Network systems security by mort anvari lecture3

Cryptography Network Systems Security Mort Anvari Cryptography  Can be characterized by  type of encryption operations used   number of keys used   substitution / transposition / product single-key or shared / two-key or public way in which plaintext is processed  block / stream 8/31/2004 Security of Cryptography  Unconditional security   no matter how much computer power 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 8/31/2004 Cryptographic Tools To Be Used  Shared keys  Public and private keys  Hashing functions and message digest  Nonces 8/31/2004 Symmetric Encryption  Sender and receiver share a common key  All classical encryption algorithms belong to this type  Was only type prior to invention of public-key in 1970’s 8/31/2004 Basic Terminology          plaintext - the original message ciphertext - the 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) – restoring plaintext from ciphertext cryptography - study of encryption principles/methods cryptanalysis (codebreaking) - the study of principles/ methods of deciphering ciphertext without knowing key cryptology - the field of both cryptography and cryptanalysis 8/31/2004 Types of Cryptanalytic Attacks      Ciphertext only  only know algorithm, ciphertext, and statistics  can identify plaintext Known plaintext  know/suspect plaintext and ciphertext Chosen plaintext  select plaintext and obtain ciphertext Chosen ciphertext  select ciphertext and obtain plaintext Chosen text  select either plaintext or ciphertext to encrypt or decrypt 8/31/2004 Symmetric Cipher Model 8/31/2004 Requirements  Two requirements for secure use of symmetric encryption     a strong encryption algorithm a secret key K known only to sender and receiver Y = EK(X) X = DK(Y) Assume encryption algorithm is known Imply a secure channel used to distribute key 8/31/2004 Classical Substitution Ciphers  Letters of plaintext are replaced by other letters, by numbers, or by symbols  If plaintext is viewed as a sequence of bits, then substitution involves replacing plaintext bit patterns with ciphertext bit patterns 8/31/2004 10 Encrypting and Decrypting  plaintext encrypted two letters at a time if a pair is a repeated letter, insert a filler like “X” eg “balloon” encrypts as “ba lx lo on” if both letters fall in the same row, replace each with letter to right (wrapping back to start from end) eg “ar” encrypts as “RM” if both letters fall in the same column, replace each with the letter below it (again wrapping to top from bottom) eg “mu” encrypts to “CM” otherwise each letter is replaced by the one in its row in the column of the other letter of the pair eg “hs” encrypts to “BP”, and “ea” to “IM” or “JM” (as desired) 8/31/2004 22 Security of Playfair Cipher      Security much improved over monoalphabetic Have 26 x 26 = 676 digrams Would need a 676 entry frequency table to analyze (verses 26 for a monoalphabetic) Also need correspondingly more ciphertexts Can still be broken since still has much of plaintext structure 8/31/2004 23 Polyalphabetic Ciphers      Use multiple cipher alphabets to improve security Make cryptanalysis harder with more alphabets to guess and flatter frequency distribution Use a key to select which alphabet is used for each letter of the message Use each alphabet in turn Repeat from start after end of key is reached 8/31/2004 24 Vigenère Cipher      Simplest polyalphabetic substitution cipher which effectively multiply Caesar ciphers Key is multiple letters long K = k1 k2 kd ith letter specifies ith alphabet to use Use each key letter as a Caesar cipher key Eg using keyword deceptive key: deceptivedeceptivedeceptive plaintext: wearediscoveredsaveyourself ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ 8/31/2004 25 Security of Vigenère Ciphers      Have multiple ciphertext letters for each plaintext letter: obscure letter frequencies a bit Start with letter frequencies  see if look monoalphabetic or not  if not, need to determine number of alphabets Repetitions in ciphertext give clues to period Find same plaintext an exact period apart which results in the same ciphertext (could also be random fluke) Eg repeated “VTW” in previous example suggests size of or 8/31/2004 26 Autokey Cipher     To eliminate periodic nature of keyword, prefix keyword to message as key Knowing keyword can recover the first few letters Use these in turn on the rest of the message Eg given key deceptive key: deceptivewearediscoveredsav plaintext: wearediscoveredsaveyourself ciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLA  Still have frequency characteristics to attack 8/31/2004 27 One-Time Pad   If use a truly random key as long as plaintext, cipher will be secure Unbreakable     ciphertext bears no statistical relationship to the plaintext for any plaintext and any ciphertext there exists a key mapping one to other Can only use the key once Problems   overhead of making large number pf random keys safe distribution of key 8/31/2004 28 Transposition Ciphers  Hide message by rearranging order of letters  Without altering the actual letters used  Can recognize these since they have the same frequency distribution as the original text 8/31/2004 29 Rail Fence Cipher    Write message letters out diagonally over a number of rows Then read off cipher row by row For example, write message out as m e m a t r h t g p r y e t e f e t e o a a t and get ciphertext as MEMATRHTGPRYETEFETEOAAT 8/31/2004 30 Row Transposition Ciphers    A more complex scheme Write letters of message out in rows over a specified number of columns Then reorder the columns according to some key before reading off the rows Key: Plaintext: 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 z Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ 8/31/2004 31 Product Ciphers    Ciphers using substitutions or transpositions are not secure because of language characteristics Hence consider using several ciphers in succession to make it harder to break  two substitutions make a more complex substitution, but still a substitution  two transpositions make a more complex transposition, but still a transposition  but a substitution followed by a transposition makes a new much harder cipher Significance: bridge from classical to modern ciphers 8/31/2004 32 Rotor Machines   Before modern ciphers, rotor machines were most common product cipher Were widely used in WW2     German Enigma, Allied Hagelin, Japanese Purple Implement a very complex, varying substitution cipher Use a series of cylinders, each giving one substitution, which rotate and change after each letter was encrypted With cylinders, have 263=17576 alphabets 8/31/2004 33 An Example of Rotor Machine 8/31/2004 34 Steganography   An alternative to encryption Hide existence of message     using only a subset of letters/words in a longer message marked in some way using invisible ink hiding in LSB in graphic image or sound file Has drawbacks  high overhead to hide relatively few info bits 8/31/2004 35 Next Class  Block ciphers  Modern symmetric encryption standard 8/31/2004 36 ... key 8/31/2004 Classical Substitution Ciphers  Letters of plaintext are replaced by other letters, by numbers, or by symbols  If plaintext is viewed as a sequence of bits, then substitution involves... replaced by the one in its row in the column of the other letter of the pair eg “hs” encrypts to “BP”, and “ea” to “IM” or “JM” (as desired) 8/31/2004 22 Security of Playfair Cipher      Security. .. Cipher      Earliest known substitution cipher Invented by Julius Caesar First attested use in military affairs Replace each letter by letter three places down the alphabet For example, meet