering to modify the codes?
ThanksIn addition to what others said encryption is totally different from what you want.(called as 'obfuscation').Not necessarily. You could encrypt the class files, but then you would have to provide an unencrypted ClassLoader (and possibly Main if you didn't want to put the ClassLoader in the bootstrap classpath) that is capable of decrypting them. Then, someone could decompile this ClassLoader (obfuscated or not) and either simply take the decryption algorithm and decrypt the classes, or even easier, simply modify the ClassLoader to write out the decrypted class files itself.
So, why would you? ;-)

Java .class files low-level (hex) editing?

Hello, all!
I have a problem:
There is an application (namely a J2ME instant messanger for Nokia mobiles). It is a non-commercial application. But unfortunately, there is a bad bug in the application. It is not even a bug, it is more a feature - but it does not support Russian (cyrillic) characters encoding. Russian characters in all incoming messages are displayed with a kind of hierogliphs.
I wrote to developers (and not only I did - I know at least 3 russian people who also wrote a developer about this) - but unsuccessful. Developer keeps silent.
So what I decided is to fix the bug by myself :) By decompilation, e t.c.
Unfortunally, the midlet is hard obfuscated :( So it took me some hours of reading obfuscated code, and now I see some ways which I must try to solve the problem. I think I must write a code which processes the desired String variables (Despite obfuscation, I can see which Strings to process to obtain effect), and inject this code somewhere to be executed, and from where it could reach the desired Strings.
But the great problem is that obfuscated classes produce a lot of erroneus code, which can not be compiled back. So I decide to reject the decompilation at all, or at least of the big classes (where it is a lot of errors, which I can not correct manually).
In some moments, the application stores the desired Strings in the java.lang.Hashmap. So if I could place my processing code into the Hashmap methods, then I could easily process what I need.
Evidently, I can not edit the real basic java classes. But why not to make the applicaton using my class with identical functionality, instead of java.jang.Hashmap?
My idea now is to edit the .class file of the midlet (yeah, edit with notepad), replacing all the references to "java/lang/Hashmap" or "java.lang.Hasmap" with "java/lang/Fuckmap" and "java.lang.Fuckmap".
So did I (replaced it in norepad).
I read the Sun's documentation about .class file structure, and it seems that nothing is against it. There is no encryption, no checksums, e t.c.
Then I produced "Fuckmap" class this way:
import java.lang.Hashmap;
class Fuckmap extends Hashmap {}
(compiled and preverified it of course)
And placed it into java/lang folder inside the application jar.
I instead of working properly, I get the "Application error - invalid constant pool entry"). What do I do wrong?

2 lexzeus, mlk:
I understood you.
I jave just tried it, and used "jaga/util/Fuckmap" (jaga, not java). But still the same ("Invalid constant pool entry). I tried two times (maybe I could make something wrong?), both times took "clean" jar package, e t.c.
Do you really think jav.alang.Hashmap should help now? I have ony little hope :(

How can I hide constant variable value in class file?

hi,everybody.
I am having a problem with constant variable which define connectted with database(URL,username,password).
when I used UltraEdit to open class file, I can see constant variable value.
Thanks!

OK, let's see. Firstly, if I may correct your terminology, the phrase "constant variable" is a paradox (I think that is the right word). You have either one of the other. You declaration is either 'constant' or 'variable' (ie: it can change at run-time or it doesn't). People often use the term 'variable' and 'declaration' interchangably which is where the confusion lies.
Anyway, onto the real problem. It seems that you want to protect your connection details (in particular the password, I would guess). Unfortunately, Java compiles not to machine-code, but byte-code which is semi-human-readable. So people, if they are inquisitive enough, will always be able to see it if they try hard enough.
You can do one of two things:
(1) Get an 'obfusticator'. An obfusticator is something that you run over Java source files and it completely messes it up (without modifying functionality). You then compile and the original source is such gibberish that the byte-code is very difficult to understand. However, the password will still be in there somewhere.
(2) Don't put sensitive information into your source. Have this kind of info in external files (encrypted) and allow your class to read this file, decrypt, and use it.
Hope that helps.
Ben

Protect class Files from Decryption?

