Java Decryption Example
The following Java code, using the Bouncy Castle API and bcprov-jdk15on-149.jar provides an example of decrypting the payload.
Note: Encryption in Java requires Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction
Policy files.
private static final String CIPHER_ALGORITHM = "AES/CBC/PKCS5Padding";
private static final String HASH_ALGORITHM = "SHA-256";
private static final String HMAC_ALGORITHM = "HmacSHA256";
private static final int IV_LENGTH = 16, HMAC_LENGTH = 32;
private static final Charset utf8 = Charset.forName("UTF-8");
private static final Provider bcProvider;
static {
bcProvider = new BouncyCastleProvider();
if (Security.getProvider(BouncyCastleProvider.PROVIDER_NAME) == null) {
Security.addProvider(bcProvider);
}
}
private static byte[] decrypt(byte[] key,byte[] data) throws GeneralSecurityException{
byte[] decodedData = Base64.decode(data);
if (decodedData == null || decodedData.length <= IV_LENGTH) {
throw new RuntimeException("Bad input data.");
}
byte[] hmac = new byte[HMAC_LENGTH];
System.arraycopy(decodedData, 0, hmac, 0, HMAC_LENGTH);
if (!Arrays.equals(hmac,
hmac(key, decodedData, HMAC_LENGTH, decodedData.length - HMAC_LENGTH))) {
throw new RuntimeException("HMAC validation failed.");
}
byte[] iv = new byte[IV_LENGTH];
System.arraycopy(decodedData, HMAC_LENGTH, iv, 0, IV_LENGTH);
Cipher cipher = Cipher.getInstance(CIPHER_ALGORITHM, bcProvider);
cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(hash(key), "AES"),
new IvParameterSpec(iv));
return cipher.doFinal(decodedData, HMAC_LENGTH + IV_LENGTH,
decodedData.length - HMAC_LENGTH - IV_LENGTH);
}
private static byte[] hash(byte[] key) throws NoSuchAlgorithmException {
MessageDigest md = MessageDigest.getInstance(HASH_ALGORITHM);
md.update(key);
return md.digest();
}
private static byte[] hmac(byte[] key, byte[] data, int offset, int length)
throws GeneralSecurityException {
Mac mac = Mac.getInstance(HMAC_ALGORITHM, bcProvider);
mac.init(new SecretKeySpec(key, HMAC_ALGORITHM));
mac.update(data, offset, length);
return mac.doFinal();
}
