1. Introduction to Cryptography
Lecture 7

2. Transposition Cipher
Letters stay the same
Order is different

3. Rail Fence Cipher
Definition: Write the plaintext downwards on successive "rails" of an imaginary fence. When you get to the bottom start moving up. Write the message line by line.

4. Rail Fence Cipher Example: The message: WRIORFEOEEESVELANADCEDETC. W R

5. Columnar Transposition
Definition: Write the message in rows of a fixed length, and then read out again column by column. The columns are chosen in some scrambled order. Both the length of the rows and the permutation of the columns are usually defined by a keyword.

6. Key Columnar Transposition
Example: Let the key word be: ZEBRA.
The message:
EODAEASRENEIELORCEECWDVFT. Z E B R A W D I S C O V F L T N

7. Route Cipher
Definition: The plaintext is first written out in a grid of given dimensions, then we read it off in a pattern given in the key.

8. Route Cipher
Example: The key say: read message from top right corner down and to the left.
The message:
TONCEADOEFLEEECRVERSADIEW. W D V F T E I L O A S R N C

9. Double Transposition
By guessing the block size it can be easy to decrypt the message
Is it more secure to use transposition cipher twice?
Should we use same key word?

10. Permutations Can be used as key word for a key transposition cipher.
Example: Let the message be: WE ARE DISCOVERED FLEE AT ONCE. Let permutation be:
The message:
EIELORCEECWDVFTEODAEASREN. x 1 2 3 4 5
p(x)

11. Incomplete Columns How can we handle incomplete columns?
We will write some letters at the end to complete the row/column.
Should it be same letter or different letters?
Will the person who gets it understand what the original message said?
Will it make attack more complicated?

12. Polyalphabetic Substitutions
Definition: Let be different substitution ciphers. Then to encrypt the message apply
If the length of the message is longer than number of different ciphers, then repeat same ciphers in the same order.

13. Homework Read pg.96-105. Exercises: 2, 6, 7 on pg.106- 107.
Those questions will be a part of your collected homework.