Can you read this? PDWK LV IXQ

This is an example of a simple crypto system, a Caesar cipher, a type of substitution cipher called a shift cipher. Symbolically,
f(x)= x+3(mod 26) or C=(M+3) mod26

The message to be communicated is called the plaintext. The message is submitted to an encryption or enciphering device using an encyption key that transforms the plaintext into ciphertext
(Ek(P)= C).
The decryption of deciphering device involves a decrypting key that undoes the enciphering
(Dk(C)= Dk (Ek (P))= P).

1. Encrypt the following message using a shift cipher of 5.

Plaintext: Cryptography is discrete mathematics.

Ciphertext:______________________________________

2. Encrypt the following message using a shift cipher of -3

Plaintext: Mathematics holds the key.

Ciphertext:______________________________________

3. Decode the following:

a)Ciphertext: riih lipt

Plaintext:______________________________________ What is the key?

b)Ciphertext: myxn xn als

Plaintext:______________________________________ What is the key?

4. Encode your own message and send it to a classmate.

You may use your graphing calculator to help encrypt or decipher messages.

I. Encryption

1. Store the numerical value of each letter to the letter key.
0sto=> A: 1sto=> B; 2sto=> C etc.

2. Enter your message to the home screen as a list and store it in list 1 (L1).

{C, R, Y, P, T, O.....}sto=> L1

Output: 2 17 24 15 19.....(Plaintext)

3. From the home screen or Stat Editor, define List 2 (L2) in terms of the encryption scheme:

L1+3 sto =>L2

Output: 5 20 27 18 22.....(Cyphertext)

II. Decryption: Define L3 by the inverse operation and repeat steps 2 and 3 above.

III. Use the TI-83 program ASHIFT

1. Input shift s= (type in the value of the shift, for example 3) <enter>

2. Input message m= (type in the letter of the message) <enter>
Output will appear as 2 (plaintext value) and 5 (ciphertext value).

3. Continue inputting each letter of the message.

Consider the following:

1. How would you use teh ASHIFT program if you receive ciphertext?

2. How secure is your communication if the encryption scheme is known to be a shift cipher?

3. Can you devise a more secure encryption scheme?

4. Can you write an efficient program that will encode the plaintext and display the ciphertext in letters?