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Steganography

Steganography

mask existence of message

Cryptography

mask content of message

Kerckhoff's Principle

security of system depends on keeping the key secret, not algorithm

Transposition Cipher

letters change position, but keep identify

Andy's Dopey Transposition Cipher

write plaintext vertically in set # of rows

Substitution Cipher

letters keep position, but change identity

Monoalphabetic Substitution Cipher

1-1 mapping of symbols in plaintext to cipher text

Caesar Shift Cipher

shift alphabet a certain # of places

MSC w/ keyphrase

pick keyphrase -> remove duplicate letters/spaces -> start alphabet @ last letter of keyphrase

Frequency Analysis

statistical properties of letters

cipher

symbol to symbol substitution

code

symbol to word substitution

nomenclator

symbol to letters & words

MSC w/ homophones

one to many mapping, assign more symbols to more frequent letters to even out frequency

one to one mapping

each symbol in plaintext maps to single symbol in ciphertext

one to many mapping

symbols in plaintext can map to more than one symbol in ciphertext

many to many (polyalphabetic) mapping

symbols in plaintext map to more than one symbol in ciphertext & vice versa

Vigenere Cipher

many to many mapping using square table w/ all possible caesar shift ciphers

Babbage-Kasiski Method

breaks vigenere cipher, find common factor of distances between each repetition in ciphertext

one-time pad ciphers

key is as long as ciphertext & randomly generated

Book cipher

Use well known text to encrypt and decrypt

Binary Transposition Cipher

transpose each pair of bits (switch places)

binary substitution cipher (XOR)

line up plaintext w/ keyphrase -> if digits are the same write 0, if different write 1

Modular Arithmetic

x (mod y) = remainder of x/y

one way functions

easy to compute in forward direction, but hard to reverse

modular exponentiation

z = y^x (mod p)

Diffie-Hellman key exchange

public = y & p, secret = a & b (solves key distribution problem)

symmetric cipher

encryption key = decryption key

asymmetric key

encryption & decryption key are different

RSA

2 one way functions: product of large primes & modular exponentiation

fixed-length code

each symbol is represented by same # of bits

minimal fixed-length code

smallest possible fixed-length code (to be able to represent ALL letters of alphabet)

variable length codes

different symbols may be assigned representations w/ different lengths

prefix-free codes

there can be no case in which the entire representation for one symbol is a prefix of the representation of another

huffman coding

optimal code, tree representation

lossless compression

no info is lost during compression (reversing the process recovers info you started w/)

lossy compression

info is lost during compression (no reversing process can recover lost info)

spectral test

frequency analysis on different combos of symbols -> see whether results have distribution we would expect of random data

spectral test: level 1

distribution of single bits

spectral test: level 2

distribution of pairs of bits

spectral test: level 3

distribution of triplets of bits

checksum code

8 bit representation of sum to catch any errors

sum (mod 2^N)

equation to break up checksum if sum is > 8 bits, N = # of bits

parity bits

error detection, won't catch any two-bit errors, 8th bit = sum of 7 bits (mod 2)

luhn algorithm

credit card checksum, won't catch transposition errors between 0 & 9

ISBN codes

10 digit error detecting, 2 methods, catches all single digit & transposition errors (but can't correct)

hamming (7 ,4)

3 tests, catches & fixes all single-bit errors within a 7-bit block, d = data bit, p = parity bit

method 1 to check ISBNs

starting from RIGHT, multiply each number with their place in the sequence. Check that Y(mod 11) = 0

method 2 to check ISBNs

starting from the LEFT, multiply each number with their place in the sequence, last digit is the check sum. Check that Y(mod 11) = check digit

test 1 and 2 fail, test 3 passes

D1 is wrong

test 1 and 3 fail, test 2 passes

D2 is wrong

test 2 and 3 fail, test 1 passes

D3 is wrong

tests 1, 2 and 3 fails

D4 is wrong

Phonetics

study of speech sounds

Phonology

study of the rules governing speech sound distribution

Morphology

study of words and their components

Syntax

study of the organization of words into phrases and sentences

Semantics

study of meaning: words, phrases, sentences

Pragmatics

study of meaning in context