How to encrypt / decrypt text files using ElGamal

Question

I am trying to encrypt and decrypt text files using ElGamal for my research, but it seems that I could not get it to work correctly. I have a group of text files ranging in size from 1kb to 1mb, and I use 512bit for my key size. I already know that, like RSA, ELGamal cannot encrypt values ​​more than its module, since my initial decision, I decided to split each file into pieces (which is smaller than its module) so that I can encrypt it and, fortunately, This solution works for encryption. My problem is that when I tried to decrypt it, the outputs that were generated are not the actual result that I expect to see. I don’t know what is the cause of my problem, and I really need to find a solution in a few days.

I will show you some of my code snippets to make this clear.

I created my key pair with the following

KeyPairGenerator keyGen = KeyPairGenerator.getInstance("ElGamal", "BC";
keyGen.initialize(512);

I encrypt by calling

public static void encryptFile(String srcFileName, String destFileName, PublicKey key) throws Exception
{
    encryptDecryptFile(srcFileName,destFileName, key, Cipher.ENCRYPT_MODE);
}

and I will decrypt, causing

public static void decryptFile(String srcFileName, String destFileName, PrivateKey key) throws Exception
{
    encryptDecryptFile(srcFileName,destFileName, key, Cipher.DECRYPT_MODE);
}

Here is the definition of the encryptDecryptFile (..) method

public static void encryptDecryptFile(String srcFileName, String destFileName, Key key, int cipherMode) throws Exception
    {
        OutputStream outputWriter = null;
        InputStream inputReader = null;
        try
        {
            Cipher cipher = Cipher.getInstance("ElGamal/None/NoPadding", "BC"");
            String textLine = null;
    //buffer(my chunks) depends wether it is encyption or decryption
            byte[] buf = (cipherMode == Cipher.ENCRYPT_MODE? new byte[50] : new byte[64]);
            int bufl;
            // init the Cipher object for Encryption...
            cipher.init(cipherMode, key);

            // start FileIO
            outputWriter = new FileOutputStream(destFileName);
            inputReader = new FileInputStream(srcFileName);
            while ( (bufl = inputReader.read(buf)) != -1)
            {
                byte[] encText = null;
                if (cipherMode == Cipher.ENCRYPT_MODE)
                {
                      encText = encrypt(copyBytes(buf,bufl),(PublicKey)key);
                }
                else
                {
                    if (_log.isDebugEnabled())
                    {
                        System.out.println("buf = " + new String(buf));
                    }
                    encText = decrypt(copyBytes(buf,bufl),(PrivateKey)key);
                }
                outputWriter.write(encText);
                if (_log.isDebugEnabled())
                {
                    System.out.println("encText = " + new String(encText));
                }
            }
            outputWriter.flush();

        }
        catch (Exception e)
        {
            _log.error(e,e);
            throw e;
        }
        finally
        {
            try
            {
                if (outputWriter != null)
                {
                    outputWriter.close();
                }
                if (inputReader != null)
                {
                    inputReader.close();
                }
            }
            catch (Exception e)
            {
                // do nothing...
            } // end of inner try, catch (Exception)...
        }
    }

For copyBytes:

public static byte[] copyBytes(byte[] arr, int length)
{
    byte[] newArr = null;
    if (arr.length == length)
    {
        newArr = arr;
    }
    else
    {
        newArr = new byte[length];
        for (int i = 0; i < length; i++)
        {
            newArr[i] = (byte) arr[i];
        }
    }
    return newArr;
}

For encypt (...)

    public static byte[] encrypt(byte[] text, PublicKey key) throws Exception
{
    byte[] cipherText = null;
    try
    {

        Cipher cipher = Cipher.getInstance("ElGamal/None/NoPadding", "BC"");
        if (_log.isDebugEnabled())
        {
            _log.debug("\nProvider is: " + cipher.getProvider().getInfo());
            _log.debug("\nStart encryption with public key");
        }

        // encrypt the plaintext using the public key
        cipher.init(Cipher.ENCRYPT_MODE, key);
        cipherText = cipher.doFinal(text);
    }
    catch (Exception e)
    {
        _log.error(e, e);
        throw e;
    }
    return cipherText;
}

and decrypt (..)

   public static byte[] decrypt(byte[] text, PrivateKey key) throws Exception
    {
        byte[] dectyptedText = null;
        try
        {
            // decrypt the text using the private key
            Cipher cipher = Cipher.getInstance("ElGamal/None/NoPadding", "BC"");
              cipher.init(Cipher.DECRYPT_MODE, key);
            dectyptedText = cipher.doFinal(text);
        }
        catch (Exception e)
        {
            _log.error(e, e);
            throw e;
        }
        return dectyptedText;

    }

Original Aviran Mondo Code

I think that all you need is just tell me if you want to see the full source code. Thank,

Answer 1

I finally got a solution, anyway, I'll just put it here in case someone else has the same problem with me. All you have to do is replace

byte[] buf = (cipherMode == Cipher.ENCRYPT_MODE? new byte[50] : new byte[64]);

in encryptDecryptFile (..) method with

byte[] buf = (cipherMode == Cipher.ENCRYPT_MODE? new byte[50] : new byte[128]);

since ElGamal with a key size of 512 generates 128b with 50b encryption. Hope this is clear enough.

Answer 2

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