CS 112 Project 4
Encryption
Due Friday, Feb. 29nd 2008

Cesar Cipher

One of the earliest cryptographic systems is the Cesar Cipher.  In this encoding scheme, each letter is shifted a set number of elements forward in the alphabet, wrapping around past z.  For instance, if we use a shift of 3, then A would be relpaced with D, B would be replaced with E, and so on.  So a Cesar Cipher with a shift of 5 would be encoded using the table:

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
F
 G
 H
 I
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 K
 L
 M
 N
 O
 P
 Q
 R
 S
 T
 U
 V
 W
 X
 Y
 Z
 A
 B
 C
 D
 E

And the message:

ATTACK AT DAWN

would be encoded as:

FYYFHP FY IFBS

The problem with a Cesar Cipher is that the plaintext letter "e" will always be represented by the same encrypted letter.  Thus cesar ciphers are very vunerable to letter frequency attacks -- we merely look for the most frequent letter in the encrypted text, and try the most frequent letters in the English (or, in Cesar's case, Latin) language -- greatly cutting down the search space.  2000 years ago, however, it seemed to work fine!

You can find an HTML cesar encoder/decoder online here.

Autokey Ciphers

We can avoid frequency attacks by using a variant of the Cesar cipher where the shift changes with each letter of the plaintext.  In Autokey ciphers, the message itself is used as a key to encrypt the message.  We will be using the Autokey variant of the Vigenere cipher.

This is easiest to see with an example.

 Let's say that we wanted to encrypt the message ATTACK AT DAWN.  First, we pick a word to use as our initial encryption key -- let's use QUEEN.  We line up QUEEN under ATTACK AT DAWN:


A
 T
 T
 A
 C
 K
   
A
 T
   
D
 A
 W
 N
Q
 U
 E
 E
 N









Next, we fill up the rest of the letters under our message with the message shifted over:

A
 T
 T
 A
 C
 K
   
 A
 T
   
D
 A
 W
 N
Q
 U
 E
 E
 N
 A
T
 T
A
 C
 K
 A

Now we can use the Key to tell us which Cesar cypher to use for each letter using the table below.  For instance, to encode the first "A" from ATTACK AT DAWN,  we look at row "A" of the table below, and column "Q", (or row "Q", column "A", the matrix is symmetric) to get "Q".  For the first "T", we use row "T", column "U", to get N.  The encrypted message is thus:

QNXEPK TM DCGN


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You can find a web based Vigenere Autokey encrypter/decrypter here

Assignment

For Project 4, you will create a program that can encrypt using both a simple Cesar cypher, and the Vigenere Autokey cipher described above.  Your main program should take 5 inputs from the command line (in this order):
So, to encrypt the file "foo.txt", writing to "foo.encrypt.txt",  using a Cesar cypher with an offset of 5, you would call your program with the command line parameters:

         C 5 E foo.txt foo.encrypt.txt

To decrypt this file, saving the result to bar.txt, you would use the parameters:

         C 5 D foo.encrypt.txt bar.txt

Capitalization, Non-letter characters

For both the Cesar Cipher and the Autokey cipher, your project should preserve capitalization, and leave non-letter characters unchanged.  For example, if you use a Cesar Cipher with an offset of 1, the message

One if by LAND,
Two if by SEA!

Should be encrypted as:

Pof jg cz MBOE,
Uxp jg cz TFB!

(Of course, if you wanted stronger encryption, you might remove spaces and punctuation, or encrypt them as well.  For this project, however, we will retain punctuation, so that if we encrypt then decrypt a file, we will be left with a file identical to the one we started with.  Even leaving in punctuation, the Vigenere Autokey cipher is remarkably strong)

Program Decomposition


Your program should be built to reuse code.  For example, you should have an abstract superclass Encrypter, that has a sublcass for encrypting/decrypting using the Cesar Cipher, and another subclass for encrypting/decrypting using the AutoKey ciper.  Your main loop should use a variable of type Encrypter to do the encription and decryption -- instantiate a Cesar Ciper or AutoKey depending upon input parameters.

Implementation Details

Converting between Characters and Integers

If you cast a character as an integer, you can get its ASCII value.  The ASCII values for letters are all contiguous (that is, the ASCII for 'b' is one larger than the ASCII for 'a', and so on).  So, we could convert from a character in the range of 'A' - 'Z' to an integer in the range 0-25 as follows:

        char c;
        //  Assign c a character in the range 'A' - 'Z'
        int i = (int) c - ((int) 'A');

Likewise, we can convert an integer in the range 0..25 to a character in the range 'A' .. 'Z':

        int i;
        //  Asign i an integer value in the range 0..25
        char c = (char) (i + (int) 'A');

Building Tables

While you are wecome to build a 2D table for for calculating the next encrypted letter, you don't need to.  Instead, you can just take the plaintext letter, convert it to a value between 0 and 25, convert the key letter to a value between 0 and 25, add then together (mod (%) 26), and then convert the resulting integer to a character.

Encrypting using Autokey

If you have a keyword of length 5 (like QUEEN), you only need to store 5 characters of the key, no matter how long the plaintext message is.  How?  Let's say you are  encrypting "ATTACK AT DAWN" using QUEEN:

Message:  ATTACK AT DAWN
Key: QUEEN

Output:  A + Q mod 26 = Q  (after doing appropriate letter / number conversions described above)

After using the first letter of the key to encrypt the plaintext, remove the first letter of the key, and append the plaintext letter you just encrypted to the end of the key:

Message:  TTACK AT DAWN
Key: UEENA

Output:  T + U mod 26 = N  (after doing appropriate letter / number conversions described above)

Again, after using the first letter of the key to encrypt the plaintext, remove the first letter of the key, and append the plaintext letter you just encrypted to the end of the key:

Message:  TACK AT DAWN
Key: EENAT

Output = T + E mod 26 = X

Continue until the entire message has been encrypted

Decrypting using Autokey

Decruption using Vigenere's Autokey is very similar to encryption -- to encrypt you were adding the key value to the plaintext to get the encrypted text -- how could you reverse that operation?

Grading

Extra Credit

You can earn up to 20 points of extra credit by adding a further encryption algorithm, such as the random encryption algorithm discussed in class on Friday.  If you do use the random encryption algorithm, be sure to write your own version of Random -- so that a change in the Java Random class doesn't render all of your encrypted files unreadable!  If you want to do a different addtional encryption scheme, let me know -- I'll likely approve it (assuming it is non-trivial).

Submission

All file(s) required for your project should be in the folder https://www.cs.usfca.edu/svn/<username>/cs112/Project4/