• Message number one must convey (in approximately 30 characters, or more) the favorite type of fruit for your group, and this message must be encrypted with an affine cipher. Of course, the multiplicative part of your key (normally referred to as "a") must have a multiplicative inverse mod 26. This message should have all blanks and punctuation stripped away, nothing but uppercase characters A to Z should be present in your cipher.

 
• Message number two will be encrypted with a more general type of monoalphabetic-substitution cipher known as a keyword substitution cipher. The key is a phrase and a single letter. Let's suppose we select charlie brown loves pumpkins as the phrase and "r" as the letter for a key. We write out the alphabet in its normal order (abcd etc.) and under it, our key phrase. We must delete all duplicate letters that occur in our key phrase, and the key phrase is written with its first letter directly under the letter of our key. Here is what we'd have for our example:

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

W

N

V

S

P

U

M

K

C

H

A

R

L

I

E

B

O

Next, one simply types out the remaining letters of the alphabet (those not used in the key phrase) in their natural order, to fill in the remaining pairings. For our example, we start with ABCDEFGHIJKLMNOPQRSTUVWXYZ and delete all the letters that appear in our key's phrase, leaving D,F,G,J,Q,T,X,Y,Z. Inserting these letters in the table above (in their natural order) gives the substitution pattern:

 

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

W

N

V

S

P

U

M

K

D

F

G

J

Q

T

X

Y

Z

C

H

A

R

L

I

E

B

O

Finally, each "a" in our plain text becomes "W" in the cipher; all plain text "b"s become "N"s, and so on. Your keyword phrase must include the name of your favorite cartoon character. Your key's letter can be anything you like. Encrypt message two with a keyword substitution cipher, where message two addresses the issue of what your favorite foods are for breakfast, lunch, and dinner. Message two should include at least one sentence about each of the three major meals (breakfast, lunch, and supper). Leave in the blanks, or spaces, to help the decryption process, but remove all punctuation.
 

• Message three must be encrypted with either a keyword substitution cipher, or an affine cipher, however, if you select an affine cipher, you are allowed to add "gibberish text" to either the beginning or the end (or both) of your message (see homework problem 1.1.16). This nonsense text cannot exceed 10% of the length of your real message (for each 100 characters of real message, you are allowed to add 10 characters of noise to the ends of your message). Furthermore, blanks, or spaces (and punctuation) should be deleted from your message before you encrypt it. The plain text of message three must state whether your group prefers diet pop or regular pop, and whether you prefer beverages with caffeine in them, or caffeine-free beverages. Message three must be at least 60 characters in length.

 

Part 2: Focusing primarily on letter frequencies, digram frequencies and trigram frequencies, give a well described account of your attacking the other group's ciphers. Do not over-use computational power (as in simply trying all 12*26 keys for an affine cipher), but focus on using the predicted frequencies of English plaintext. You will be graded on both a correct decryption and your explanation of how you decrypted the other messages. Give a detailed write up of decrypting message one, stating the guesses at letters you made and showing the "solving of equations". Likewise, give a fairly detailed description of your attack on message two, and determine what the other groups like to eat for the three standard meals of the day. For message three, your goal is simply to decide if the other groups like diet or regular pop, and whether they like beverages with or without caffeine.