javax.crypto.BadPaddingException: Decryption Error - Not able to decrypt multiple blocks from file

Question

Hoping somebody can point me in the right direction here.

I'm not particularly familiar with encryption (in Java or otherwise), but I said that I'd help somebody out with a task they have that specifies using RSA encrypt/decrypt.

To familiarise myself with what is trying to be accomplished, I've put together a little example program that:

• Generates 6 random numbers, and puts them in a comma-separated string
• Encrypts the sequence of numbers
• Appends the encrypted numbers sequence to a file - the same file can be written to more than once if the program is run multiple times
• Reads the encrypted file - should get all sequences that have been written
• Decrypt each sequence and print out a collection of comma-separated strings

This works fine on the first attempt - i.e. when the first sequence is written into an empty file - and the correct number sequence is returned.

When the file contains more than one encrypted sequence, this causes the decrypt routine to crash with a BadPaddingException. I've done a step-through at this point, and each of the byte[] (representing an encrypted sequence) to be decrypted is 128 bytes, so it's not like an irregular number of bytes is causing the problem.

I know that RSA isn't the recommended way to go about it anymore, but this is the specification that I'm having to get my head around. I'm also aware that there's a ton of SO questions to do with RSA and BadPaddingException, but I haven't come across one that deals with this issue.

My example code is below:

public class EncryptDecrypt {
    
    private static Cipher cipher;
    private static KeyPair keyPair;

    public static void main(String[] args)
    {
        String[] numbers = getNumbers();
        String numbersStr = String.join(", ", numbers);
        System.out.println("Generated: " + numbersStr + ", NumBytes: " + numbersStr.getBytes().length);
        
        byte[] encryptedNumbers = encrypt(numbersStr);
        System.out.println("Encrypted: " + encryptedNumbers.toString() + ", NumBytes: " + encryptedNumbers.length);
        
        writeToFile(encryptedNumbers);
        System.out.println("Encrypted numbers written to data.txt");
        
        ArrayList<byte[]> encryptedData = readFromFile();
        System.out.println("Encrypted numbers read from data.txt, NumSequences: " + encryptedData.size());

        ArrayList<String> decryptedSequences = decrypt(encryptedData);
        for (int i = 0; i < decryptedSequences.size(); i++)
        {
            String sequence = decryptedSequences.get(i);
            System.out.println("Sequence " + i + ": " + sequence);
        }
    }
    
    private static String[] getNumbers()
    {
        String[] numbers = new String[6];
        
        int min = 1;
        int max = 60;
        
        for (int i = 0; i < numbers.length; i++)
        {
            double number = (Math.random() * (max - min) + min);
            numbers[i] = number >= 10 ? Integer.toString((int) number) : "0" + Integer.toString((int) number);
        }
        
        return numbers;
    }
    
    private static byte[] encrypt(String data)
    {
        try {
            KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
            keyPairGenerator.initialize(2048);
            keyPair = keyPairGenerator.generateKeyPair();
            PublicKey publicKey = keyPair.getPublic();
            cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
            
            cipher.init(Cipher.ENCRYPT_MODE, publicKey);
            cipher.update(data.getBytes());
            byte[] encrypted = cipher.doFinal();
            
            return encrypted;
        } catch (NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException | IllegalBlockSizeException | BadPaddingException ex) {
            Logger.getLogger(EncryptDecrypt.class.getName()).log(Level.SEVERE, null, ex);
        }
        
        return null;
    }
    
    private static void writeToFile(byte[] data)
    {
        FileOutputStream fileOut = null;
        try {
            File file = new File("data.txt");
            file.createNewFile();
            fileOut = new FileOutputStream(file, true);
            
            fileOut.write(data);
            fileOut.flush();
            fileOut.close();
        } catch (FileNotFoundException ex) {
            Logger.getLogger(EncryptDecrypt.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IOException ex) {
            Logger.getLogger(EncryptDecrypt.class.getName()).log(Level.SEVERE, null, ex);
        } finally {
            try {
                fileOut.close();
            } catch (IOException ex) {
                Logger.getLogger(EncryptDecrypt.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }
    
    private static ArrayList<byte[]> readFromFile()
    {
        File file = new File("data.txt");
        if (file.exists())
        {
            try {
                ArrayList<byte[]> encryptedSequences = new ArrayList<>();
                FileInputStream fileIn = new FileInputStream(file);
                int blockSize = 128;
                int numBlocks = fileIn.available() / blockSize;
                
                for (int i = 0; i < numBlocks; i++)
                {
                    byte[] encryptedSequence = new byte[blockSize];
                    fileIn.read(encryptedSequence);
                    
                    encryptedSequences.add(encryptedSequence);
                }
                
                fileIn.close();
                
                return encryptedSequences;
            } catch (FileNotFoundException ex) {
                Logger.getLogger(EncryptDecrypt.class.getName()).log(Level.SEVERE, null, ex);
            } catch (IOException ex) {
                Logger.getLogger(EncryptDecrypt.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
        
        return null;
    }
    
    private static ArrayList<String> decrypt(ArrayList<byte[]> data)
    {
        try {
            cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
            PrivateKey privateKey = keyPair.getPrivate();
            
            cipher.init(Cipher.DECRYPT_MODE, privateKey);
            
            ArrayList<String> decryptedStrings = new ArrayList<>();
            
            for (byte[] sequence : data)
            {
                byte[] decryptedBytes = cipher.doFinal(sequence);
                String decryptedString = new String(decryptedBytes);
                decryptedStrings.add(decryptedString);
            }
            
            return decryptedStrings;
        } catch (NoSuchAlgorithmException | NoSuchPaddingException | InvalidKeyException | IllegalBlockSizeException | BadPaddingException ex) {
            Logger.getLogger(EncryptDecrypt.class.getName()).log(Level.SEVERE, null, ex);
        }
        
        return null;
    }
}

If anyone can spot what's wrong with this, I'd really appreciate it!

Thanks