Hey. I have a couple of Java classes I wish to share with some people, but I would like to add a copyright/credit giving message into the program. This code is the kind of code people would want to steal and take credit for, which is why I'm only (if I can encrypt them) giving out the .class files and not the source code. The thing is, it took me about 5 seconds of searching on Google to find programs that can easily be used to decrypt .class files and get a re-compilable source from them.
So I was wondering, is there a [somewhat] foolproof way to protect against people decrypting the .class files to get the source? Is there some program I can run them through that will make it so they can't be decompiled, but will still run on a machine with a standard Java runtime installed? I probably won't be giving out the classes if there isn't a way to do this.
Thanks

There are programs out their that semi pseudo scramble your code but if someone wanted to they can always reverse it and have a look at your source. What you have to admit is that your code is nothing special and nobody would really want to steal your code. If you are paranoid about it, about the most you can do is add some copyright message and threaten them with legal action.

Problem in Packages while compailing the code "bad class file"

Hi,
I wrote the code for encrypt the password in .java file and i used package also
i'm importing that package and i'm compiling the .java file it throws the error like this.
regi1.java:23: cannot access EncryptPassword
bad class file: .\EncryptPassword.java
file does not contain class EncryptPassword
Please remove or make sure it appears in the correct subdirectory of the claspath
EncryptPassword enc = new EncryptPassword();
                        ^
1 error
EncryptPassword.java
package encrypt;
import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import sun.misc.BASE64Decoder;
import sun.misc.BASE64Encoder;
import sun.misc.CharacterEncoder;
public class EncryptPassword
     public EncryptPassword() { }
     public EncryptPassword instance;
     public String getEncrypt(String plaintext) throws Exception
     MessageDigest md = null;
     try
     md = MessageDigest.getInstance("SHA"); //step 2
     catch(NoSuchAlgorithmException e)
     throw new Exception(e.getMessage());
     try
     md.update(plaintext.getBytes("UTF-8")); //step 3
     catch(UnsupportedEncodingException e)
     throw new Exception(e.getMessage());
     byte raw[] = md.digest(); //step 4
     String hash = (new BASE64Encoder()).encode(raw); //step 5
     return hash; //step 6
     public EncryptPassword getInstance() //step 1
     if(instance == null)
     return new EncryptPassword();
     else
     return instance;
}regi1.java
import encrypt.*;
import java.io.*;
import java.sql.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class regi1 extends HttpServlet
     public regi1(){     }
      public void service(HttpServletRequest httpservletrequest, HttpServletResponse httpservletresponse)
        throws ServletException, IOException
        java.io.PrintWriter printwriter = httpservletresponse.getWriter();
        HttpSession httpsession = httpservletrequest.getSession(true);
        httpservletresponse.setContentType("text/html");
        try
        {     EncryptPassword enc = new EncryptPassword();
               String Email = httpservletrequest.getParameter("email");
               String Password = httpservletrequest.getParameter("password");
*--- some code for database connection---*
String insQry1 = "INSERT INTO register (Email, Pass);
pStmt.setString(1,Email);
pStmt.setString(2,enc.getEncrypt(Password));
          catch( Exception exception)
               exception.printStackTrace();
               System.out.println(exception);
          httpservletrequest.getRequestDispatcher("./FileSubmit.jsp").forward(httpservletrequest,httpservletresponse);
{code}

Multipost: http://forum.java.sun.com/thread.jspa?threadID=5225097&tstart=0
Please continue the thread there and don't multipost!
People on the forum help others voluntarily, it's not their job.
Help them help you.
Learn how to ask questions first: http://faq.javaranch.com/java/HowToAskQuestionsOnJavaRanch
(Yes I know it's on JavaRanch but I think it applies everywhere)
----------------------------------------------------------------

