Incorrect DES File Decryption

Question

I am using the following code to create an encrypted file. I am doing some research for my thesis and really only care about the encrypted file, however,
I wanted to test the encrypted file by decrypting it. The decrypted file is never correct. So the million dollar question is "What am I doing wrong?"
private static void encrypted(
               FileInputStream inputFile, FileOutputStream outputFile)
          Cipher DESCipher;
          Cipher TestDESCipher;
          try
               SecretKey desKey = getDESKey();
               DESCipher = Cipher.getInstance("DES");
               TestDESCipher = Cipher.getInstance("DES");
               // Initialize the cipher for encryption
               DESCipher.init(Cipher.ENCRYPT_MODE, desKey);
               TestDESCipher.init(Cipher.DECRYPT_MODE, desKey);
               CipherOutputStream desCipherStream = new CipherOutputStream(
                         outputFile, DESCipher);
               byte[] b = new byte[8];
               int i = inputFile.read(b);
               while (i > 0)
                    if (debug)
                         System.out.write(b);
                    desCipherStream.write(b, 0, i);
                    i = inputFile.read(b);
               inputFile.close();
               desCipherStream.close();
               outputFile.close();
               // //////////////////////////////////////////start decrypt test
               FileInputStream decInFile = new FileInputStream(
                         "C:\\Documents and Settings\\Walter\\My Documents\\AFIT\\Thesis Work\\Encryption Work\\Text\\DES\\text test.txt");
               FileOutputStream decOutFile = new FileOutputStream(
                         "C:\\Documents and Settings\\Walter\\My Documents\\AFIT\\Thesis Work\\Encryption Work\\Text\\DES\\text test decrypted.txt");
               CipherInputStream desDecryptCipherStream = new CipherInputStream(
                         decInFile, TestDESCipher);
               byte[] c = new byte[8];
               i = desDecryptCipherStream.read(c);
               while (i > 0)
                    System.out.write(c);
                    decOutFile.write(c, 0, i);
                    i = desDecryptCipherStream.read(c);
               decInFile.close();
                        desDecryptCipherStream.close();
               decOutFile.close();
               // ////////////////////////////////////////// end decrypt test
          catch (Exception e)
               e.getMessage();
               e.printStackTrace();
     }Here is the contents of the plaintext input file:
This is an example.Here is the contents of the encrypted file:
��3���f�g�b`��C��3��8wHere is the contents of the decrypted file:
�P�e�a�C�v,(���The program encrypted and decrypted the following correctly:
bytes[] plaintext = "This is an example.".getBytes();But when I tried to put the text in a file and tried to use CipherOutputStream the problem arises.
I've tried to use a single cipher that was reinitialized to DECRYPT, and I've tried to use two separate ciphers, but the results were the same.
I know I am using the same key and padding scheme, so what is going wrong?

Answer 1

<snip>
My intent was not to upset you! I didn't want to
clutter the forum with 500 lines of code that won't
help to discover the problem. Which is exactly why you should provide a short testable example. I had to add about 20 lines of code to get your method testable.
The main method simply
sets up the FileInputStream and the FileOutputStream
and passes them to the encryptDES method posted in
the original post. Exactly so why did you not post one?
The getDESKey is just
an abstraction for KeyGenerator, however, the key is
stored in desKey variable and passed to both init
methods, so I am assuming the key is not
the issue, but here is the getDESKey method just in
case.I agree.
>
private static final SecretKey getDESKey()
throws NoSuchAlgorithmException,
on, NoSuchPaddingException
          byte[] key = new byte[8];
KeyGenerator desKeyGen =
= KeyGenerator.getInstance("DES");
          SecretKey desKey = desKeyGen.generateKey();
return desKey;
I'm running JRE 1.5.0_04 and am using Eclipse3.2.1
for Windows XP.
So you are able to encrypt and decrypt from and
to a
file? Have your tried more complicated files? What is a 'more complicated' file?
A 'more complicated' file is one containing more then
just the simple phrase "This is an example." as
stated in the
original post.  Presumably this would be a 3-4 Kb
text file.  Thus a file where the encryption
algorithm would span
several hundred blocks.My test file was about 10 KBytes so about 1,200 blocks.
>
>>>
Assuming the code is working fine for you, thenwhat
would prevent it from working for me?I have no real idea! That is why you need tocreate
a test harness that shows exactly how you areusing
it. One possibility is that your getDESKey()
method
returns a different key value each time it is
called
but without seeing the rest of your code how am I
to
know.
You are correct, the getDESKey() does return a
different key, but that is how I designed it. As
stated earlier, this
method is only called once and the results are stored
in a local variable. OK.
>
I would be more than happy to send you a zip file
containing the .java file and the file structure so
you can
further analysis the code. I just didn't see a way
to attach a file to a post, so you can email me at
[email protected] and I'll reply with the
attached zip file.It is not a good idea to publish your email address on a public forum.
>
There was one thing I noticed that needed fixingbut
it did does not cause a problem with encryptingand
decrypting and that is that
System.out.write(c);             
decOutFile.write(c, 0, i);needs changing to
System.out.write(c, 0, i);
decOutFile.write(c, 0, i);but this will only show a problem with yourprintout,
not your decrypted file. You did check the contentof
the decrypted file didn't you?The System.out.write(c) will write the full
length of the byte array to the output stream and is
only being used for
debugging purpose. So the System.out.write(c, 0,
i) will prevent a repeat of old data from being
printed out,
but since this is not going to the output file it
shouldn�t be causing any problems. Which is what I said but it does present an opportunity to misinterpret the output.
>
Yes I did check the decrypted file. In fact, the
contents of each file were listed in the original
post. What good is the ASCII of the encrypted data without knowledge of the the key used and the character code used to convert the encrypted bytes to a String.
>
Come on give me a little credit, I did graduate with
distinction with a Bachelor Degree in Computer
Science,
and I am pursuing a Master's Degree in Computer
Science with an emphases in Software Engineering
while
maintaining a GPA > 3.5.There are aspects to your code that don't indicate such good qualifications. For example, your encryption method takes a FileInputStream and a FileOuputStream. As part of a 'design to interface' policy I would expect these to be InputStream and OutputStream. Also, I would not expect you to close the OutputStream because the encryption method did not create the OutputStream and outside of the encryption method the user may want to provide a prefix and a suffix. This is what I do with RSA where my prefix is an IV and the RSA encrypted symetric key and the suffix is a hash.
Also, I assume you know that it is very important to design for testability so I would have expect you to provide the test harness for this method. In this way you would not have has to post 500 lines.
>
And your credentials speak for themselves.I agree! I am one of life's 'has beens'. My MSc is now 34 years old and not worth the paper the certificate is printed on.