ICOM6045 Fundamentals of ECommerce Security
Topic 1 Cryptography
Definition
 Process of transforming information to make it unreadable to anyone except those possessing the key
Purpose
 Data confidentiality
Transpositions/Permutations
 An encryption in which the letters of the message are rearranged
 Function
 Try to break established patterns
 Example
 Columnar transposition
 Rearrangement of the characters of the plaintext into columns
 Based on characteristic patterns of pairs of adjacent letters, called digrams
 Analysis
 Compute the letter frequencies
 Break the text into columns by compare a block of ciphertext characters against characters successively farther away in the ciphertext.

 Do common digrams appear.

 Do most of the digram look reasonable
 Compute the letter frequencies
 Columnar transposition
 Complexity
 No additional work
 Require storage for all characters of the message
 Not good for long message
 Alternative
 Permute the characters of the plaintext with a fixed period d
Confusion
 Cipher that makes relationship between the plaintext/key pair and the ciphertext as complex as possible
 Good confusion
 polyalphabetic substitution with a long key
 Bad confusion
 Caesar cipher
Diffusion
 Cipher that spreads the information from the plaintext over the entire ciphertext
 Change in the plaintext should affect many parts of the ciphertext
 Good diffusion
 DES
 Transposition cipher
 Bad diffusion
 Substitutin cipher
Cryptanalysis
 Index of coincidence (A tool to rate how wella particular distribution matches the distribution of letter in English)
 Procedure
 Measure of roughness(variance)
 If the distribution is perfectly flat
 Examine
 Is it encrypted
 How is it encrypted
 What is the key
Types
Symmetric Key Encryption
 Procedure (Single key)
 Original message
 Key > Encryption algorithm
 Encrypted message
 Encrypted message sent over Internet
 Encrypted message arrives destination
 Key > Decryption algorithm
 Original message
 Algorithms
 DES(Data Encryption Standard)
 Most commonly used block cipher
 Purpose
 Facilitate hardware implementation
 Form
 A block cipher with 56bit key (64bit including parity bits)
 “Feistel” network structure
 AES(Advanced Encryption Standard)
 RC4
 DES(Data Encryption Standard)
 Stream cipher
 Definition
 Convert one symbol of plaintext immediately into a symbol of ciphertext
 Advantage
 Speed of transformation
 Low error propagation
 Disadvantage
 Low diffusion
 Possible for malicious insertions and modifications
 Definition
 Block cipher
 Definition
 Encrypt a group of plaintext symbol as one block
 Advantage
 Diffusion
 Immunity to insertion
 Disadvantage
 Slowness of encryption
 Error propagation
 Definition
 Kasiski method
 Search for repeated sequence of characters
 Example
 3 occurrences of the 11character sequence
 Distance between first 2 sequence = 141 90 = 51
 Distance between second 2 sequences = 213  141 = 72
 The common divisor between 51 and 72 is 3
 Estimated key length is 3
 “Perfect” substitution cipher
 Definition
 Many alphabets for an unrecognizable distribution
 No apparent pattern for the choice of an alphabet at a particular point
 Function
 Confuse the Kasiski method
 Index of coincidence would be close to 0.038
 Definition
 Application
 Caesar cipher
 Definition
 The message is enciphered with a 27symbol alphabet (A>Z) and the blank, the blank is translated to itself
 Permutation
 Each letter is translated to a fixed number of letters after it in the alphabet
 The “real” Caessar cipher by Julius Caesar used a shift of 3
 Definition
 Monoalphabetic substitutions
 Definition
 The alphabet is scrambled, and each plaintext letter maps to a unique ciphertext letter
 Permutation
 A permutation is a recording of the elements of a series
 A permutation can be a function
 Some permutations can’t be represented as simple equation
 Weakness
 Frequency distribution
 Definition
 Polyalphabetic substitutions
 Definition
 Combine distributions that are high with ones that are low
 Analysis
 Use Kasiski method to predict likely numbers of enciphering alphabets
 If no numbers emerge fairly regularly, may not a polyalphabetic substitution
 Compute the index of coincidence to validate the predictions from step 1
 When step 1 and 3 indicate a promising value, separate the ciphertext into appropriate subsets and independently compute index of coincidence of each subset
 Example
 Rotor Machines
 Definition
 Vigenere cipher
 Definition
 Vigenere tableau
 A collection of 26 permutations
 Written in a 26*26 matrix
 Vigenere tableau
 Permutation
 Use a key (keyword) > select particular permutaion
 Definition
 Onetime pad
 Definition
 Based on a large nonrepeating set of keys (written on paper and glued together into a pad)
 Procedure
 Sender writes key one time above the letters of the plaintext
 Encipher the plaintext with a chart like Vigenere tableau
 Sender destroys the key
 Receiver takes the appropriate number of keys
 Decipher the message
 Example
 Vernam cipher
 Involves an arbitrarily long nonrepeating sequence of numbers that are combined with the plaintext
 Possible attack
 Random number generator
 Vernam cipher
 Definition
 Caesar cipher
Public Key Encryption
 Procedure (Everyone has 2 keys)
 Original message
 Encryption key > Encryption algorithm
 Plaintext < Encryption
 The original form of a message
 Ciphertext < Decryption
 The encrypted form a message
 Original plaintext
 Plaintext < Encryption
 Encrypted message
 Encrypted message sent over Internet
 Encrypted message arrives destination
 Decryption key > Decryption algorithm
 Original message
 Algorithms
 RSA