Cryptography

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

class EncryptAndDecrypt {

public static void main (String[] args) throws Exception{
KeyPairGenerator keygenerator = KeyPairGenerator.getInstance("RSA");
SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");
keygenerator.initialize(1024, random);

KeyPair keypair = keygenerator.generateKeyPair();
PrivateKey privateKey = keypair.getPrivate();
PublicKey publicKey = keypair.getPublic();
Cipher cipher = Cipher.getInstance("RSA");

cipher.init(Cipher.ENCRYPT_MODE, publicKey);
BufferedReader br=new BufferedReader(new FileReader(new File("data.txt")));
String st,str;
while((st=br.readLine()) != null)   {
    str+=st+" ";
}
byte[] cleartext = null;
cleartext = str.getBytes();
byte[] ciphertext = null;
ciphertext = cipher.doFinal(cleartext);
System.out.println("the encrypted text is: " + ciphertext.toString());

cipher.init(Cipher.DECRYPT_MODE, privateKey);
byte[] cleartext1 = cipher.doFinal(ciphertext);
System.out.println("the decrypted cleartext is: " + new String(cleartext1));
}
}

import java.security.*; 
import javax.crypto.*;
import javax.crypto.spec.DESKeySpec;

class EncryptAndDecrypt {

public static void main (String[] args) throws Exception{ 
    SecretKeyFactory key = SecretKeyFactory.getInstance("DES");
    SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");

    byte[] desKeyData = { (byte)0x01, (byte)0x02, (byte)0x03, (byte)0x04, (byte)0x05, (byte)0x06, (byte)0x07, (byte)0x08 }; 
    DESKeySpec desKeySpec = new DESKeySpec(desKeyData);

    Key keys = key.generateSecret(desKeySpec);
    Cipher cipher = Cipher.getInstance("DES");
    cipher.init(Cipher.ENCRYPT_MODE, keys);
    String st= "Welcome";
    byte[] cleartext = null;
    cleartext = st.getBytes();
    byte[] ciphertext = null;
    ciphertext = cipher.doFinal(cleartext); 
    System.out.println("the encrypted text is: " + ciphertext.toString());

    cipher.init(Cipher.DECRYPT_MODE, keys); 
    byte[] cleartext1 = cipher.doFinal(ciphertext);
    System.out.println("the decrypted cleartext is: " + new String(cleartext1));
    }
}

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

   public class EncryptAndDecrypt {
     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="Welcome";
       KeyGenerator kgen = KeyGenerator.getInstance("AES");
       kgen.init(128); 
       SecretKey skey = kgen.generateKey();
       byte[] raw = skey.getEncoded();

       SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
       Cipher cipher = Cipher.getInstance("AES");

       cipher.init(Cipher.ENCRYPT_MODE, skeySpec);

       byte[] encrypted = cipher.doFinal((args.length == 0 ?"Welcome" : args[0]).getBytes());
       System.out.println("encrypted string: " + asHex(encrypted));

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