Securing class files

Hello,
I read all these topics about securing class-files,
and about encryptors and stuff like that
so I tought this could be possible :
I've made an application and you can run it by using an exe-file.
In the same directory you find the class-files.
Now I archived the class files with winrar, and set a password on it.
I tried to use the exe-file to run the application, but it can't.
Obvisiously, it can't find the mainclass.
So I was wondering if there is a way to make clear to the exe -file, that the main class is in that zip-file, and that you need <this password> to get in to the zip-file.
I think it's possible, but I don' know how to do it.
I thought Google would know it, but he don't.
...

The collision wasn't "stumbled" across it was found
because the researchers
found a way (from your link) to "reduce the search
space".I didn't say "the collision was "stumbled" across"...
>
This means that, under certain circumstances, inputs
producing the same hash
can be found for other inputs + hashes.That is what I said...
>
Did you have a look at the PDF ?
http://eprint.iacr.org/2004/199.pdf
This doesn't damage the use of MD% for verifyingthat
file contents are unchanged, so to OP - go aheadand
use it.Well, it does because it means that there is the
potential for another
file that is not the same to return the same
sum - hence the program
won't realise the difference.And the situation is no worse now that before the duplication was found. Any encryption/hashing routine can create duplicate hashes - because none of them have an infinitely large numberspace.

Secure the class-file - Make decompile impossible?

Hi everybody!
I have "basic" (?) question concerning Java-Class-Files.
As our company is currently developing an Web-Intranet-Solution, we have to give the source-Code (class-files, jsp-pages and the jar-files) tio the customer.
But our problem is, that we do not want him to be able to read the source or manipulate it. Now it is very easy to do so, I am just thinking of jad - a java decompiler, for example...
How can we prevent this?
What are common approaches for this problem?
Thx in Advance!
Stef

You will be better off using obfustacting softwares, rather than customized class loader with encrypted classes as the nature of bytecodes will leave the decompiling process with an intercepting point always. Thaw will make the decompiling trivial regardless of the encryption strength you use.
For obfuscation, Proguard is a GPL one, and Retroguard is suitable for commercial shipments, but I suppose there are more than a bunch of them out there, which I am not aware of.

Encrypting configuration files

I have an application and a configuration file which is xml format.To prevent the user from fiddling with the xml file with a text editor I want to only provide an encrypted version of the file in the released product which I decrypt in memory.
Ive done this using the DES algorithm but of course to decrypt the file you need to use the same key that you encrypted it with so Ive converted the key to a transparent key and written it to another file. So when the application starts it reads the key from the file and then uses it to decrypt the config file on the fly.
This works but because the key is in a plain text file it would be fairly easy for someone to recognise it as a key try it with the various known key algorithms and decode the config file. How can I avoid this, (I could encrypt the key but then I would need to store the key I encrypted it with in another file and so on,,)
Thanks Paul

