Java – how to attach to AES encrypted files

I'm writing some recorders that generate encrypted log files Unfortunately, cryptography is not my strong point Now I can write to the file and close it Then I can open it, attach some messages, close it again. After decryption, I see the filled bytes in the middle of the file Is there a way to use encrypted files without having to attach some messages every time I decrypt?

Editor: more details The current implementation uses cipheroutputstream As far as I know, there is no way to seek to use it If I control that the output data size can be divided by the block size, can I use the 'nopadding' option?

Solution

If you use AES in CBC mode, you can use the second last block as IV to decrypt the last block, which may be only partially full, and then encrypt the plaintext of the last block, and then the new plaintext

This is a proof of concept:

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.io.RandomAccessFile;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;


public class AppendAES {

    public static void appendAES(File file,byte[] data,byte[] key) throws IOException,NoSuchAlgorithmException,NoSuchPaddingException,InvalidKeyException,InvalidAlgorithmParameterException,IllegalBlockSizeException,BadPaddingException {
        RandomAccessFile rfile = new RandomAccessFile(file,"rw");
        byte[] iv = new byte[16];
        byte[] lastBlock = null;
        if (rfile.length() % 16L != 0L) {
            throw new IllegalArgumentException("Invalid file length (not a multiple of block size)");
        } else if (rfile.length() == 16) {
            throw new IllegalArgumentException("Invalid file length (need 2 blocks for iv and data)");
        } else if (rfile.length() == 0L) { 
            // new file: start by appending an IV
            new SecureRandom().nextBytes(iv);
            rfile.write(iv);
            // we have our iv,and there's no prior data to reencrypt
        } else { 
            // file length is at least 2 blocks
            rfile.seek(rfile.length()-32); // second to last block
            rfile.read(iv); // get iv
            byte[] lastBlockEnc = new byte[16]; 
                // last block
                // it's padded,so we'll decrypt it and 
                // save it for the beginning of our data
            rfile.read(lastBlockEnc);
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            cipher.init(Cipher.DECRYPT_MODE,new SecretKeySpec(key,"AES"),new IvParameterSpec(iv));
            lastBlock = cipher.doFinal(lastBlockEnc);
            rfile.seek(rfile.length()-16); 
                // position ourselves to overwrite the last block
        } 
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        cipher.init(Cipher.ENCRYPT_MODE,new IvParameterSpec(iv));
        byte[] out;
        if (lastBlock != null) { // lastBlock is null if we're starting a new file
            out = cipher.update(lastBlock);
            if (out != null) rfile.write(out);
        }
        out = cipher.doFinal(data);
        rfile.write(out);
        rfile.close();
    }

    public static void decryptAES(File file,OutputStream out,BadPaddingException {
        // nothing special here,decrypt as usual
        FileInputStream fin = new FileInputStream(file);
        byte[] iv = new byte[16];
        if (fin.read(iv) < 16) {
            throw new IllegalArgumentException("Invalid file length (needs a full block for iv)");
        };
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        cipher.init(Cipher.DECRYPT_MODE,new IvParameterSpec(iv));
        byte[] buff = new byte[1<<13]; //8kiB
        while (true) {
            int count = fin.read(buff);
            if (count == buff.length) {
                out.write(cipher.update(buff));
            } else {
                out.write(cipher.doFinal(buff,count));
                break;
            }
        }
        fin.close();
    }

    public static void main(String[] args) throws Exception {
        byte[] key = new byte[]{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
        for (int i = 0; i<1000; i++) {
            appendAES(new File("log.aes"),"All work and no play makes Jack a dull boy. ".getBytes("UTF-8"),key);
        }
        decryptAES(new File("log.aes"),new FileOutputStream("plain.txt"),key);
    }

}

I want to point out that the output is no different from the output generated by encryption in one run This is not a custom form of encryption - it is the standard of AES / CBC / pkcs5 padding The only implementation detail is that in the case of a blank file, I wrote IV before starting the data

Edit: improved (to my taste) solution using cipheroutputstream:

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.CipherOutputStream;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;


public class AppendAES {
    public static CipherOutputStream appendAES(File file,SecretKeySpec key) throws IOException,BadPaddingException {
        return appendAES(file,key,null);
    }

    public static CipherOutputStream appendAES(File file,SecretKeySpec key,SecureRandom sr) throws IOException,"rw");
        byte[] iv = new byte[16];
        byte[] lastBlock = null;
        if (rfile.length() % 16L != 0L) {
            throw new IllegalArgumentException("Invalid file length (not a multiple of block size)");
        } else if (rfile.length() == 16) {
            throw new IllegalArgumentException("Invalid file length (need 2 blocks for iv and data)");
        } else if (rfile.length() == 0L) { 
            // new file: start by appending an IV
            if (sr == null) sr = new SecureRandom();
            sr.nextBytes(iv);
            rfile.write(iv);
        } else { 
            // file length is at least 2 blocks
            rfile.seek(rfile.length()-32);
            rfile.read(iv);
            byte[] lastBlockEnc = new byte[16];
            rfile.read(lastBlockEnc);
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
            cipher.init(Cipher.DECRYPT_MODE,new IvParameterSpec(iv));
            lastBlock = cipher.doFinal(lastBlockEnc);
            rfile.seek(rfile.length()-16);
        } 
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        cipher.init(Cipher.ENCRYPT_MODE,new IvParameterSpec(iv));
        byte[] out;
        if (lastBlock != null) {
            out = cipher.update(lastBlock);
            if (out != null) rfile.write(out);
        }
        CipherOutputStream cos = new CipherOutputStream(new FileOutputStream(rfile.getFD()),cipher);
        return cos;
    }

    public static CipherInputStream decryptAES(File file,BadPaddingException {
        FileInputStream fin = new FileInputStream(file);
        byte[] iv = new byte[16];
        if (fin.read(iv) < 16) {
            throw new IllegalArgumentException("Invalid file length (needs a full block for iv)");
        };
        Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
        cipher.init(Cipher.DECRYPT_MODE,new IvParameterSpec(iv));
        CipherInputStream cis = new CipherInputStream(fin,cipher);
        return cis;
    }

    public static void main(String[] args) throws Exception {
        byte[] keyBytes = new byte[]{
            0,15
        };
        SecretKeySpec key = new SecretKeySpec(keyBytes,"AES");

        for (int i = 0; i<100; i++) {
            CipherOutputStream cos = appendAES(new File("log.aes"),key);
            cos.write("All work and no play ".getBytes("UTF-8"));
            cos.write("makes Jack a dull boy.  \n".getBytes("UTF-8"));
            cos.close();
        }

        CipherInputStream cis = decryptAES(new File("log.aes"),key);
        BufferedReader bread = new BufferedReader(new InputStreamReader(cis,"UTF-8"));
        System.out.println(bread.readLine());
        cis.close();
    }

}