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How do I "decompile" Java class files? [closed]
(19 answers)
Closed 8 years ago.
When i create a runnable jar file from a java source code file from eclipse, i believe it creates a class file which then can be run by the JVM.
The reason i am asking this is because i am going to be making an java application that keeps all my passwords. The app is going consist of an interface that asks for a password and then if the password is correct show the passwords. Here are my questions on this subject:
What exactly does a runnable *jar file* consist of?
If the runnable jar file consists of a class file, can that class file be interpreted in anyway to be able to see the source code which would revile the passwords?
when you run the runnable jar file from cmd and type "java -jar xxx". and "xxx" meaning the file name, does "-jar" mean you are going to run a jar file and "java" means run this following file in the JVM?
Is this like .exe files, which can't be un done to readable source code when turned into the .exe file.
For the unasked question: If your password is in the source code it will be in the class file in a pretty easy to find way.
A runnable jar file is a jar file (i.e. zip file with jar suffix) containing class files and special file containing information about which class to start.
You can decompile byte code: http://www.program-transformation.org/Transform/JavaDecompilers to get source code. But you can actually see the password in the byte code without decompiling it
yes. in java -jar xxx java means run the jvm using a jar file with name xxx
If you know the language and tools created you should be able to decompile exe file just as class files. And even if not passwords in the source code will be easy to find in the .exe file. So yes jar files are kind of like exe files, but they are different then what you describe.
If you want to make an application to maintain your passwords, make it so that it encrypts the stored passwords using a master password that you provide on startup as a user of the application. Never ever store passwords in source code.
Java class files can be disassembled back to readable Java code. Even easier, though, is that you can extract all passwords from the compiled class file because they are stored as plain text (try for example the strings command on a GNU machine).
So, no, the passwords are not inherently safe inside a class file (and therefore not in a jar archive, because jars can easily be unpacked).
What you need to do is implement some sort of encryption and let your program save the passwords in an encrypted file. Your program will then need you to input the decryption key to even be able to unpack the passwords. The key should not be included in the Java program, it should be provided by you each time you run the program.
Basically, however hard you try, it can be easily decompiled and read by anyone.
If you can, don't store the password, compare it online on your server (using asynchronous encryption) or store it using a master password given to the application every time.
If you can't, don't store the password as-is. The best thing I can think of is storing (and checking against) the hashcode of the password. User enters the password, it is then SHA-256'd and the resulting hashcode is compared to your stored hashcode of the password. Add salt to protect against rainbow tables.
The idea of this is that the hash function is only one-way, a password can't be recovered from its hash. Therefore, nobody should be able to get your password in a reasonable time if it's strong enough.
A very good read on this are the wiki links - but most notably this article on jasypt.org.
That said - if you tried to encrypt the file containing the hash to add another layer of security, the decrypted password could still be found using a decompiler and a debugger, so don't really count on it being supereffective against someone who would really want to get through. Therefore, it's more a security through obscurity than a real encryption.
Related
My java application stores data in a folder at user home directory.
How can i encrypt that folder but access the encrypted folder with an application?
I tried to encrypt the folder with java, (Java Cryptography Extension (JCE)), but cannot access this folder from within my application without decrypting it.
I need the user to have no access to this folder, only within my application.
Note: This is going to be a bit of an unorthodox answer, and might not be exactly what the OP is looking for.
One possible solution is to zip the files in your folder, and then encrypt the zip file.
Reading its contents will be slower of course, but is perfectly feasible with Java's NIO Zip File system provider. You will need to decrypt it back so that it is back to zip format.
https://docs.oracle.com/javase/8/docs/technotes/guides/io/fsp/zipfilesystemprovider.html
Not sure how frequently the files will change by the application, and how feasible it is to update the zip file and re-encrypt it for what is needed.
I am trying to implement a simple encryption utility for educational pourpuses, it works, at least with simple files, but when I succesfully encrypt one file, i'd like not to encrypt it again, because that could lead to loose my data if i encrypt/decrypt it in a wrong way... Is there a way to prevent me from do it?
I am using java, and the default encryption library.
Thanks in advance
The answer to what you want to know here depends very much on how your encrypting the files in the first place.
