Cipher.getOutputSize method unnecessarily add output size in DECRYPT mode. recovered text size is less than or equal to the size of cipher text. AES cipher can not distiguish the ENCRYPT mode and DECRYPT mode, getOutputSize method always add some length upto 16 more even in decryption case. The following program demonstrates this.

//test the length of recovered text

import java.io.PrintStream;
import java.security.*;
import java.security.spec.*;
import java.util.Random;

import javax.crypto.*;
import javax.crypto.spec.*;
import java.security.Provider;
import com.sun.advcrypto.provider.SunAES;

public class test {

     public static void main( String argv[] ) {
    
     String algorithm = "AES";
     String mode = "ECB";
    byte[] iv = null;
    String padding = "PKCS5Padding";
     int keyStrength=128;
     AlgorithmParameterSpec aps = null;
    
     try{
    
     // Initialization
Random rdm = new Random();
byte[] plainText=new byte[125];
rdm.nextBytes(plainText);
System.out.println("plain text length is "+plainText.length);
        
  Cipher ci = Cipher.getInstance( algorithm + "/" + mode + "/" + padding);
 
     KeyGenerator kg = KeyGenerator.getInstance( algorithm);
     kg.init(keyStrength);
     SecretKey key = kg.generateKey();

                // encrypt
     ci.init( Cipher.ENCRYPT_MODE, key, aps);
     byte[] cipherText = new byte[ci.getOutputSize(plainText.length)];
System.out.println("cipherText length is "+cipherText.length);
     int offset = ci.update(plainText, 0, plainText.length, cipherText, 0);
     ci.doFinal( cipherText, offset );

//Decrypt
ci.init( Cipher.DECRYPT_MODE, key, aps);
  byte[] recoveredText = new byte[ci.getOutputSize(cipherText.length)];
System.out.println("Recovertext length is "+recoveredText.length);
     int len = ci.doFinal( cipherText, 0, cipherText.length, recoveredText );
System.out.println("The length actually needed is "+len);

     }catch(Exception ex){
     System.out.println("FAILED");
     ex.printStackTrace();
    
     }
     }
}

lwang:/home/lw129730/AES/testcode/test( 117 )%java test
plain text length is 125
cipherText length is 128
Recovertext length is 144
The length actually needed is 125