11/21/2015 1. 2 Session Plan Overview of Classical encryption techniques: Symmetric cipher model Substitution Transposition Steganography.

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Session PlanOverview of Classical encryption techniques: Symmetric cipher modelSubstitutionTranspositionSteganography

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Cryptography Crypto

secret key public key cryptographic hashes

Used for authentication, integrity protection,

encryption

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plaintext - original message ciphertext - coded message cipher - algorithm for transforming plaintext to

ciphertext key - info used in cipher known only to sender/receiver encipher (encrypt) - converting plaintext to

ciphertext decipher (decrypt) - recovering ciphertext from

plaintextcryptography - study of encryption

principles/methodscryptanalysis (codebreaking) - study of principles/

methods of deciphering ciphertext without knowing key

cryptology - field of both cryptography and cryptanalysis

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Cryptography Crypto

secret key public key cryptographic hashes

Used for authentication, integrity protection,

encryption

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Components of symmetric encryptionPlain textEncryption algorithmSecret keyCipher textDecryption algorithm

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Important factors of symmetric encryptionType of operations(substitution & transpositions)Number of keys usedProcessing mode ( block cipher & stream cipher)Attacks on encryption system Crypt analysis – To get plain text or key using algorithm Brute-force attack – Try every possible key to get plain

text/key Cryptography + cryptanalysis = cryptology








ciphertext only only knows algorithm & ciphertext

known plaintext know/suspect plaintext & ciphertext

chosen plaintext select plaintext and obtain ciphertext

chosen ciphertext select ciphertext and obtain plaintext

chosen text select plaintext or ciphertext to en/decrypt


unconditional security no matter how much computer power or time

is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext

computational security given limited computing resources (eg time

needed for calculations is greater than age of universe), the cipher cannot be broken


always possible to simply try every key most basic attack, proportional to key size assume either know / recognise plaintext

Key Size (bits)

Number of Alternative Keys

Time required at 1 decryption/µs

Time required at 106 decryptions/µs

32 232 = 4.3 109 231 µs = 35.8 minutes

2.15 milliseconds

56 256 = 7.2 1016 255 µs = 1142 years 10.01 hours

128 2128 = 3.4 1038 2127 µs = 5.4 1024 years

5.4 1018 years

168 2168 = 3.7 1050 2167 µs = 5.9 1036 years

5.9 1030 years

26 characters

(permutation)

26! = 4 1026 2 1026 µs = 6.4 1012 years

6.4 106 years


Caesar cipherMono alphabetic cipherPlayfair cipherPloyalphabetic ciphersOne-time pad


Each character of a message is replaced by a character three po-

sition down in the alphabet.plaintext: are you readyciphertext: DUH BRX UHDGBciphertext can be expressed asc = E(3, p) = (p + 3) mod 26where E() stands for encryption

c = E(k, p) = (p + k) mod 26• The formula for decryption would be

p = D(k, c) = (c − k) mod 26


In a monoalphabetic cipher, our substitution characters are arandom permutation of the 26 letters of the alphabet:

plaintext letters: a b c d e f .....substitution letters: t h i j a b .....Note that there are 26! permutations of the

alphabet. That is a number larger than 4 × 1026.Pairs of adjacent characters are referred to as

digrams, andtriples of characters as trigrams.


human languages are redundant eg "th lrd s m shphrd shll nt wnt" letters are not equally commonly used in English E is by far the most common

letter followed by T,R,N,I,O,A,S

other letters like Z,J,K,Q,X are fairly rare have tables of single, double & triple

letter frequencies for various languages



In Playfair cipher, you first choose an encryption key. You then enter the letters of the key in the cells of a 5 × 5 matrix in a left to right fashion starting with the first cell at the top-left corner.


Rulesyou must insert a chosen “filler” letter (let’s say it is

‘x’) between any repeating letters in the plaintext.Two plaintext letters that fall in the same row of the

5 × 5 matrix are replaced by letters to the right of each in the row.