I'll list a couple of different approaches that might help you however
Approach 1 - Scripting
If your using a 3rd party tool such as an encryption util written by another java programmer, and if your running this tool in some kind of a shell session, your best bet might be to wrap the invocation of said tool in a shell script.
If your running on windows this could be a batch file, on Linux a bash script.
Essentially you use this approach by working out ahead of time what command you need to use, then putting that command into said shell script while substituting any parameters you need to change.
Following on from the wrapped command you could then provide further commands to rename the file, or embed some kind of information in it's file properties or file name, a possible example might be something like:
IF NOT EXIST %%0.encrypted(
encrypt %%0 -a -b -c -d
rename %%0 %%0.encrypted) ELSE (
)
ELSE
()
NOTE: These are just theoretical examples as I don't know what your OS is
If this was saved in a file called 'myencrypt.bat', then you could just type
myencrypt.bat afile.ext
Approach 2 - Custom Bytes
If you have direct control of the source code and consequently the application that performs this encryption, then why not make a pseudo file format.
Add some kind of a marker into the file that your program then checks for.
By way of an example, you could perhaps
add the following string to the front of the file
ENCFOriginalFile.Ext
Turn that into a set of bytes, then load the file in, encrypt it, add the bytes from the text on and save it back out, maybe with a custom file extension.
When you come to encrypt a file again, all you then need to do is read the first 4 bytes and if they are equal to ENCF you know that the file is already encrypted.
Those are just 2 ideas I can think of off the top of my head, but it's late here and I'm tired. If I was more awake I could probably come up with a whole page full.
Since it is encrypted it cannot be opened in the default program for that file type, so you can savely rename the file. This can be done for example by adding .enc as the extension. Doing so will make it easy to spot the encrypted file for you and your java application.
Depending on your use case you can also let your java application manage a database of encrypted files.
I have made an encryption system and I'm looking for a way to add some way to integrate it with file system on windows(if possible also on linux). I don't want to rise a debate whether it is needed, that it already exists etc...
I was hoping to find a way to mount a virtual disk drive that will be able to access the files in the decrypted form, encrypt and decrypt on the fly using my software, it is currently written in java, but if needed I can port it to c++.
I have found one way to do it, which is to run a java ssh server and use another software to mount it in windows, but it doesn't work work well, constant crashes or it sometimes just doesn't mount the drive.
I need it as I want to access the files using IDE and other programs without coping the files as it decreases the security and doubles the disk space used.
Has anyone found a way to do it preferably in java?
Is there some kind of API for it (all I need is list files, get parent, read file, write file)?
Or is there a good java lib what works with another program to do it?
I am making the setup of java swing application by using Inno Setup as an exe i am selecting the jar file of my project, I am also adding other necessary resources as folder.
When I am installing the setup on the client side. it is putting the jar and other
resources in program files folder but there client can extract the my java classes
and other resources from jar. I want that client can only use the resources by
application program but he could not extract the resources. How is it possible?
There is literally nothing you can do to entirely prevent someone from extracting the resources.
The best you can do is to make the process a bit difficult; e.g. by storing the resources in the JAR file in encrypted form. The problem is that your program would need to decrypt the resources in order to use them. Someone with sufficient skills and patience can reverse engineer your decryption code and capture the unencrypted resources.
By the way, this is not a Java-specific problem. Any application that you provide to a user as an executable can be reverse engineered ... assuming that the user has the wherewithal to run it in the first place.
The bottom line is that if you are not prepared for the possibility that someone might extract the resources, you should not distribute the executable.
I am working on a Swing application in which i have to give the HTML files to client but I dont want the client to get access to them.
Is there any way that I can put my files either in password protected folder or I could encrypt the file and my program should access them back in the Swing application.
You could encrypt them with a shared hidden key.
One for your application, hardcoded in the software, one for yourself, to encrypt the html files with.
That's about all you can do about it. Once decrypted it is fairly easy to get the contents from it because they can always write their own class which implements your class that simply shows the content(it's about 15 lines of code really, java decompilers work pretty good these days)
Hehe, well that's of course technically possible, but at some point the cleartext will be available to your client - if not by other means, then at least by network sniffing...
Use your resources to write good code and don't bother encrypting your HTML.
Cheers,