Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is...

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Introduction to Cryptography Lecture 7

Transcript of Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is...

Page 1: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

Introduction to Cryptography

Lecture 7

Page 2: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

Transposition Cipher

Letters stay the same Order is different

Page 3: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

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.

Page 4: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

Example:

The message:

WRIORFEOEEESVELANADCEDETC.

Rail Fence Cipher

W R I O R F E O E

E E S V E L A N

A D C E D E T C

Page 5: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

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.

Page 6: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

Key Columnar Transposition

Example: Let the key word be: ZEBRA.

The message:

EODAEASRENEIELORCEECWDVFT.

Z E B R A

W E A R E

D I S C O

V E R E D

F L E E A

T O N C E

Page 7: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

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.

Page 8: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

Route CipherExample: The key say: read message from

top right corner down and to the left.

The message:

TONCEADOEFLEEECRVERSADIEW.

W D V F T

E I E L O

A S R E N

R C E E C

E O D A E

Page 9: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

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?

Page 10: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

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) 3 1 5 2 4

Page 11: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

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?

Page 12: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

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.

nEEE ,...,, 21

nppp ,...,, 21 )( ii pE

Page 13: Introduction to Cryptography Lecture 7. Transposition Cipher Letters stay the same Order is different.

Homework

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

collected homework.


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