What are the AES parameters used and steps performed internally by crypto-js while encrypting a message with a password?

Solution 1:

CryptoJS uses the non-standardized OpenSSL KDF for key derivation (EvpKDF) with MD5 as the hashing algorithm and 1 iteration. The IV is also derived from the password which means that only the actual ciphertext, the password and the salt are needed to decrypt this on Java side.

In other words, PBKDF2 is not used for key derivation in password mode of CryptoJS. By default AES-256 is used in CBC mode with PKCS5 padding (which is the same as PKCS7 padding). Keep in mind that you might need the JCE Unlimited Strength Jurisdiction Policy Files. See also Why there are limitations on using encryption with keys beyond certain length?

The following code recreates the KDF in Java (keySize and ivSize are 8 respectively 4 for AES-256 and come from ).

public static byte[] evpKDF(byte[] password, int keySize, int ivSize, byte[] salt, int iterations, String hashAlgorithm, byte[] resultKey, byte[] resultIv) throws NoSuchAlgorithmException {
    int targetKeySize = keySize + ivSize;
    byte[] derivedBytes = new byte[targetKeySize * 4];
    int numberOfDerivedWords = 0;
    byte[] block = null;
    MessageDigest hasher = MessageDigest.getInstance(hashAlgorithm);
    while (numberOfDerivedWords < targetKeySize) {
        if (block != null) {
            hasher.update(block);
        }
        hasher.update(password);
        block = hasher.digest(salt);
        hasher.reset();

        // Iterations
        for (int i = 1; i < iterations; i++) {
            block = hasher.digest(block);
            hasher.reset();
        }

        System.arraycopy(block, 0, derivedBytes, numberOfDerivedWords * 4,
                Math.min(block.length, (targetKeySize - numberOfDerivedWords) * 4));

        numberOfDerivedWords += block.length/4;
    }

    System.arraycopy(derivedBytes, 0, resultKey, 0, keySize * 4);
    System.arraycopy(derivedBytes, keySize * 4, resultIv, 0, ivSize * 4);

    return derivedBytes; // key + iv
}

Here is the complete class for reference:

public class RecreateEVPkdfFromCryptoJS {
    public static void main(String[] args) throws UnsupportedEncodingException, GeneralSecurityException {
        String msg = "hello";
        String password = "mypassword";
        String ivHex = "aab7d6aca0cc6ffc18f9f5909753aa5f";
        int keySize = 8; // 8 words = 256-bit
        int ivSize = 4; // 4 words = 128-bit
        String keyHex = "844a86d27d96acf3147aa460f535e20e989d1f8b5d79c0403b4a0f34cebb093b";
        String saltHex = "ca35168ed6b82778";
        String openSslFormattedCipherTextString = "U2FsdGVkX1/KNRaO1rgneK9S3zuYaYZcdXmVKJGqVqk=";
        String cipherTextHex = "af52df3b9869865c7579952891aa56a9";
        String padding = "PKCS5Padding";

        byte[] key = hexStringToByteArray(keyHex);
        byte[] iv = hexStringToByteArray(ivHex);
        byte[] salt = hexStringToByteArray(saltHex);
        byte[] cipherText = hexStringToByteArray(cipherTextHex);

        byte[] javaKey = new byte[keySize * 4];
        byte[] javaIv = new byte[ivSize * 4];
        evpKDF(password.getBytes("UTF-8"), keySize, ivSize, salt, javaKey, javaIv);
        System.out.println(Arrays.equals(key, javaKey) + " " + Arrays.equals(iv, javaIv));

        Cipher aesCipherForEncryption = Cipher.getInstance("AES/CBC/PKCS5Padding"); // Must specify the mode explicitly as most JCE providers default to ECB mode!!

        IvParameterSpec ivSpec = new IvParameterSpec(javaIv);
        aesCipherForEncryption.init(Cipher.DECRYPT_MODE, new SecretKeySpec(javaKey, "AES"), ivSpec);

        byte[] byteMsg = aesCipherForEncryption.doFinal(cipherText);
        System.out.println(Arrays.equals(byteMsg, msg.getBytes("UTF-8")));
    }

    public static byte[] evpKDF(byte[] password, int keySize, int ivSize, byte[] salt, byte[] resultKey, byte[] resultIv) throws NoSuchAlgorithmException {
        return evpKDF(password, keySize, ivSize, salt, 1, "MD5", resultKey, resultIv);
    }

    public static byte[] evpKDF(byte[] password, int keySize, int ivSize, byte[] salt, int iterations, String hashAlgorithm, byte[] resultKey, byte[] resultIv) throws NoSuchAlgorithmException {
        int targetKeySize = keySize + ivSize;
        byte[] derivedBytes = new byte[targetKeySize * 4];
        int numberOfDerivedWords = 0;
        byte[] block = null;
        MessageDigest hasher = MessageDigest.getInstance(hashAlgorithm);
        while (numberOfDerivedWords < targetKeySize) {
            if (block != null) {
                hasher.update(block);
            }
            hasher.update(password);
            block = hasher.digest(salt);
            hasher.reset();

