Java encrypt files with password

Question

Hi I'm a new user and this is my first question: I state that I have no extensive knowledge of cryptography. I'm trying to encrypt files with a user-supplied password and I have found this method:

fileProcessor(Cipher.ENCRYPT_MODE,key,inputFile,newFile);

static void fileProcessor(int cipherMode,String key,File inputFile,File outputFile) {
    try {
        Key secretKey = new SecretKeySpec(key.getBytes(), "AES");
        Cipher cipher = Cipher.getInstance("AES");
        cipher.init(cipherMode, secretKey);

        FileInputStream inputStream = new FileInputStream(inputFile);
        byte[] inputBytes = new byte[(int) inputFile.length()];
        inputStream.read(inputBytes);

        byte[] outputBytes = cipher.doFinal(inputBytes);

        FileOutputStream outputStream = new FileOutputStream(outputFile);
        outputStream.write(outputBytes);

        inputStream.close();
        outputStream.close();

        } catch (NoSuchPaddingException | NoSuchAlgorithmException 
                     | InvalidKeyException | BadPaddingException
                 | IllegalBlockSizeException | IOException e) {
        e.printStackTrace();
            }
}

The problem is that the program only works if I enter a 16 byte password (I think even a multiple of it is fine). How can I use a password that is not necessarily a multiple of 16 bytes?

Answer 1

I'm trying to encrypt files with a user-supplied password
How can I use a password that is not necessarily a multiple of 16 bytes?

To create an encryption key from a user provided password you may check some examples, generally search for "password based encryption"

Here is en example how to create an encryption key using a user password

 private static final String PBKDF_ALG = "PBKDF2WithHmacSHA256";
 private static final int PBKDF_INTERATIONS = 800000;

// create key from password
 SecretKeyFactory secKeyFactory = SecretKeyFactory.getInstance(PBKDF_ALG);
 KeySpec pbeSpec = new PBEKeySpec(password.toCharArray(), psswdSalt, PBKDF_INTERATIONS, SYMMETRIC_KEY.length*8);
 SecretKey pbeSecretKey = secKeyFactory.generateSecret(pbeSpec);
SecretKey secKey = new SecretKeySpec(pbeSecretKey.getEncoded(), SYMMETRIC_KEY_ALG);

Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(cipherMode, secKey) ;

Answer 2

A key (SecretKeySpec) is a cryptographic key and not a simple plaintext password supplied by user. AES standard specifies the following key sizes: 128, 192 or 256 bits. A key can be created from a text password using a key derivation function, for example PBKDF2.

As Maarten-reinstateMonica mentioned in the comment, Cipher.getInstance("AES") results in AES encryption in ECB mode that is insecure. AES-GCM is strong approved authenticated encryption modes based on AES algorithm.

Also, you need to understand the following concepts before proceeding to the sample code:

• salting
• PBKDF2 password hasing algorithm
• what is confidentiality, integrity and authenticity in Information security
• password strength recommendations

Sample code:

// The number of times that the password is hashed during the derivation of the symmetric key
private static final int PBKDF2_ITERATION_COUNT = 300_000;
private static final int PBKDF2_SALT_LENGTH = 16; //128 bits
private static final int AES_KEY_LENGTH = 256; //in bits
// An initialization vector size
private static final int GCM_NONCE_LENGTH = 12; //96 bits
// An authentication tag size
private static final int GCM_TAG_LENGTH = 128; //in bits

private static byte[] encryptAES256(byte[] input, String password) {
  try {
    SecureRandom secureRandom = SecureRandom.getInstanceStrong();
    // Derive the key, given password and salt
    SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA512");
    // A salt is a unique, randomly generated string
    // that is added to each password as part of the hashing process
    byte[] salt = new byte[PBKDF2_SALT_LENGTH];
    secureRandom.nextBytes(salt);
    KeySpec keySpec =
        new PBEKeySpec(password.toCharArray(), salt, PBKDF2_ITERATION_COUNT, AES_KEY_LENGTH);
    byte[] secret = factory.generateSecret(keySpec).getEncoded();
    SecretKey key = new SecretKeySpec(secret, "AES");

    // AES-GCM encryption
    Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
    // A nonce or an initialization vector is a random value chosen at encryption time
    // and meant to be used only once
    byte[] nonce = new byte[GCM_NONCE_LENGTH];
    secureRandom.nextBytes(nonce);
    // An authentication tag
    GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_LENGTH, nonce);
    cipher.init(Cipher.ENCRYPT_MODE, key, gcmParameterSpec);
    byte[] encrypted = cipher.doFinal(input);
    // Salt and nonce can be stored together with the encrypted data
    // Both salt and nonce have fixed length, so can be prefixed to the encrypted data
    ByteBuffer byteBuffer = ByteBuffer.allocate(salt.length + nonce.length + encrypted.length);
    byteBuffer.put(salt);
    byteBuffer.put(nonce);
    byteBuffer.put(encrypted);
    return byteBuffer.array();
  } catch (Exception e) {
    throw new RuntimeException(e);
  }
}

private static byte[] decryptAES256(byte[] encrypted, String password) {
  try {
    // Salt and nonce have to be extracted
    ByteBuffer byteBuffer = ByteBuffer.wrap(encrypted);
    byte[] salt = new byte[PBKDF2_SALT_LENGTH];
    byteBuffer.get(salt);
    byte[] nonce = new byte[GCM_NONCE_LENGTH];
    byteBuffer.get(nonce);
    byte[] cipherBytes = new byte[byteBuffer.remaining()];
    byteBuffer.get(cipherBytes);

    SecretKeyFactory factory = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA512");
    KeySpec keySpec =
        new PBEKeySpec(password.toCharArray(), salt, PBKDF2_ITERATION_COUNT, AES_KEY_LENGTH);
    byte[] secret = factory.generateSecret(keySpec).getEncoded();
    SecretKey key = new SecretKeySpec(secret, "AES");

    Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
    // If encrypted data is altered, during decryption authentication tag verification will fail
    // resulting in AEADBadTagException
    GCMParameterSpec gcmParameterSpec = new GCMParameterSpec(GCM_TAG_LENGTH, nonce);
    cipher.init(Cipher.DECRYPT_MODE, key, gcmParameterSpec);
    return cipher.doFinal(cipherBytes);
  } catch (Exception e) {
    throw new RuntimeException(e);
  }
}

public static void main(String[] args) throws Exception {
  String password = "Q8yRrM^AvV5r8Yx+"; //Password still has to be strong ehough
  String input = "Sample text to encrypt";
  byte[] encrypted = encryptAES256(input.getBytes(UTF_8), password);
  System.out.println(Base64.getEncoder().encodeToString(encrypted));
  //s+AwwowLdSb3rFZ6jJlxSXBvzGz7uB6+g2e97QXGRKUY5sHPgf94AOoybkzuR3rNREMj56Ik1+Co682s4vT2sAQ/
  byte[] decrypted = decryptAES256(encrypted, password);
  System.out.println(new String(decrypted, UTF_8));
  //Sample text to encrypt
}

A few more words about random nonces. If only a few records are encrypted with the same key, then a random nonce does not pose a risk. However, if a large number of records is encrypted with the same key, the risk may become relevant.

A single repeated nonce is usually enough to fully recover the connection’s authentication key. In such faulty implementations, authenticity is lost and an attacker is able to manipulate TLS-protected content.

For safety reasons random nonces should be avoided and a counter should be used.