2. Classical Cryptosystems Classical.ppt

Cryptography and
Network Security
Classical Cryptosystems

Symmetric Encryption
 or conventional / private-key / single-key
 sender and recipient share a common key
 all classical encryption algorithms are
private-key
 was only type prior to invention of public-
key in 1970’s
 and by far most widely used

Some Basic Terminology
 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 plaintext
 cryptography - study of encryption principles/methods
 cryptanalysis (codebreaking) - study of principles/
methods of deciphering ciphertext without knowing key
 cryptology - field of both cryptography and cryptanalysis

Requirements
 two requirements for secure use of
symmetric encryption:
 a strong encryption algorithm
 a secret key known only to sender / receiver
 mathematically have:
Y = EK(X)
X = DK(Y)
 assume encryption algorithm is known
 implies a secure channel to distribute key

Cryptography
 characterize cryptographic system by:
 type of encryption operations used
• substitution / transposition / product
 number of keys used
• single-key or private / two-key or public
 way in which plaintext is processed
• block / stream

Cryptanalysis
 objective to recover key not just message
 general approaches:
 cryptanalytic attack
 brute-force attack

Cryptanalytic Attacks
 ciphertext only
 only know algorithm & ciphertext, is statistical,
know or can identify plaintext
 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

More Definitions
 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

Brute Force Search
 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

Classical Substitution
Ciphers
 where letters of plaintext are replaced by
other letters or by numbers or symbols
 or if plaintext is viewed as a sequence of
bits, then substitution involves replacing
plaintext bit patterns with ciphertext bit
patterns

Caesar Cipher
 earliest known substitution cipher
 by Julius Caesar
 first attested use in military affairs
 replaces each letter by 3rd letter on
 example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB

Caesar Cipher
 can define transformation as:
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
D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
 mathematically give each letter a number
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
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
 then have Caesar cipher as:
c = E(p) = (p + k) mod (26)
p = D(c) = (c – k) mod (26)

Cryptanalysis of Caesar
Cipher
 only have 26 possible ciphers
 A maps to A,B,..Z
 could simply try each in turn
 a brute force search
 given ciphertext, just try all shifts of letters
 do need to recognize when have plaintext
 eg. break ciphertext "GCUA VQ DTGCM"

Monoalphabetic Cipher
 rather than just shifting the alphabet
 could shuffle (jumble) the letters arbitrarily
 each plaintext letter maps to a different random
ciphertext letter
 hence key is 26 letters long
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Plaintext: ifwewishtoreplaceletters
Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA

Monoalphabetic Cipher
Security
 now have a total of 26! = 4 x 1026 keys
 with so many keys, might think is secure
 but would be !!!WRONG!!!
 problem is language characteristics

Language Redundancy and
Cryptanalysis
 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

Use in Cryptanalysis
 key concept - monoalphabetic substitution
ciphers do not change relative letter frequencies
 discovered by Arabian scientists in 9th century
 calculate letter frequencies for ciphertext
 compare counts/plots against known values
 if caesar cipher look for common peaks/troughs
 peaks at: A-E-I triple, NO pair, RST triple
 troughs at: JK, X-Z
 for monoalphabetic must identify each letter
 tables of common double/triple letters help

Example Cryptanalysis
 given ciphertext:
UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ
VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX
EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ
 count relative letter frequencies (see text)
 guess P & Z are e and t
 guess ZW is th and hence ZWP is the
 proceeding with trial and error finally get:
it was disclosed yesterday that several informal but
direct contacts have been made with political
representatives of the viet cong in moscow

Playfair Cipher
 not even the large number of keys in a
monoalphabetic cipher provides security
 one approach to improving security was to
encrypt multiple letters
 the Playfair Cipher is an example
 invented by Charles Wheatstone in 1854,
but named after his friend Baron Playfair

Playfair Key Matrix
 a 5X5 matrix of letters based on a keyword
 fill in letters of keyword (sans duplicates)
 fill rest of matrix with other letters
 eg. using the keyword MONARCHY
M O N A R
C H Y B D
E F G I/J K
L P Q S T
U V W X Z

Encrypting and Decrypting
 plaintext is encrypted two letters at a time
1. if a pair is a repeated letter, insert filler like 'X’
2. if both letters fall in the same row, replace
each with letter to right (wrapping back to start
from end)
3. if both letters fall in the same column, replace
each with the letter below it (again wrapping to
top from bottom)
4. otherwise each letter is replaced by the letter
in the same row and in the column of the other
letter of the pair

Security of Playfair Cipher
 security much improved over monoalphabetic
 since have 26 x 26 = 676 digrams
 would need a 676 entry frequency table to
analyse (verses 26 for a monoalphabetic)
 and correspondingly more ciphertext
 was widely used for many years
 eg. by US & British military in WW1
 it can be broken, given a few hundred letters
 since still has much of plaintext structure

Polyalphabetic Ciphers
 polyalphabetic substitution ciphers
 improve security using multiple cipher alphabets
 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

Vigenère Cipher
 simplest polyalphabetic substitution cipher
 effectively multiple caesar ciphers
 key is multiple letters long K = k1 k2 ... kd
 ith letter specifies ith alphabet to use
 use each alphabet in turn
 repeat from start after d letters in message
 decryption simply works in reverse

Example of Vigenère Cipher
 write the plaintext out
 write the keyword repeated above it
 use each key letter as a caesar cipher key
 encrypt the corresponding plaintext letter
 eg using keyword deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ

Security of Vigenère Ciphers
 have multiple ciphertext letters for each
plaintext letter
 hence letter frequencies are obscured
 but not totally lost
 start with letter frequencies
 see if look monoalphabetic or not
 if not, then need to determine number of
alphabets, since then can attach each

Kasiski Method
 method developed by Babbage / Kasiski
 repetitions in ciphertext give clues to period
 so find same plaintext an exact period apart
 which results in the same ciphertext
 of course, could also be random fluke
 eg repeated “VTW” in previous example
 suggests size of 3 or 9
 then attack each monoalphabetic cipher
individually using same techniques as before

Autokey Cipher
 ideally want a key as long as the message
 Vigenère proposed the autokey cipher
 with keyword is prefixed to message as key
 knowing keyword can recover the first few letters
 use these in turn on the rest of the message
 but still have frequency characteristics to attack
 eg. given key deceptive
key: deceptivewearediscoveredsav
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLA

One-Time Pad
 if a truly random key as long as the message is
used, the cipher will be secure
 called a One-Time pad
 is unbreakable since ciphertext bears no
statistical relationship to the plaintext
 since for any plaintext & any ciphertext there
exists a key mapping one to other
 can only use the key once though
 problems in generation & safe distribution of key

Transposition Ciphers
 now consider classical transposition or
permutation ciphers
 these hide the message by rearranging
the letter order
 without altering the actual letters used
 can recognise these since have the same
frequency distribution as the original text

Rail Fence cipher
 write message letters out diagonally over a
number of rows
 then read off cipher row by row
 eg. 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
 giving ciphertext
MEMATRHTGPRYETEFETEOAAT

Row Transposition Ciphers
 a more complex transposition
 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: 3 4 2 1 5 6 7
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

Product Ciphers
 ciphers using substitutions or transpositions are
not secure because of language characteristics
 hence consider using several ciphers in
succession to make harder, but:
 two substitutions make a more complex substitution
 two transpositions make more complex transposition
 but a substitution followed by a transposition makes a
new much harder cipher
 this is bridge from classical to modern ciphers

Steganography
 an alternative to encryption
 hides 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

2. Classical Cryptosystems Classical.ppt

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