            // Iterations
            for (int i = 1; i < iterations; i++) {
                block = hasher.digest(block);
                hasher.reset();
            }

            System.arraycopy(block, 0, derivedBytes, numberOfDerivedWords * 4,
                    Math.min(block.length, (targetKeySize - numberOfDerivedWords) * 4));

            numberOfDerivedWords += block.length/4;
        }

        System.arraycopy(derivedBytes, 0, resultKey, 0, keySize * 4);
        System.arraycopy(derivedBytes, keySize * 4, resultIv, 0, ivSize * 4);

        return derivedBytes; // key + iv
    }

    /**
     * Copied from http://stackoverflow.com/a/140861
     * */
    public static byte[] hexStringToByteArray(String s) {
        int len = s.length();
        byte[] data = new byte[len / 2];
        for (int i = 0; i < len; i += 2) {
            data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                    + Character.digit(s.charAt(i+1), 16));
        }
        return data;
    }
}

and the JavaScript code which was used for the generation of the values in the Java code:

var msg = "hello";
var password = "mypassword"; // must be present on the server
var encrypted = CryptoJS.AES.encrypt( msg, password );
var ivHex = encrypted.iv.toString();
var ivSize = encrypted.algorithm.ivSize; // same as the blockSize
var keySize = encrypted.algorithm.keySize;
var keyHex = encrypted.key.toString();
var saltHex = encrypted.salt.toString(); // must be sent as well
var openSslFormattedCipherTextString = encrypted.toString(); // not used
var cipherTextHex = encrypted.ciphertext.toString(); // must be sent

Solution 2:

Following @Artjom B's great answer both on this question and here for python users, I am joining the full java code that helped me decrypt a string that was encrypted this way

var encrypted = CryptoJS.AES.encrypt(message, password).toString();

This piece of Java code is useful when you only know the password (i.e. salt was not sent with the encrypted string):

public String decrypt(String encrypted, String password) throws Exception {
    int keySize = 8;
    int ivSize = 4;
    // Start by decoding the encrypted string (Base64)
    // Here I used the Android implementation (other Java implementations might exist)
    byte[] cipherText = Base64.decode(encrypted, Base64.DEFAULT);
    // prefix (first 8 bytes) is not actually useful for decryption, but you should probably check that it is equal to the string "Salted__"
    byte[] prefix = new byte[8];
    System.arraycopy(cipherText, 0, prefix, 0, 8);
    // Check here that prefix is equal to "Salted__"
    // Extract salt (next 8 bytes)
    byte[] salt = new byte[8];
    System.arraycopy(cipherText, 8, salt, 0, 8);
    // Extract the actual cipher text (the rest of the bytes)
    byte[] trueCipherText = new byte[cipherText.length - 16];
    System.arraycopy(cipherText, 16, trueCipherText, 0, cipherText.length - 16);
    byte[] javaKey = new byte[keySize * 4];
    byte[] javaIv = new byte[ivSize * 4];
    evpKDF(password.getBytes("UTF-8"), keySize, ivSize, salt, javaKey, javaIv);
    Cipher aesCipherForEncryption = Cipher.getInstance("AES/CBC/PKCS5Padding");
    IvParameterSpec ivSpec = new IvParameterSpec(javaIv);
    aesCipherForEncryption.init(Cipher.DECRYPT_MODE, new SecretKeySpec(javaKey, "AES"), ivSpec);

    byte[] byteMsg = aesCipherForEncryption.doFinal(trueCipherText);
    return new String(byteMsg, "UTF-8");
}

public  byte[] evpKDF(byte[] password, int keySize, int ivSize, byte[] salt, byte[] resultKey, byte[] resultIv) throws NoSuchAlgorithmException {
    return evpKDF(password, keySize, ivSize, salt, 1, "MD5", resultKey, resultIv);
}

public  byte[] evpKDF(byte[] password, int keySize, int ivSize, byte[] salt, int iterations, String hashAlgorithm, byte[] resultKey, byte[] resultIv) throws NoSuchAlgorithmException {
    int targetKeySize = keySize + ivSize;
    byte[] derivedBytes = new byte[targetKeySize * 4];
    int numberOfDerivedWords = 0;
    byte[] block = null;
    MessageDigest hasher = MessageDigest.getInstance(hashAlgorithm);
    while (numberOfDerivedWords < targetKeySize) {
        if (block != null) {
            hasher.update(block);
        }
        hasher.update(password);
        block = hasher.digest(salt);
        hasher.reset();

        // Iterations
        for (int i = 1; i < iterations; i++) {
            block = hasher.digest(block);
            hasher.reset();
        }

        System.arraycopy(block, 0, derivedBytes, numberOfDerivedWords * 4,
                Math.min(block.length, (targetKeySize - numberOfDerivedWords) * 4));

        numberOfDerivedWords += block.length/4;
    }

    System.arraycopy(derivedBytes, 0, resultKey, 0, keySize * 4);
    System.arraycopy(derivedBytes, keySize * 4, resultIv, 0, ivSize * 4);

    return derivedBytes; // key + iv
}