OK,sorry I dont quite get the multiple key thing
The program below encode a file using key defined in the file then decrypts the file,if you have any spare time I would be grateful if you could explain how would I modify this t o support mutiple keys,
public class ConvertFile
    private static byte[]    rawKey={111,50,78,45,12,78,90,99};
    public static void main(String[] args)
        throws Exception
        encryptFile(new File("originalfile"),
                    new File("encryptedfile"));
        decryptFile(new File("settings.xml.decrypt"),
                    new File("encryptedfile"));
    public static SecretKey retrieveKey()
        try
            //Rebuild Transparent key and convert to secret key
            DESKeySpec keySpec = new DESKeySpec(rawKey);
            SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DES");
            SecretKey key = keyFactory.generateSecret(keySpec);
            return key;
        catch(Exception e)
            e.printStackTrace();
            return null;
    public static void createKey()
        try
            //Create Secret key
            KeyGenerator keyGen = KeyGenerator.getInstance("DES");
            SecretKey key = keyGen.generateKey();
            //Now get transparent form of key
            SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DES");
            DESKeySpec keySpec =(DESKeySpec)keyFactory.getKeySpec(key,javax.crypto.spec.DESKeySpec.class);
            byte[] rawKey = keySpec.getKey();
        catch (Exception e)
            e.printStackTrace();
    /** Encrypt a file */
    public static void encryptFile(File plainFile, File encryptedFile)
        try
            byte[] buf = new byte[1024];
            int len;
            //Create Cipher
            Cipher cipher = Cipher.getInstance("DES");
            cipher.init(Cipher.ENCRYPT_MODE, retrieveKey());
            //Encrypt the Data
            FileInputStream fis = new FileInputStream(plainFile);
            FileOutputStream fos = new FileOutputStream(encryptedFile);
            while ( (len = fis.read(buf)) != -1)
                fos.write(cipher.update(buf, 0, len));
            fos.write(cipher.doFinal());
            fos.close();
            fis.close();
        catch (Exception e)
            e.printStackTrace();
    /** Decrypt a file */
    public static void decryptFile(File plainFile, File encryptedFile)
        try
            byte[] buf = new byte[1024];
            int len;
            //Create Cipher
            Cipher cipher = Cipher.getInstance("DES");
            cipher.init(Cipher.DECRYPT_MODE,retrieveKey());
            //Decrypt the Data
            FileInputStream fis = new FileInputStream(encryptedFile);
            FileOutputStream fos = new FileOutputStream(plainFile);
            while ( (len = fis.read(buf)) != -1)
                fos.write(cipher.update(buf, 0, len));
            fos.write(cipher.doFinal());
            fos.close();
            fis.close();
        catch (Exception e)
            e.printStackTrace();
}

Concealing strings in .class files

How do I conceal strings in the .class file which have been defined in the source file during compilation, in order to hide theses strings (passwords etc.) from people using the programme?

If you are storing passwords then store them as MD5 encrypted strings. When the users input the password, you MD5 encrypt it as well, and check the two MD5 encrypted strings up against eachother. If they are the same, the password is correct - if not, it is false.
This is a so-called "one-way" encryption algorithm, which means that you cannot decrypt the password once it has been encrypted - so it is mostly useful with passwords.
You can use Java's MessageDigest class, or you can check out this MD5 library which may be easier to use:
http://ostermiller.org/utils/MD5.html
Best regards,
Bjorn Borresen

DrClap, schapel, jschell whats better architecture jar files or class files

What would be a better architecture for deployment. Jar files or class files. We were recently having a debate at work as to generate installation scripts that put jar files on the system where the app is being installed or to make the script generate class files at the desired location.
It would be good to know as to what you ppl think and what are the pros and cons of both.
Some claimed with class files it would be easier to put in patches later on and I kinda agree with that. However your thoughts would be appreciated.
Come on DrClap, schapel, jschell, jsalonen....... I am a fan of yours already

This, of course, is why topics shouldn't be addressed to individuals - they are likely to never see it. Today is the first time I have seen this, and that is due solely to a search that I was running.
Why would you use one over the other?
----Distribution costs
It takes more time to send a bunch of class files down the wire than a single jar file. However, it takes more time to send a jar than a single class file. So there is a delicate trade off in a web application where one is using java. If you send the jar every time it is modified the entire jar must be moved. Class files take less time, but all of them must be moved for a new person.
So in this case if you have a fixed based of users (internal use), and a majority of the files do not change then class files are better. If the user base is transitory and large (external use) then a jar file is probably better.
----Usage:
You can use reflection to dynamically add functionality to an application. If you use a jar file then the application must be restarted for the changes to take effect. If you use class files then the application does not have to be restarted. Obviously the second only works if you have some way to dynamically notify the application that new classes are available - like a database.
This becomes very relevant if you do not want to restart the application and you do not have a distributed application base in place.
Also the older classes, even if not used, are still loaded in the application unless it is restarted (presuming a class loader is not used.) Most of the time this does not make difference.
Since in the above it is likely that there is not a large distribution problem, when the usage is a concern then the best strategy is as class files. They allow the most flexibility and there is unlikely to be any concern at all how it is packaged.
-------Misc
Speed. Someone suggested speed. Guessing I would say that the jar file is going to be faster. Once the Sun jvm gets the file it doesn't let go. And as pointed out opening files is rather costly. However, I doubt this is going to be a significant cost on any system. Startup costs, although sometimes annoying, are seldom significant in terms of application performance.
Security. The is a certain level of security with a jar file. You have one jar file and distribute it to the help desk people. And one of them deletes it accidently. It is going to be a lot easier to figure that out than if they deleted one class file. Additionally it is probably easier to encrypt/decrypt a jar file (if you really must) than class files.
Dynamic class loading. If you are storing classes in non-traditional places, like a database rather than files, then it is probably going to be easier if you do it as class files.
Testing. It is easier to replace a class file in a testing environment to fix a bug than a jar file. Particularily if the make process for the jar file rebuilds everything.

