Worksheet II One-time Pads

The most secure encryption method is a one-time pad; that is a key used once and then discarded. You can generate a one-time pad using the rand function in the math/prob menu of the TI-83.

Ex) RandInt(0,25) might generate the sequence 23, 10, 16, 21, 0, 15, 1, 22.....

The first letter of the message is shifted 23, the second 10, the third 16, etc. Of course, your recipient's calculator might generate a different sequence of random numbers. To share the one time key, you must seed the random integer function with the same value.

Ex) Using 3 as a seed, 3sto=> rand (under math/prob), RandInt(0, 25) returns 6, 14, 17, 10, 1, 6, 6.....

These numbers can be stored in a list: 3sto=> rand <enter> RandInt(0, 25, n) sto=> L1 where n is the number of letters in your message.

To encode message, add the randomly generated numbers to each letter in the message.

Plaintext:cat  20019
   +61417
    81436=10mod26

Your recipient will use the one time pad to subtract from the ciphertext.

Try these:

1. Encode: message___________

2. Decode: 9 2 23 ___________

3. Change seed values and exchange messages with a friend. Be sure to share the key!