CPSC 120 Program 4
Encryption
Due Friday, Dec 6

The Caesar Cipher

In this assignment you will write a class Crypt that has methods to encrypt and decrypt text using a Caesar cipher. In the Caesar cipher, each character in the original text ("plaintext" in cryptography parlance) is shifted by a fixed amount. So if the plaintext is HELLO and the shift is 3, the encrypted text is KHOOR. If the end of the range of characters is reached during the shift, just wrap around to the beginning; so if you're dealing only with capital letters, YOU becomes ARX under a shift of 3. Decryption is just the reverse of encryption; to decrypt text that was shift by 3, just shift it by -3. But since it wraps around, every negative shift has a corresponding positive shift -- for a range of 26 characters such as the capitals-only example, a shift of -3 is the same as a shift of +23.

Of course, restricting the range to only capital letters is quite limiting. It is also unnecessary; we could choose any range of the Unicode character set to deal with. Most of the common printable Unicode characters have codes in the range 32..126 (this includes upper and lower case letters, digits, space and most punctuation), so we'll use that range. In general to shift a character in the range LO..HI (inclusive) you would do the following:

       (char) ((c-LO + shift)%((HI-LO+1) + LO)

This wraps around appropriately while staying in the given range.

Program Structure

You will need the following methods in your Crypt class. Your Crypt class is essentially an encryption machine that just performs the tasks given to it without maintaining any state, so all of these methods will be static.

A public encrypt method String encrypt(String phrase) that takes a phrase, encrypts it using a randomly generated shift, and returns the encrypted phrase. Note that the shift should be in the range 1..(HI-LO). This method will call the private encrypt method below to do the real work.
A private encrypt method String encrypt(String phrase, int shift) that encrypts a phrase using the given shift. This is easy; it just runs through the characters in the phrase, shifting each as described above and placing the new character into a new string, which is then returned.
A public decrypt method String decrypt(String phrase) that takes an encrypted phrase and decrypts it. It should do this by doing the following for each of the (HI-LO) possible shifts:
- Use the shift to "decrypt" the phrase. (Note that for the Caesar cipher encryption is the same as decryption, so you can use your encrypt method.)
- Use a StringTokenizer to get the first word from the maybe-decrypted phrase. Use the lookup method below to look this word up in the system dictionary. If you find it, display the decrypted string to the user and ask if it is correct. If so, return the decrypted string; if not, continue trying other possible shifts. If no shift produces a match (or the user rejects all matches), print a message saying you can't crack the code and return the string that was passed in.
A private method boolean lookup(String s) that takes a word and looks it up in the dictionary. Like most Unix systems, Riddler has a dictionary at /usr/share/dict/words. You may assume that the dictionary stores words one per line. Read the words from the dictionary using the a BufferedReader, which you should set up as follows:
```
BufferedReader in = new Bufferedreader(new FileReader(DICT_PATH));
```
DICT_PATH should be a constant in the Crypt class that holds the path (above) to the dictionary. You can then read words using the readLine method, e.g.,
```
String nextWord = in.readLine();
```
A BufferedReader returns null when it reaches end of file, so you should check for that in your loop.

You will also need to write a TestCrypt class that prompts the user for a phrase, displays the encrypted phrase, then asks if the user would like to try to decrypt it. If the response is yes, the program should attempt to decrypt it as described above; if the response is no, the program should terminate.

Input and Output

All I/O should be done using dialog boxes. Remember that when using dialog boxes you have to use Ctrl-C to get the prompt back when your program ends.

What to Turn In

Turn in hardcopy of your Crypt and TestCrypt classes. Tar your prog4 directory and e-mail it to me with cpsc120 prog4 in the Subject line.

CPSC 120 Program 4 EncryptionDue Friday, Dec 6

The Caesar Cipher

Program Structure

Input and Output

What to Turn In

CPSC 120 Program 4
Encryption
Due Friday, Dec 6