Java AES encryption and decryption using a predefined 128bit key

Question

The following code in java attempts to decrypt a string encoded in a QR code, encrypted in C# code. it seems to fail to decrypt the string. is there an easy way of doing this???

   //string encrypted contains the string of the encoded characters. 

     String encrypted = intent.getStringExtra("SCAN_RESULT");

     //converting the string into a byte array          
     byte[] byteEncrypted = encrypted.getBytes();

     //instantiating the AES cipher object
     Cipher cipher = Cipher.getInstance("AES");

     //Predefined public-key 

     byte[] skey = new byte[] { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a,  0x0b, 0x0c, 0x0d, 0x0e, 0x0f };

     //creating a secretKeySpec       
     SecretKeySpec skeyspec = new SecretKeySpec(skey, "AES");

 //initializing the cipher to Decrypt               
     cipher.init(Cipher.DECRYPT_MODE, skeyspec);
  final byte[] decrypt  = cipher.doFinal(byteEncrypted);

 //decrypting the string                
 String contents = new String(decrypt, "UTF-8");

Find out how the C# code did this encryption and encoding the result into a QR-code. Without this, there is only wild guessing. — Paŭlo Ebermann
– Paŭlo Ebermann, Commented Nov 29, 2011 at 17:47
I removed the public-key-encryption tag since I see no evidence of this question having anything to do with public-key encryption, despite the comment in the code. — Fantius
– Fantius, Commented Dec 6, 2011 at 19:41

erickson · Accepted Answer · 2011-11-28 22:03:12Z

For starters, you generally can't convert a cipher text stored as text, and convert it directly to bytes with a call to getBytes().

AES cipher text contains bytes with values from 0 to 255; I know of no character set encoding that maps all 256 values to a character, and even if there is one, it's unlikely to be your platform default encoding, and you aren't specifying it in your text-to-byte conversion.

The most common byte-to-text transformation for cipher text is Base-64 encoding. If that's what you are using here, you'll have to find or write a base-64 decoding utility.

You should also specify a complete transformation when creating the Cipher instance; otherwise, a provider specific default is used, and that might not match the sender's choices.

Since you don't show any IV, you might be using ECB as the mode. For most messages, this is insecure. It can only be safe if your message is a large random number (like a session identifier).

Most likely the padding is PKCS #5 padding (called PKCS7Padding in .NET), but you might have no padding, or some home-brew padding algorithm.

Assuming ECB and PKCS #5 padding, your cipher creation should look like this:

Cipher cipher = Cipher.getInstance("AES/ECB/PKCS5Padding");

Collectives™ on Stack Overflow

Java AES encryption and decryption using a predefined 128bit key

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related