Linear Ciphers - Introduction to Cryptography with Java Applets - page 123

FIGURE 5.6

EXERCISES

1. Writing a program to encipher and decipher individual characters using affine trans-

formations is very easy. All you need do is use a method that computes inverses mod
n , and the rest is as easy as writing the Caesar cipher. Write a TestAffineCipherApplet

class which tests the two static methods affineEncipher(), and affineDecipher(). These
last two methods should accept an array of bytes (a message), along with the enci-
phering key(s), and return an array of bytes. You must use primitive ints, so you need
to write a method to compute inverses modulo

n for ints.

2. Encipher, and then decipher, the following messages using Caesar shift transforma-

tions with the ordinary alphabet, and where the shift b = 23.

a. KENNEDY IS DEAD

b. HITLER IN PERU

c. BIG BROTHER

3. Repeat the previous exercise but use a single character affine cipher with a multiplier

of 15 and a shift of 6.

4. Repeat the previous exercise, but use a block affine cipher with a block size of 4 char-

acters per block. Use 25252526, the appropriate value for the modulus (using the ordi-
nary alphabet), and use 1542327 for the multiplier, and 9923411 for the shift. Pad
messages that are not multiples of the block size with the letter X.