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Companion code to my article about AES+CBC with Encrypt-then-MAC
package at.favre.lib.armadillo;
import org.junit.Test;
import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.security.SecureRandom;
import java.util.Arrays;
import javax.crypto.Cipher;
import javax.crypto.Mac;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import at.favre.lib.crypto.HKDF;
import static org.junit.Assert.assertArrayEquals;
import static org.junit.Assert.assertFalse;
/**
* Companion code to my article about AES+CBC with Encrypt-then-MAC
*/
public class AesCbcExample {
private final SecureRandom secureRandom = new SecureRandom();
@Test
public void testEncryption() throws Exception {
// create a random key
SecureRandom secureRandom = new SecureRandom();
byte[] key = new byte[16];
secureRandom.nextBytes(key);
// the possible plain text
byte[] plainText = "A secret message we created.".getBytes(StandardCharsets.UTF_8);
// data to add to the authentication tag - possibly protocol version
byte[] aad = new byte[] {0x01, 0x02};
byte[] cipherText = encrypt(key, plainText, aad);
byte[] decrypted = decrypt(key, cipherText, aad);
// plaintext and decrypted must be equal
assertArrayEquals(plainText, decrypted);
// plaintext must not be equal to cipher text
assertFalse(Arrays.equals(plainText, cipherText));
}
/**
* Encrpyt given plaintext with given key.
*
* @param key must be strong 16, 24 or 32 byte secret key
* @param plainText to encrypt
* @param associatedData optional data added to the authentication tag
* @return encrypted message including mac & iv
* @throws Exception
*/
private byte[] encrypt(byte[] key, byte[] plainText, byte[] associatedData) throws Exception {
byte[] iv = new byte[16];
secureRandom.nextBytes(iv);
byte[] encKey = HKDF.fromHmacSha256().expand(key, "encKey".getBytes(StandardCharsets.UTF_8), 16);
byte[] authKey = HKDF.fromHmacSha256().expand(key, "authKey".getBytes(StandardCharsets.UTF_8), 32); //HMAC-SHA256 key is 32 byte
final Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding"); //actually uses PKCS#7
cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(encKey, "AES"), new IvParameterSpec(iv));
byte[] cipherText = cipher.doFinal(plainText);
SecretKey macKey = new SecretKeySpec(authKey, "HmacSHA256");
Mac hmac = Mac.getInstance("HmacSHA256");
hmac.init(macKey);
hmac.update(iv);
hmac.update(cipherText);
if (associatedData != null) {
hmac.update(associatedData);
}
byte[] mac = hmac.doFinal();
ByteBuffer byteBuffer = ByteBuffer.allocate(1 + iv.length + 1 + mac.length + cipherText.length);
byteBuffer.put((byte) iv.length);
byteBuffer.put(iv);
byteBuffer.put((byte) mac.length);
byteBuffer.put(mac);
byteBuffer.put(cipherText);
byte[] cipherMessage = byteBuffer.array();
Arrays.fill(authKey, (byte) 0);
Arrays.fill(encKey, (byte) 0);
return cipherMessage;
}
/**
* Decrypt previously encrypted message with {@link #encrypt(byte[], byte[], byte[])}.
*
* @param key same secret used during encrpytion
* @param cipherMessage the message returned by encrypt
* @param associatedData optional data added to the authentication tag
* @return the plain text
* @throws Exception
*/
private byte[] decrypt(byte[] key, byte[] cipherMessage, byte[] associatedData) throws Exception {
ByteBuffer byteBuffer = ByteBuffer.wrap(cipherMessage);
int ivLength = (byteBuffer.get());
if (ivLength != 16) { // check input parameter
throw new IllegalArgumentException("invalid iv length");
}
byte[] iv = new byte[ivLength];
byteBuffer.get(iv);
int macLength = (byteBuffer.get());
if (macLength != 32) { // check input parameter
throw new IllegalArgumentException("invalid mac length");
}
byte[] mac = new byte[macLength];
byteBuffer.get(mac);
byte[] cipherText = new byte[byteBuffer.remaining()];
byteBuffer.get(cipherText);
byte[] encKey = HKDF.fromHmacSha256().expand(key, "encKey".getBytes(StandardCharsets.UTF_8), 16);
byte[] authKey = HKDF.fromHmacSha256().expand(key, "authKey".getBytes(StandardCharsets.UTF_8), 32);
SecretKey macKey = new SecretKeySpec(authKey, "HmacSHA256");
Mac hmac = Mac.getInstance("HmacSHA256");
hmac.init(macKey);
hmac.update(iv);
hmac.update(cipherText);
if (associatedData != null) {
hmac.update(associatedData);
}
byte[] refMac = hmac.doFinal();
if (!MessageDigest.isEqual(refMac, mac)) {
throw new SecurityException("could not authenticate");
}
final Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(encKey, "AES"), new IvParameterSpec(iv));
byte[] plainText = cipher.doFinal(cipherText);
return plainText;
}
}
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