Simple Codes MTH 440

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Transcription:

Simple Codes MTH 440

Not all codes are for the purpose of secrecy Morse Code ASCII Zip codes Area codes Library book codes Credit Cards

ASCII Code

Steganography: Hidden in plain sight (example from http://www.bbc.co.uk/news/10 480477) Logos 2

Permutation Ciphers The plaintext is all there, it is just mixed up in some way. Scytale Block and Rotate Transposition Ciphers: Columnar Transposition

Columnar Transposition The secret key is the width of a rectangle The message is written across the rectangle and the ciphertext is read down the columns:

Plaintext: The answers to the test are abcbbadeac Rectangle width: 7 1 2 3 4 5 6 7 T H E A N S W E R S T O T H E T E S T A R E A B C B B A D E A C X X X Ciphertext: TEEEDHRTAEESEBAATSCCNOTBXSTABXWHRAX

Plaintext: The answers to the test are abcbbadeac Rectangle width: 7 T H E A N S W E R S T O T H E T E S T A R E A B C B B A D E A C X X X Ciphertext: TEEEDHRTAEESEBAATSCCNOTBXSTABXWHRAX

Plaintext: The answers to the test are abcbbadeac Rectangle width: 7 T H E A N S W E R S T O T H E T E S T A R E A B C B B A D E A C Ciphertext: TEEEDHRTAEESEBAATSCCNOTBSTABWHRA

Your turn decipher the following (width = 11; no padding) keonamoklwsilbseutdtegwneissspd

Width unknown DECRYPT: (Assume rows were not permuted.) TOQOIEOUTOEHFUFDQTAHTETATEUHREESHRHSAEE HNUEEEILSOYUMSSSSTQFPS Guess the number of rows/columns & check (there are online applets for this) OR Look: TOQOIEOUTOEHFUFDQTAHTETATEUHREESHRHSAEE HNUEEEILSOYUMSSSSTQFPS TOQOIEOUTOEHFUFDQTAHTETATEUHREESHRHSAEE HNUEEEILSOYUMSSSSTQFPS

COLUMNAR transposition TOQOIEOUTOEHFUFDQTAHTETATEUHREESHRHSAEEHNUEEE ILSOYUMSSSSTQFPS T O Q O I E O U We either have 5 full rows or 4 full rows and one partial row. There are 61 letters. Since 61 is not divisible by 5 we have 4 full rows and a partial. 61 = 4 x 15 + 1. So we have 4 rows of 15 columns and the last row just has one column

COLUMNAR transposition TOQOIEOUTOEHFUFDQTAHTETATEUHREESHRHS AEEHNUEEEILSOYUMSSSSTQFPS 1 2 3 4 5 6 7 8 9 1 0 T E O U T E E E R E U I Y S Q O O E F A T U E H E E L U S F Q U H D H A H S S H E S M S P O T F Q T T R H A N E O S T S I This doesn t look promising 1 1 1 2 1 3 1 4 1 5

COLUMNAR transposition TOQOIEOUTOEHFUFDQTAHTETATEUHREESHRHS AEEHNUEEEILSOYUMSSSSTQFPS T O Q O I E O U T O E H F U So we have 11 full rows or 10 full rows and one partial row. Since 61 is not divisible by 11 we have 10 full rows and one partial: 61 = 10x6 + 1 So if this is correct, we have 6 columns.

COLUMNAR transposition TOQOIEOUTOEHFUFDQTAHTETATEUHREESHRHS AEEHNUEEEILSOYUMSSSSTQFPS 1 2 3 4 5 6 T H E S U M O F T H E S Q U A R E S O F T H E S I D E S I S E Q U A L T O T H E S Q U A R E O F T H E H Y P O T E N U S E

To easy? Add a codeword and alphabetize the columns P Y T H A G T H E S U M O F T H E S Q U A R E S O F T H E S I D E S I S E Q U A L T O T H E S Q U A R E O F T H E H Y P O T E N U S E A G H P T Y U M S T E H E S H O T F E S R Q A U E S H O T F I S S I E D L T A E U Q S Q E O H T O F E U R A Y P H T E H U S N O E T E Ciphertext: ueeeilsoyumsssstqfpsshrhsaeehntoqoieoutoeetateuhreehfufdqtaht

Simple Substitution Ciphers Substitute each letter by another letter or symbol

PigPen cipher

Back Jacket Cover of Dan Brown s The Lost Symbol

Too easy? Add a codeword Codeword Davinci (don t use repeated letters, followed by unused letters alphabetically) D A V G H J I N C K L M B E F O P Q R S U T

Decrypt (homework) what is the codeword and plaintext?

Breaking simple substitution ciphers Frequency analysis: Most common English letters: E T A O I N S H R D L U Most common double letters: SS, EE, TT, FF, LL, MM, OO Most common digraphs: th er on an re he in ed nd ha at en es of or nt ea ti to it st io le is ou ar as de rt ve Single letter words: A, I http://www.wiley.com/college/mat/gilbert139343/java /java11_s.html

Polybuis Checkerboard 1 2 3 4 5 1 A B C D E 2 F G H I/J K 3 L M N O P 4 Q R S T U 5 V W X Y Z Decipher: 23 15 31 31 34

Polybuis Checkerboard with codeword 1 2 3 4 5 1 M A T H E 2 I/J C S B D 3 F G K L N 4 O P Q R U 5 V W X Y Z Homework: Encipher Mathematics is fun using the codeword Cryptology

Playfair Cipher pairs of letter are enciphered together 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 Encipher: I am brilliant Decipher: zdtxptbqcz

Playfair Cipher mix up the letters (code word or pattern) A Q P O N B R Y X M C S Z W L D T U V K E F G H I/J Write the grid for the playfair cipher using the codeword Wendesday then encrypt Tuesday

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