How to make a Java app compute an MD5sum of itself

I am trying to write an open-source online Java game which will feature, among other things, an online economy system. The problem with this is security: I need a way to verify that the client app accessing the server has not been modified in any way. The client will be available as both an applet and as a standard Java application, making this somewhat more difficult.
My current plan is this: when the client connects to my server, it will download a heavily-obfuscated JAR file (the security module of the program does not need to be open-source), and that JAR file will tell the client to compute an MD5sum of itself, as well as gathering a bunch of other data and heavily encrypting it, and it will send this data back to the server. Unfortunately, I do not know how to accomplish either the loading of the classes in the downloaded JAR or the making the program take an MD5sum of itself. I cannot have the program take an MD5sum of the JAR file on disk, because its name may have been changed, and it would not work for the applet version. Somehow I would have to compute a checksum of the code stored in memory, and send that. Is this even possible? Or is there a better, less convoluted way to accomplish what I'm trying to do?

ejp wrote:
You need to read the [Signed JAR file specification|http://java.sun.com/j2se/1.5.0/docs/guide/jar/jar.html#Signed%20JAR%20File]. The .class files are digested. The digests are listed in a supplement to the manifest. The manifest is signed with a certificate. All that has to be reproducible at the recipient otherwise the JAR is corrupt. It can only be reproduced if the bytecodes agree with the digests and the signature of the digests agrees with the signature in the manifest, in other words essentially if the JAR is signed. And if it is signed by your certificate, it is your JAR file. Otherwise it is either corrupt or somebody else's JAR file. And only you can sign with your certificate because it is done with your private key, which is, well, private.Now it makes sense, thanks. But there's still one piece missing: how do you tell the server that you're running the signed version of the app? And how does the server know that the client is telling the truth?

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First I will give an example and then do the question.
Ex1.
We have any function, eg.y = x ^ 2 + 1 (read 'y' equals 'x' high to the square), a function of the second degree.
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We can use any numbers, but arfer interval [-3.3]
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2 | y = 5
3 | y = 10
Now I begin to explain my doubts.
See this program:
Ex2
* To change this template, choose Tools | Templates
* And open the template in the editor.
Encontrando_o_valor_de_y package;
* @ Author des Soldat Gottes
Import javax.swing.JOptionPane;
Public class (Main
* @ Param args the command line arguments
Public static void main (String [] args) (
Int x, y;
String x1;
X1 = JOptionPane.showInputDialog ( "We have the function y = x + 1 \ n" +
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X = Integer.parseInt (x1); / / tranforma String in int
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/ / Displays the value of 'y'
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In: x1 = JOptionPane.showInputDialog ( "We have the function y = x + 1 \ n" +
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The entry is a number and that number is assigned aa ja existing function in the (y = x + 1).
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Ex: the program ask: DIGITE THE FUNCTION?
The USUARIO DIGITARIA A FUNCTION ANY, TYPE: y = x ^ 2 +1
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It would be possible that?
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Thank you for your attention!
God bless!

rafaelmenezes wrote:
Thanks for the explanation, could understand what fly said.
But as it applied to a program?
How to create a program that recognizes that the entry coefficients?Are you asking about how to parse out the coefficients from the string "3x^4 + 4x^3 - 8x^2 + 5x^1 + 2x^0"? If you define the format to strictly follow that example, this should get you started:
Strip out the spaces
Split the String on "x^"
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jschell wrote:
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How to make a Java app compute an MD5sum of itself

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