Cryptography

Cryptography, the study of secret writing, has been around for a very long time, from simplistic techniques to sophisticated mathematical techniques. No matter what the form however, there are some underlying things that must be done - encrypt the message and decrypt the encoded message.

One of the earliest and simplest methods ever used to encrypt and decrypt messages is called the Caesar cipher method, used by Julius Caesar during the Gallic war. According to this method, letters of the alphabet are shifted by three, wrapping around at the end of the alphabet. For example,

PlainTest: a b c d e f g h i j k l m n o p q r s t u v w x y z

Caesar shift: d e f g h I j k l m n o p q r s t u v w x y z a b c

When encrypting a message, you take each letter of the message and replace it with its corresponding letter from the shifted alphabet. To decrypt an encoded message, you simply reverse the operation. That is, you take the letter from the shifted alphabet and replace it with the corresponding letter from the plaintext alphabet. Thus the string the quick brown fox becomes wkh txlfn eurzq ira

Another type of cipher is known as Transposition cipher. In this type of cipher, letters in the original message are re-arranged in some methodical way - for instance, reverse the letters in each string. Thus the string the quick brown fox becomes eht kciuq nworb xof

Still yet another cipher method is the Reverser cipher. This method does not only reverses the letters in each word, but as does the Transposition cipher, but it also reverses the result generated from the Transposition cipher. Hence the original message the quick brown fox becomes xof nworb kciuq eht

Class design

All three methods - Caesar, Transposition and Reverser - have something in common. They encrypt and decrypt messages. That is, they take a string of words and translate each word using the encoding algorithm appropriate for that cipher. Thus each class cipher will need polymorphic encode() and decode() methods, which take a word and encodes and decodes it according to the rule of the particular cipher.

From a design perspective, the encrypt() method and the decrypt() methods will be the same for every class. They simply break message into words and have each word encode or decode. However, the encode and decode methods will be different for each cipher. Figure 1 shows a hierarchy of the classes.

Figure 1. Inheritance hierarchy

From the above analysis a partial abstract class Cipher is depicted be by Listing 1.

public abstract class Cipher

{

private String message;

public Cipher(String text)

{ ... }

public String encrypt()

{

String word = encode(...);

return ....;

}

public String decrypt(String message)

{

String word = decode(...);

return ....;

}

public abstract String encode(String s);

public abstract String decode(String s);

}

Listing 1.

The class Caesar inherits the abstract class Cipher. This class defines the methods code and decode. The method encode takes a String parameter and returns a String result. It takes each character of the parameter and performs a Caesar shift on the character. That is, the a shift with possible wrap around can be coded as follows:

char ch = word.charAt(i);

ch = (char)('a' + ch - 'a' + 3) % 26);

The method decode does the reverse. Listing 2 shows a partial definition of this class - Caesar

public class Caeser extends Cipher

{

public Caeser(String s)

{

super(s);

}

public String encode(String word)

{

StringBuffer result = new StringBuffer();

for (int i = 0; i < word.length(); i++)

{

char ch = word.charAt(i);

ch = determineCharacter(ch, Constants. ENCODE_SHIFT);

result.append(ch);

}

return result.toString();

}

public String decode(String word)

{

return ....;

}

public char determineCharacter(char ch, int shift)

{

if(Character.isLowerCase(ch))

ch = (char)('a' + (ch - 'a' + shift) % Constants.WRAP_AROUND);

return ch;

}

}

Listing 2.

In similar fashion, the class Transpose inherits Cipher and defines the methods code and encode. Listing 3 shows an incomplete definition of the class Transpose.

public class Transpose extends Cipher

{

Transpose(String s)

{