using CipherOutputStream in Java, encrypted file ends up corrupted

Question

I am doing my bachelor thesis on the topic of cryptography and its costs.

A part of that is to compare different crypto Algorithms and Cipher Modes in terms of Runtime and Resource cost. For that I wrote a small tool that should work in four steps:

1. Read Input File
2. Encrypt the Input File and write it to a new file.
3. Read and decrypt the just written encrypted file.
4. Write another copy of the decrypted file onto file system.
5. Compare initial input file and decrypted file to see if they are equal.

It works just fine with a small .txt input File. But for some reason it doesn’t work with any other kind of file. If I take an Image as input File, the first few pixels are good, the rest ends up corrupted. So as far as I understand the problem should be somehow when I initialize the Cipher or when I use the Streams. I also tried to out comment the lines encrypting and decrypting and it also works then in just making plain copies of the input file which are identical.

Any suggestions are welcome, I will try to test them out asap and report back what results showed up. I do apologies for the “Hungarian notations”. The p is just use for public and the l for local. It is the way we do it in our company.

So here is my class:

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.Key;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;
import java.util.Date;
import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.CipherOutputStream;
import javax.crypto.KeyGenerator;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.spec.IvParameterSpec;


public class AES_Cipher_Test {


    public String pLocalRef = "E:\\Test.txt";
    public String pLocalRefOutput = "E:\\Test-crypted.txt";
    public String pLocalCopyOutput = "E:\\Test-Neu.txt";
    public Key pKeyAES = null;
    public int pBitKey = 128;
    public Cipher pCipher;  
    public FileOutputStream pFos;
    public FileInputStream pFis;
    public CipherOutputStream pCos;
    public CipherInputStream pCis;
    public File pInputFile = new File(this.pLocalRef);
    public File pOutputFile = new File(this.pLocalRefOutput);
    public File pGeneratedFile = new File(this.pLocalCopyOutput);

    public AES_Cipher_Test() {
        crypt_decrypt_write_File();
    }

    public void crypt_decrypt_write_File() {
        byte[] lLoadedFile = null;
        byte[] lGeneratedFileByte = null;
        try {

            // generate new random AES Key
            KeyGenerator lKeygen = KeyGenerator.getInstance("AES");
            lKeygen.init(this.pBitKey);
            this.pKeyAES = lKeygen.generateKey();


            // read input File
            this.pFis = new FileInputStream(this.pInputFile);
            FileInputStream tempStream = new FileInputStream(this.pInputFile);
            int count = 0;
            while (tempStream.read() != -1){
                count ++;
            }
            lLoadedFile = new byte[count]; // new byte[this.pFis.available()]
            this.pFis.read(lLoadedFile);
            System.err.println("lLoadedFile.legth  " + lLoadedFile.length);
            this.pFis.close();

            //init Cipher with AES Encrypt Mode CFB8 oder CTR
            this.pCipher = Cipher.getInstance("AES/CTR/PKCS5Padding");
            this.pCipher.init(Cipher.ENCRYPT_MODE, this.pKeyAES);


            // build cipher stream from FileOutputStream
            this.pFos = new FileOutputStream(this.pOutputFile);
            this.pCos = new CipherOutputStream(this.pFos, this.pCipher);

            //write encrypted Data to stream
            this.pCos.write(lLoadedFile);
            this.pCos.close();
            this.pFos.close();

            // init Cipher for decrypt Mode
            this.pCipher.init(Cipher.DECRYPT_MODE, this.pKeyAES, new IvParameterSpec(this.pCipher.getIV()));


            // read just written localFile and decrypt
            this.pFis = new FileInputStream(this.pOutputFile);
            tempStream = new FileInputStream(this.pOutputFile);
            count = 0;
            while (tempStream.read() != -1){
                count ++;
            }
            byte[] lBytes = new byte[count];// new byte[this.pFis.available()]
            this.pCis = new CipherInputStream(this.pFis, this.pCipher);
            int lBytesRead = this.pCis.read(lBytes);
            while (lBytesRead > -1) {
                lBytesRead = this.pCis.read(lBytes);
            }
            this.pCis.close();
            this.pFis.close();
            System.err.println("lBytes.length " + lBytes.length);

            // write new not crypted File to see if procedure works
            this.pFos = new FileOutputStream(this.pLocalCopyOutput);
            this.pFos.write(lBytes);
            this.pFos.close();


            //compare Input File and Output File
            this.pFis = new FileInputStream(this.pGeneratedFile);
            tempStream = new FileInputStream(this.pGeneratedFile);
            count = 0;
            while (tempStream.read() != -1){
                count ++;
            }
            lGeneratedFileByte = new byte[count]; // new byte[this.pFis.available()]
            int i = this.pFis.read(lGeneratedFileByte);
            this.pFis.close();

            System.err.println("lGeneratedFileByte.length " + i);
            System.err.println("Test if initial File and new File are identical = " + Arrays.equals(lGeneratedFileByte, lLoadedFile));


        } catch (FileNotFoundException e) {
            throw new RuntimeException("FILE_DOES_NOT_EXIST", e);
        } catch (NoSuchAlgorithmException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (NoSuchPaddingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InvalidKeyException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InvalidAlgorithmParameterException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }


    public static void main(String args[]) {
        System.err.println("Start AES_Cipher_Test");
        long start = new Date().getTime();
        new AES_Cipher_Test();
        long runningTime = new Date().getTime() - start;
        System.err.println("End AES_Cipher_Test");
        System.err.println("Runtime: " + runningTime);

    }
}

Answer 1

Usual series of mistakes.

• read() isn't specified to fill the buffer. It is only specified to transfer at least one byte, or else return -1 indicating end of stream. You have to loop:

  while ((count = in.read(buffer)) > 0)
  {
      out.write(buffer, 0, count);
  }
  

Your existing loop while (!(lBytesRead < lBytes.length)) is basically nonsense.

• available() explicitly is not the total number of bytes in the stream, and any usage of it to allocate a buffer of such a size is explicitly stated in the Javadoc to be incorrect. Again, you have to loop, see above. There are few if any uses of available(), and this isn't one of them.