How to convert class file

Hi all, I am new in java card development.
This is my environment setting:
@echo off
set JC_HOME=C:\JavaCard\java_card_kit-2_2
set JAVA_HOME=C:\j2sdk14103
set PATH=.;%JC_HOME%\bin;%PATH%
I have created the Wallet applet according to 'Zhiqun Chen" text book and named it WalletApp.java.
I have compiled this file to a class file.
This is where is I saved my file
C:\JavaCard\java_card_kit-2_2\samples\src\com\sun\javacard\samples\WalletApp\WalletApp.java.
But I don't really understand how to convert the class file.
May I know what is this for?
-out EXP JCA CAP
-exportpath
-applet 0xa0:0x0:0x0:0x0:0x62:0x3:0x1:0xc:0x1:0x1
com.sun.javacard.samples.HelloWorld.HelloWorld
com.sun.javacard.samples.HelloWorld
0xa0:0x0:0x0:0x0:0x62:0x3:0x1:0xc:0x1 1.0
must save this in what file?
I tried to type in the command line below (and the result):
C:\JavaCard\java_card_kit-2_2\samples>converter -config scr\com\sun\javacard\sap
les\Wallet\Wallet.opt
error: file scr\com\sun\javacard\saples\Wallet\Wallet.opt could not be found
Usage: converter <options> package_name package_aid major_version.minor_ver
sion
OR
converter -config <filename>
use file for all options and parameters to converter
Where options include:
-classdir <the root directory of the class hierarchy>
set the root directory where the Converter
will look for classes
-i support the 32-bit integer type
-exportpath <list of directories>
list the root directories where the Converter
will look for export files
-exportmap use the token mapping from the pre-defined export
file of the package being converted. The converter
will look for the export file in the exportpath
-applet <AID class_name>
set the applet AID and the class that defines the
install method for the applet
-d <the root directory for output>
-out [CAP] [EXP] [JCA]
tell the Converter to output the CAP file,
and/or the JCA file, and/or the export file
-V, -version print the Converter version string
-v, -verbose enable verbose output
-help print out this message
-nowarn instruct the Converter to not report warning messages
-mask indicate this package is for mask, so restrictions on
native methods are relaxed
-debug enable generation of debugging information
-nobanner suppress all standard output messages
-noverify turn off verification. Verification is default
*********************************************************May I know What is the correct command line to convert the class file and what must I do to before converting the class file?
I saw some article saying we must use JDK1.3, is it a must?
Your solution is highly appreciated.
Thank you!