Two plaintext letters that fall in the same column are replaced by the letters just below them in the column.

Otherwise, for each plaintext letter in a pair, replace it with the letter that is in the same row but in the column of the other letter.


if a truly random key as long as the message is used, the cipher will be secure

called a One-Time padis unbreakable since ciphertext bears no

statistical relationship to the plaintextsince for any plaintext & any ciphertext

there exists a key mapping one to othercan only use the key once thoughproblems in generation & safe distribution of

key


Mapping is achieved permutation on plain text letters.

Transposition means rearranging the order of appearance of the elements of the plaintext.

Rail fenceRow transposition


A very simple form of [transposition cipher] is the rail fence, named for its fencelike appearance, which is the result of aligning rows of letters, then shifting them. The key is the number of rails and the order in which they are taken off.


Three rows


a more complex transpositionwrite letters of message out in rows over a

specified number of columnsthen reorder the columns according to

some key before reading off the rowsKey: 3 4 2 1 5 6 7Plaintext: a t t a c k p o s t p o n e d u n t i l t w o a m x y zCiphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ


before modern ciphers, rotor machines were most common complex ciphers in use

widely used in WW2German Enigma, Allied Hagelin, Japanese

Purpleimplemented a very complex, varying

substitution cipherused a series of cylinders, each giving

one substitution, which rotated and changed after each letter was encrypted




Steganography is the science of hiding informationOne of the oldest stego schemes was to shave the

head of a messenger and tattoo a message on the messenger's head. After the hair grows back, the messenger can be sent to the intended recipient, where the messenger's head can be shaved and the message recovered. This method is decidingly clever, patient, and very low-tech, and goes right to the heart of steganography's literal meaning of "covered writing."


Character marking – visible at an angle to bright lightInvisible ink – visible on heat or chemical is appliedpin punctures – visible in front of lightType writer correction ribbon – visible in strong lightDeliberate misspelling to mark words in the messageUse of small changes in spacing to indicate significant

letters or words in a hidden message Use of a slightly different font in a typeset message to

indicate the hidden letters Not only restricted to written forms of communication


Consider this cablegram that might have been sent by a journalist/spy from the U.S. to Europe during World War I:PRESIDENT'S EMBARGO RULING SHOULD HAVE IMMEDIATE NOTICE. GRAVE SITUATION AFFECTING INTERNATIONAL LAW. STATEMENT FORESHADOWS RUIN OF MANY NEUTRALS. YELLOW JOURNALS UNIFYING NATIONAL EXCITEMENT IMMENSELY.

The first letters of each word form the character string: PERSHINGSAILSFROMNYJUNEI.

A little imagination and some spaces yields the real message: PERSHING SAILS FROM NY JUNE I.


Template (e.g., a piece of paper with holes cut in it) or a set of preselected locations on the page to hide a message

THE MOST COMMON WORK ANIMAL IS THE HORSE. THEY CAN BE USED TO FERRY EQUIPMENT TO AND FROM WORKERS OR TO PULL A PLOW. BE CAREFUL, THOUGH, BECAUSE SOME HAVE SANK UP TO THEIR KNEES IN MUD OR SAND, SUCH AS AN INCIDENT AT THE BURLINGTON FACTORY LAST YEAR. BUT HORSES REMAIN A SIGNIFICANT FIND. ON A FARM, AN ALTERNATE WORK ANIMAL MIGHT BE A BURRO BUT THEY ARE NOT AS COMFORTABLE AS A TRANSPORT ANIMAL

Applying a template or rule as to which words to read to this message might yield the following:

HORSE FERRY SANK IN BURLINGTON FIND ALTERNATE TRANSPORT




11/21/2015 1. 2 Session Plan Overview of Classical encryption techniques: Symmetric cipher model Substitution Transposition Steganography.

Documents

possible key

cryptologynetwork security

ciphertext key info

broken network security

unconditional security

endecryptnetwork security

ciphertextchosen ciphertext

algorithm bruteforce