Java Decryption

Question

I have a problem with with

javax.crypto.Cipher

When I write this lines of code

    Cipher cipher;
    byte[] bytes = null;

    try
    {
        cipher = Cipher.getInstance("AES");
        cipher.init(Cipher.DECRYPT_MODE, generateAESKey128b(key));
        bytes = cipher.doFinal(input.getBytes("UTF-8"));
    }
    catch (NoSuchAlgorithmException e)
    {
        e.printStackTrace();
    }
    catch (NoSuchPaddingException e)
    {
        e.printStackTrace();
    }
    catch (InvalidKeyException e)
    {
        e.printStackTrace();
    }
    catch (UnsupportedEncodingException e)
    {
        e.printStackTrace();
    }
    catch (IllegalBlockSizeException e)
    {
        e.printStackTrace();
    }
    catch (BadPaddingException e)
    {
        e.printStackTrace();
    }

The console give me this error

javax.crypto.IllegalBlockSizeException 
Input length must be multiple of 16 when    
decrypting with padded cipher
at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
at com.sun.crypto.provider.SunJCE_f.b(DashoA13*..)
at com.sun.crypto.provider.AESCipher.engineDoFinal(DashoA13*..)
at javax.crypto.Cipher.doFinal(DashoA13*..)
at it.unitn.se.gym.backend.utils.Security.AES128Decode(Security.java:109)
at it.unitn.se.gym.backend.utils.Security.decode_AES128_Base64(Security.java:96)
at it.unitn.se.gym.backend.WebService.main(WebService.java:42)
Exception in thread "main" java.lang.NullPointerException
at it.unitn.se.gym.backend.utils.Security.decode_AES128_Base64(Security.java:97)
at it.unitn.se.gym.backend.WebService.main(WebService.java:42)

The first 2 lines of code are correct but when I pass the attribute "text", of type byte[], to the doFinal function, it give me the error.

Can someone tell me why?

SOLVED:

Okay, problem solved

byte[] encrypted = UniversalBase64Encoder.decode(input);
Cipher cipher = Cipher.getInstance("AES");
cipher.init(Cipher.DECRYPT_MODE, generateAESKey128b(key));
byte[] originalBytes = cipher.doFinal(encrypted);

This is the right code that I wrote

The exception says you're decrypting, but the code says you're encrypting. Show us your real code. — JB Nizet
– JB Nizet, Commented Mar 15, 2013 at 12:53
This stackoverflow.com/questions/3954611/… question and answer will help you. Use Base64 library — Serkan Arıkuşu
– Serkan Arıkuşu, Commented Mar 15, 2013 at 12:54
@DavidMartinelli If you have found the solution, don't add it to your question. Create an answer and accept it. — Michaël
– Michaël, Commented Mar 20, 2013 at 8:07

Nathaniel Waisbrot · Accepted Answer · 2013-03-19 13:19:52Z

1

The problem is that you're trying to decrypt a string that was not encrypted, and in doing so are violating an assumption of the decryption algorithm (that its input size is always a multiple of 16).

Here's a block of code that encrypts and then decrypts a string. Notice that when the encrypted string is printed, it's 16 bytes long, even though the input string is not. The encryption algorithm pads the input string out to make it a multiple of 16 bytes before encrypting it. That 16-byte-long encrypted string is now a valid input for decryption.

This assumption (that the result of encrypting will be an even size) is quite standard. It not only makes the decryption/encryption algorithm easier to write, but it also prevents an attacker from knowing the length of the thing you encrypted.

byte[] keyBytes = new byte[16];
keyBytes[0] = 1;
SecretKeySpec key = new SecretKeySpec(keyBytes, "AES");
String input = "hello";
Cipher cipher;
byte[] bytes = null;
cipher = Cipher.getInstance("AES");
cipher.init(Cipher.ENCRYPT_MODE, key);
bytes = cipher.doFinal(input.getBytes("UTF-8"));

System.out.println("Encoded: "+Arrays.toString(bytes));

cipher.init(Cipher.DECRYPT_MODE, key);
byte[] decoded = cipher.doFinal(bytes);

System.out.println("Decoded: "+new String(decoded, "UTF-8"));

answered Mar 19, 2013 at 13:19

Nathaniel Waisbrot

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2 Comments

David Martinelli Over a year ago

So, if I have an encrypted string (passed from someone), and I have the shared key (key of encryption and decryption), how can I decrypt the string?

Nathaniel Waisbrot Over a year ago

@DavidMartinelli Like I'm doing in the final 3 lines. If you're getting that error from a string that somebody has told you is encrypted, then one of the following is true (1) it's not actually encrypted (2) it's encrypted using some algorithm other than what you're using (3) you have mangled the encrypted string prior to trying to decrypt it

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