Re: The difference between update() and doFinal() in ClassCipher

Question

Hi Guys,
I found this topic,
The difference between update() and doFinal() in ClassCipher
during a Google Search. I need your help, because I'm working on a project for an university exam.
It's about a Client FTP that sends and received files with the content crypted.
I'm using DES Encryption with "DES/CBC/PKCS5Padding".
I have this doubt: when it's correct to uso the doFinal method? At the last block of the file, or after it? (for encryption)
And during the decryption, how can I use the result of the doFinal operation?

Answer 1

Reposting with {code} tags
capagira87 wrote:
Ok these are the functions:
public void CodificaInvia (String cd, String filename, SecretKey Key) // it opens, codes and sends to the server the content of the file 512 bytes per time
int returncode=0;
Socket DataSock = null;
FileInputStream fis = null;
byte [] text = null;
try {
DataSock = new Socket (servAddress,22);
ObjectInputStream cin = new ObjectInputStream (DataSock.getInputStream());
ObjectOutputStream out = new ObjectOutputStream (DataSock.getOutputStream());
byte[] iv = new byte[] { (byte) 0x8E, 0x12, 0x39, (byte) 0x9C, 0x07, 0x72, 0x6F, 0x5A };
AlgorithmParameterSpec paramSpec = new IvParameterSpec(iv);
Cipher cipher = Cipher.getInstance("DES/CBC/PKCS5Padding");
cipher.init(Cipher.ENCRYPT_MODE, Key, paramSpec);
fis = new FileInputStream (cd+File.separator+filename);
int size = fis.available();
if ((size%8)==0)
text = new byte [size];
} else {
do {
size++;
} while ((size%8)!=0);
text = new byte [size];
fis.read(text,0,fis.available());
byte [] ciphertext = cipher.doFinal(text);
out.writeInt(ciphertext.length);
int numblock = ciphertext.length/512;
int resto = ciphertext.length%512;
out.writeInt(numblock);
out.writeInt(resto);
byte [] tempblock = new byte [512];
for (int conta=0; conta<numblock; conta++)
for (int i=0; i<512; i++)
tempblock=ciphertext[(conta*512)+i];
out.write(tempblock);
out.flush();
if (resto > 0) // this block works if the file size is not multiple of 512
java.util.Arrays.fill (tempblock, (byte) 0);
int h= numblock*512;
for (int i=0; i<resto; i++)
tempblock[i]=ciphertext[h+i];
out.write (tempblock);
out.flush();
System.out.println ("OK!");
} catch (Exception e)
e.printStackTrace();
public void RicezioneDecodifica (String filename, SecretKey Key) // in this function I receive the file blocks, I decrpyt them and I save the decrypted text into a file
Socket DataSock = null;
byte [] buffer=null; // it saves the received data
String Plaintext = new String (); // it saves the deciphered text
try {
ServerSocket ss2 = new ServerSocket (TRANSMISSION_PORT);
DataSock = ss2.accept();
ObjectOutputStream out = new ObjectOutputStream (DataSock.getOutputStream());
ObjectInputStream cin = new ObjectInputStream (DataSock.getInputStream());
byte[] iv = new byte[] { (byte) 0x8E, 0x12, 0x39, (byte) 0x9C, 0x07, 0x72, 0x6F, 0x5A };
AlgorithmParameterSpec paramSpec = new IvParameterSpec(iv);
Cipher dcipher = Cipher.getInstance("DES/CBC/PKCS5Padding");
dcipher.init(Cipher.DECRYPT_MODE, Key, paramSpec);
int size = cin.readInt();
int numblock = cin.readInt();
int resto = cin.readInt();
if ((size%8)!=0)
do{
size++;
} while ((size%8)!=0);
int ByteLetti=0; // counter of read bytes
buffer = new byte [size];
for (int conta=0; conta<numblock; conta++)
int a=cin.available();
byte [] tempblock = new byte [a];
cin.read(tempblock, 0, a);
for (int i=0; i<a; i++)
buffer[ByteLetti+i]=tempblock[i];
ByteLetti=ByteLetti+a;
byte[] texttmp=dcipher.update(tempblock);
String tmp = new String (texttmp, "UTF8");
Plaintext=Plaintext+tmp;
if (resto>0)
int a = cin.available();
byte [] tempblock;
if ((a%8)>0)
do {
a++;
} while ((a%8)!=0);
tempblock = new byte [a];
cin.read(tempblock, 0, cin.available());
byte[] texttmp = dcipher.doFinal(tempblock);
String tmp = new String (texttmp, "UTF8");
Plaintext = Plaintext+tmp;
ObjectOutputStream oos = new ObjectOutputStream (new FileOutputStream (filename));
oos.writeChars(Plaintext);
oos.flush();
oos.close();
} catch (Exception e)
e.printStackTrace();