In this answer I choose to approach the "Simple Java AES encrypt/decrypt example" main theme and not the specific debugging question because I think this will profit most readers.
This is a simple summary of my blog post about AES encryption in Java so I recommend reading through it before implementing anything. I will however still provide a simple example to use and give some pointers what to watch out for.
In this example I will choose to use authenticated encryption with Galois/Counter Mode or GCM mode. The reason is that in most case you want integrity and authenticity in combination with confidentiality (read more in the blog).
AES-GCM Encryption/Decryption Tutorial
Here are the steps required to encrypt/decrypt with AES-GCM with the Java Cryptography Architecture (JCA). Do not mix with other examples, as subtle differences may make your code utterly insecure.
1. Create Key
As it depends on your use-case, I will assume the simplest case: a random secret key.
SecureRandom secureRandom = new SecureRandom();
byte[] key = new byte[16];
secureRandom.nextBytes(key);
SecretKey secretKey = SecretKeySpec(key, "AES");
Important:
2. Create the Initialization Vector
An initialization vector (IV) is used so that the same secret key will create different cipher texts.
byte[] iv = new byte[12]; //NEVER REUSE THIS IV WITH SAME KEY
secureRandom.nextBytes(iv);
Important:
3. Encrypt with IV and Key
final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
GCMParameterSpec parameterSpec = new GCMParameterSpec(128, iv); //128 bit auth tag length
cipher.init(Cipher.ENCRYPT_MODE, secretKey, parameterSpec);
byte[] cipherText = cipher.doFinal(plainText);
Important:
- use 16 byte / 128 bit authentication tag (used to verify integrity/authenticity)
- the authentication tag will be automatically appended to the cipher text (in the JCA implementation)
- since GCM behaves like a stream cipher, no padding is required
- use
CipherInputStream when encrypting large chunks of data
- want additional (non-secret) data checked if it was changed? You may want to use associated data with
cipher.updateAAD(associatedData); More here.
3. Serialize to Single Message
Just append IV and ciphertext. As stated above, the IV doesn't need to be secret.
ByteBuffer byteBuffer = ByteBuffer.allocate(iv.length + cipherText.length);
byteBuffer.put(iv);
byteBuffer.put(cipherText);
byte[] cipherMessage = byteBuffer.array();
Optionally encode with Base64 if you need a string representation. Either use Android's or Java 8's built-in implementation (do not use Apache Commons Codec - it's an awful implementation). Encoding is used to "convert" byte arrays to string representation to make it ASCII safe e.g.:
String base64CipherMessage = Base64.getEncoder().encodeToString(cipherMessage);
4. Prepare Decryption: Deserialize
If you have encoded the message, first decode it to byte array:
byte[] cipherMessage = Base64.getDecoder().decode(base64CipherMessage)
Important:
5. Decrypt
Initialize the cipher and set the same parameters as with the encryption:
final Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
//use first 12 bytes for iv
AlgorithmParameterSpec gcmIv = new GCMParameterSpec(128, cipherMessage, 0, 12);
cipher.init(Cipher.DECRYPT_MODE, secretKey, gcmIv);
//use everything from 12 bytes on as ciphertext
byte[] plainText = cipher.doFinal(cipherMessage, 12, cipherMessage.length - 12);
Important:
- don't forget to add associated data with
cipher.updateAAD(associatedData); if you added it during encryption.
A working code snippet can be found in this gist.
Note that most recent Android (SDK 21+) and Java (7+) implementations should have AES-GCM. Older versions may lack it. I still choose this mode, since it is easier to implement in addition to being more efficient compared to similar mode of Encrypt-then-Mac (with e.g. AES-CBC + HMAC). See this article on how to implement AES-CBC with HMAC.
