Background: The application that I am working on is supposed to work offline. I should encrypt some text data using a password as a key at the java server side. The encrypted data is passed to the HTML5 page and at the client side using crypto-js library the server encrypted data should be decrypted.
My issue: In order to encrypt my message in such a way that the client can decrypt it with crypt-js (using a user entered password), I need to know the exact steps that crypto-js expects while encrypting a message.
What I need to know: I have the following encryption code which does the encryption of a message at the client side using crypto-js.
var message = "my message text";
var password = "user password";
var encrypted = CryptoJS.AES.encrypt( message ,password );
console.log(encrypted.toString());
I need to know the AES parameters used by CryptoJS while encrypting a message(Not sure what they are, but it sounds like: key size (256), padding (pkcs5), mode (CBC), PBE algorithm (PBKDF2), salt (random), iteration count (100)) . It would be a great help if some one could confirm it...I been trying to solve this mystery for the last few days?.
I need to know the different steps performed by CryptoJS while AES encrypting a message
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