How to use AES DECRYPT_MODE?

Hello Everybody,,

I have a code of AES encryption and decryption and it works fine for both.

When I separeate the code of AES into two codes, one for encrypt and another for decrypt.
The code of encryption works fine.

I saved output of encryption as String in a variable.
Also I saved key into a variable.

Now, the decryption code works, But it gives me no output(null)!

Here is the code of decryption:

I don't know why it gives me no output!!!

any help please!

I get the following error when I run the code:

javax.crypto.BadPaddingException: Given final block not properly padded

The problem most likely is in the skewed encrypted data, show us also code you are using for encryption.

Here is the full code that works fine for both encryption and decryption:

(*note: instead of using auto generation of key I ask user for a password and I insert it into md5. I take half of hash value and use it as 128bit key for encryption).

import java.util.*;
   import java.security.*;
   import javax.crypto.*;
   import javax.crypto.spec.*;
   import java.io.*;

/**
   * This program generates a AES key, retrieves its raw bytes, and
   * then reinstantiates a AES key from the key bytes.
   * The reinstantiated key is used to initialize a AES cipher for
   * encryption and decryption.
   */

public class AES {

/**
     * Turns array of bytes into string
     *
     * @param buf	Array of bytes to convert to hex string
     * @return	Generated hex string
     */
     public static String asHex (byte buf[]) {
      StringBuffer strbuf = new StringBuffer(buf.length * 2);
      int i;

for (i = 0; i < buf.length; i++) {
       if (((int) buf[i] & 0xff) < 0x10)
	    strbuf.append("0");

strbuf.append(Long.toString((int) buf[i] & 0xff, 16));
      }

return strbuf.toString();
     }

public static void main(String[] args) throws Exception {

//String message="This is just an example";

// Get the KeyGenerator

// KeyGenerator kgen = KeyGenerator.getInstance("AES");
      // kgen.init(128); // 192 and 256 bits may not be available

// Generate the secret key specs.
       //SecretKey skey = kgen.generateKey();
       //byte[] raw = skey.getEncoded();
       
       
        Scanner stdin=new Scanner(System.in);
        System.out.println("Enter a Password");
        String pass=stdin.nextLine();
       
       MessageDigest digest = java.security.MessageDigest.getInstance("MD5");
       digest.update(pass.getBytes());
       byte[] hash = digest.digest();
       String hash1 =asHex(hash);
       System.out.println(hash1);

char x;
        String y="";

for (int j=0; j<16; j++)
        {
        		 x=hash1.charAt(j);
        		 y=y+x;
        }
        System.out.println(y);

SecretKeySpec skeySpec = new SecretKeySpec(y.getBytes(), "AES");

// Instantiate the cipher
        //Scanner stdin=new Scanner(System.in);
        System.out.println("Enter Plaintext");
        String text=stdin.nextLine();
        
        
       Cipher cipher = Cipher.getInstance("AES");

cipher.init(Cipher.ENCRYPT_MODE, skeySpec);

byte[] encrypted =
         cipher.doFinal(text.getBytes());
       System.out.println("encrypted string: " + asHex(encrypted));
	   String mycipher=new String(encrypted);
       System.out.println("My Cipher: " + mycipher);

byte[] encrypted2 = mycipher.getBytes();

cipher.init(Cipher.DECRYPT_MODE, skeySpec);
       byte[] original =
         cipher.doFinal(encrypted2);
       String originalString = new String(original);
       System.out.println("Original string: " +
         originalString + " " + asHex(original));
     }
   }

here as one code it works fine!
but, when I save the encrypted text and try again to decrypt it it tell me the previous error that I mentioned!

any help please?

Question too difficult for “beginning”. Moving thread.

I got a solution:

I saved the ciphertext as hexadecimal!

Then at decryption I convert hexadecimal back to byte array and decrypt it,
and it works fine!

So, do you think the problem is I cannot save encrypted text as String??!!

Campbell Ritchie wrote:Question too difficult for “beginning”. Moving thread.

Excuse me, to which fourm did you move it??

Sultan Altoobi wrote:
So, do you think the problem is I cannot save encrypted text as String??!!

Encrypted data is binary data. It can't be safely stored into a String, as strings expect unicode.

Henry

Sultan Altoobi wrote:

Campbell Ritchie wrote:Question too difficult for “beginning”. Moving thread.

Excuse me, to which fourm did you move it??

It looks like it was moved to the security forum -- see top. Regardless, you'll have a link in the beginners forum for the next few days to help in regards to this.

Henry

Exceuse Henry,,

When I changed the text it gives me the following error:

javax.crypto.IllegalBlockSizeException: Input length must be multiple of 16 when decrypting with padded cipher

any idea?

This the code I use to convert from hexadecimal to byte array:

Sultan Altoobi wrote:Exceuse Henry,,

When I changed the text it gives me the following error:

javax.crypto.IllegalBlockSizeException: Input length must be multiple of 16 when decrypting with padded cipher

any idea?

I found my mistake!
now it works fine!