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Tools required for changing byte-code

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I really don�t know if this advance java question

I happen to support an application written long back...1999 I suppose, basically the developers have left the company
There is no source code avaliable in company CVS and the class files are scrambled using some file name scrambler tool.

My Requirement is

1.) I have particular module of code in the application that does a authentication based on a hash value embedded into the code itself, I don't know what hashing algorithms is being used nor the exact password to match that hash value.

2.) I have decompiled the code using JAD but as the JAD was unable to create a full class file it generated some byte code also.

3.) I need modify the particular class file at byte-code level to change the flow that particular authentication call is alone skipped.

How can I change the byte-code? The nice thing is that I need to change in only one class file

May be even skip that particular authentication routine itself or set the default return value as true for any input irrespective of the hash being matched

I just in a spur of a moment promised my manager that I will resolve this issue, but I find it tough to proceed further.

I will be gratefully for any help or directions to any byte code editor tools
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For starters, you might try a different decompiler. JODE (on SourceForge) does a better job than JAD, so that may solve your problem. Try that first, before you delve into examining or modifying class files.
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