Hi Ricardo,
I saved the file below as WalletApp.opt in the directory of scr\com\sun\javacard\samples\WalletApp
-out EXP JCA CAP
-exportpath c:\javacard\java_card_kit-2_2\api_export_files
-applet 0xa0:0x0:0x0:0x0:0x62:0x3:0x1:0xc:0x2:0x1 WalletApp.WalletApp
WalletApp 0xa0:0x0:0x0:0x0:0x62:0x3:0x1:0xc:0x2 1.0
But I still have the problem below:
C:\JavaCard\java_card_kit-2_2\samples>converter -config scr\com\sun\javacard\sam
ples\WalletApp\WalletApp.opt
error: file scr\com\sun\javacard\samples\WalletApp\WalletApp.opt could not be fo
und
Usage: converter <options> package_name package_aid major_version.minor_ver
sion
OR
converter -config <filename>
May I know What's wrong with my command or file?
Where can I download JDK because what I can find is J2SDK.
Thank you!

Error while converting class file to exp and jca file

error while converting *.class file to *.exp and *.jca file
=====================================================================================================================
linux-y60u:/home/admin/java_card_kit-2_2_1/samples/src # converter -exportpath "/home/admin/java_card_kit-2_2_1/lib/" com/sun/javacard/samples/HelloWorld 0x00:0x01:0x02:0x03:0x04:0x05:0x06:0x07:0x0b 1.0 -v -applet 0x00:0x01:0x02:0x03:0x04:0x05:0x06:0x07:0x0b:0x01 Identity
Java Card 2.2.1 Class File Converter, Version 1.3
Copyright 2003 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms.
parsing /home/admin/java_card_kit-2_2_1/samples/src/com/sun/javacard/samples/HelloWorld/HelloWorld.class
parsing /home/admin/java_card_kit-2_2_1/samples/src/com/sun/javacard/samples/HelloWorld/Identity.class
error: com.sun.javacard.samples.HelloWorld.HelloWorld: unsupported class file format of version 50.0.
error: com.sun.javacard.samples.HelloWorld.Identity: unsupported class file format of version 50.0.
conversion completed with 2 errors and 0 warnings.
=====================================================================================================================

i compile a file javacard use this command:
===
javac -source 1.3 -target 1.1 -g -classpath ./classes:../lib/api.jar:../lib/installer.jar src/com/sun/javacard/samples/Identity/Identity.java
===
and try to convert this class use this command
===
/home/xnuxerx/admin/java_card_kit-2_2_1/bin/converter -exportpath "/home/xnuxerx/admin/java_card_kit-2_2_1/lib/" com/sun/javacard/samples/Identity 0x00:0x01:0x02:0x03:0x04:0x05:0x06:0x07:0x0b 1.0 -v -applet 0x00:0x01:0x02:0x03:0x04:0x05:0x06:0x07:0x0b:0x01 Identity
===
result convert:
===
Java Card 2.2.1 Class File Converter, Version 1.3
Copyright 2003 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms.
parsing /home/xnuxerx/admin/java_card_kit-2_2_1/samples/classes/com/sun/javacard/samples/Identity/Identity.class
converting com.sun.javacard.samples.Identity.Identity
error: export file framework.exp of package javacard.framework not found.
conversion completed with 1 errors and 0 warnings.
===
why ??
please your comment for this problem.
thank 4 all.

NetBeans IDE 4.1 & a class file

Hello, I use NetBeans IDE 4.1 to write my java programs. I've been assigned to write a program that will prompt the user for an input. To make this easier, my University has written a file called "In.java" so we can easily get any type of input from the user. For example if we want the user to enter his/her name, we would write:
System.out.println ("Please enter your name: ");
String name = In.getString(); //allows user to enter a string
The "In.java" file is supposed to be kept and compiled in the directory I'm working in. I kept it in C:\j2sdk1.4.2_09 and it works fine when I write a program in notepad but not NetBeans IDE. It does not recognize the "In.class" file. Any way to get around this? Thanks in advance.

NetBeans 4.1 is bundled with J2sdk1.5.0_04. Check your NetBeans classpath and see if it is pointing to the newer sdk.
If so, it needs to be changed to point to your older SDK...
... or, get your In.class file into the current NetBeans classpath...
... or read up on, and use the new Scanner class in the new sdk.

Encrypting class files

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