AES-128 encryption in Java

Question

I have tried to encrypt some plaintext, which is in bytes, with a key, which is in bytes. However, the output I am getting is not what I am expecting.

public class AES { public static byte[] encrypt(byte[] plainText, byte[] key) { try { Cipher cipher = Cipher.getInstance("AES"); SecretKeySpec secretKey = new SecretKeySpec(key, "AES"); cipher.init(Cipher.ENCRYPT_MODE, secretKey); byte[] cipherText = cipher.doFinal(plainText); return cipherText; } catch (Exception e) { e.printStackTrace(); } return null; } public static String byteToHex(byte[] hash) { StringBuilder sb = new StringBuilder(hash.length * 2); for (byte b : hash) { sb.append(String.format("%02x", b)); } return sb.toString(); } public static void main(String args[]) { byte plaintext[] = {0x00,0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x58,0x63,(byte)0xaa,(byte)0xbb,(byte)0xcc,(byte)0xdd,(byte)0xee,(byte)0xff}; byte key[] = {0x00,0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09,0x0a,0x0b,0x0c,0x0d,0x0e,0x0f}; byte[] encrypted = encrypt(plaintext, key); System.out.println("Encrypted String : " + byteToHex(encrypted)); } }

The output I am getting is: 814064943fe05668da1f3d2269a4ee22954f64f2e4e86e9eee82d20216684899 while I am expecting 69c4e0d86a7b0430d8cdb78070b4c55a. I am using the plaintext and key from here

Do you understand what PKCS5 padding does? A hint: I will always make the result longer. So an input of 16 bytes can never result in an output of 16 bytes. — Codo
– Codo, Commented Nov 9, 2018 at 22:44
No, I have removed it, and use the default. However, I would like to know why I am getting the incorrect output — Bab
– Bab, Commented Nov 9, 2018 at 22:46
I have also tried to print the encrypted byte array with Array.toString(encrypted ) — Bab
– Bab, Commented Nov 9, 2018 at 22:51
The default padding mode of your AES provider is most likely "PKCS5". So you haven't changed anything... Read about it and you have a good idea why your result is different. — Codo
– Codo, Commented Nov 9, 2018 at 22:53
Have read about it, and I found out that I had to use NoPadding, which has given me the proper format. However, I am still getting the incorrect output. — Bab
– Bab, Commented Nov 9, 2018 at 23:00

Codo · Accepted Answer · 2018-11-09 23:11:36Z

There are two things incorrect in your code.

You use PKCS5 padding while the original example doesn't use any padding.
You incorrectly copied the plain text.

So:

Cipher cipher = Cipher.getInstance("AES/ECB/NoPadding"); ... byte plaintext[] = { 0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, (byte) 0x88, (byte) 0x99, (byte) 0xaa, (byte) 0xbb, (byte) 0xcc, (byte) 0xdd, (byte) 0xee, (byte) 0xff };

Result:

Encrypted String : 69c4e0d86a7b0430d8cdb78070b4c55a

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