tcn - lecture 6
TRANSCRIPT
-
8/7/2019 TCN - Lecture 6
1/34
CLASSICAL CIPHERS
By
Mr. Fasee UllahCUSIT
Peshawar
-
8/7/2019 TCN - Lecture 6
2/34
Agenda
Types of classical ciphers
Asymmetric cryptography
Application of Asymmetric encryption
-
8/7/2019 TCN - Lecture 6
3/34
Symmetric Cipher Model
A symmetric encryption scheme has five ingredients
Plaintext: This is the original message or data
Encryption algorithm: The encryption algorithm performs
various substitutions and transformations on theplaintext
Secret key: The secret key is also input to the encryptionalgorithm
The algorithm will produce a different output
depending on the specific key being used at the time The exact substitutions and transformations performed
by the algorithm depend on the key
-
8/7/2019 TCN - Lecture 6
4/34
Contd
Ciphertext: This is the scrambled message produced
as output
It depends on the plaintext and the secret key and,
as it stands, is unintelligible
Decryption algorithm: This is essentially the
encryption algorithm run in reverse
It takes the ciphertext and the secret key andproduces the original plaintext
-
8/7/2019 TCN - Lecture 6
5/34
Classical Substitution Ciphers
Substitution techniques use classical encryptiontechnique
A study of these techniques enables us to explain the
basic approaches to symmetric encryption used today Two basic block of encryption techniques are
substitution
Transposition
A substitution technique is one in which the letters ofplaintext are replaced by other letters or by numbersor symbols
-
8/7/2019 TCN - Lecture 6
6/34
Caesar Cipher
earliest known substitution cipher
by Julius Caesar
first attested use in military affairs
replaces each letter by 3rd letter on
meet me after the toga party
-
8/7/2019 TCN - Lecture 6
7/34
Caesar Cipher
example:Plain text: meet me after the toga
party
Cipher Text: PHHWPH DIWHUWKHWRJDSDUWB
Note: Plaintext always in lowercase;
ciphertext is in uppercase;
key values are in lowercase
-
8/7/2019 TCN - Lecture 6
8/34
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 JK L M N O P Q R S T U V W X Y Z A B C
mathematically give each letter a numbera 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)
-
8/7/2019 TCN - Lecture 6
9/34
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"
-
8/7/2019 TCN - Lecture 6
10/34
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
-
8/7/2019 TCN - Lecture 6
11/34
Monoalphabetic Cipher Security
now have a total of 26! = 4 x 1026 keys
with so many keys, might think is secure to eliminate
brute-force techniques
but would be !!!WRONG!!!
problem is language characteristics
-
8/7/2019 TCN - Lecture 6
12/34
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 letterfrequencies for various languages
-
8/7/2019 TCN - Lecture 6
13/34
English Letter Frequencies
-
8/7/2019 TCN - Lecture 6
14/34
Relative Frequencies of letters in
ciphertext
P 13.33 H 5.83 F 3.33 B 1.67 C 0.00
Z 11.67 D 5.00 W 3.33 G 1.67 K 0.00
S 8.33 E 5.00 Q 2.50 Y 1.67 L 0.00
U 8.33 V 4.17 T 2.50 I 0.83 N 0.00
O 7.50 X 4.17 A 1.67 J 0.83 R 0.00
M 6.67
-
8/7/2019 TCN - Lecture 6
15/34
Example Cryptanalysis
given ciphertext:UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ
VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX
EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ
count relative letter frequencies
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 butdirect contacts have been made with political
representatives of the viet cong in moscow
-
8/7/2019 TCN - Lecture 6
16/34
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
-
8/7/2019 TCN - Lecture 6
17/34
Playfair Key Matrix
a 5X5 matrix of letters based on a keyword
fill in letters of keyword
fill rest of matrix with other letters eg. using the keyword MONARCHY
MM OO NN AA RR
CC HH YY BB DD
EE FF GG I/JI/J KK
LL PP QQ SS TT
UU VV WW XX ZZ
-
8/7/2019 TCN - Lecture 6
18/34
Encrypting and Decrypting
plaintext is encrypted two letters at a time
wrap from right side back to left, or from bottom backto top.
1. if a pair is a repeated letter, insert a filler like 'X', eg."balloon" encrypts as "ba lx lo on"
2. if both letters fall in the same row, replace each with letterto right eg. ar" encrypts as "RM"
3. if both letters fall in the same column, replace each withthe letter below it (again wrapping to top from bottom), eg.mu" encrypts to "CM"
4. otherwise each letter is replaced by the one in its row inthe column of the other letter of the pair, eg. hs" encryptsto "BP", and ea" to "IM" or "JM" (as desired)
-
8/7/2019 TCN - Lecture 6
19/34
Security of Playfair Cipher
security much improved over monoalphabetic
since have 26 x 26 = 676 digrams
was widely used for many years eg. by US & British military in WW1
-
8/7/2019 TCN - Lecture 6
20/34
Polyalphabetic Ciphers
polyalphabetic substitution ciphers
improve security using multiple cipher
alphabets make cryptanalysis harder with more
alphabets to guess
use a key to select which alphabet is used foreach letter of the message
repeat from start after end of key is reached
-
8/7/2019 TCN - Lecture 6
21/34
Vigenre Cipher
simplest polyalphabetic substitution cipher
key is multiple letters long K = k1 k2 ...
i
th
letter specifies i
th
alphabet to use use each alphabet in turn
-
8/7/2019 TCN - Lecture 6
22/34
Example of Vigenre Cipher
write the plaintext out
write the keyword repeated above it
eg using keyword deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
-
8/7/2019 TCN - Lecture 6
23/34
Security of Vigenre Ciphers
have multiple ciphertext letters for each plaintext
letter
hence letter frequencies are obscured
but not totally lost
-
8/7/2019 TCN - Lecture 6
24/34
One-Time Pad
Each new message requires a new key of the same length as thenew message
Such a technique called One Time Pad
is unbreakable since ciphertext bears no statistical relationship to
the plaintext can only use the key once though
The one-time pad offers complete security but, in practice, has twofundamental difficulties:
1. There is the practical problem of making large quantities of randomkeys
2. And the problem of key distribution and protection, where forevery message to be sent, a key of equal length is needed byboth sender and receiver
-
8/7/2019 TCN - Lecture 6
25/34
Transposition Ciphers
now consider classical transposition or permutation
ciphers
these hide the message by rearranging the letter
order
without altering the actual letters used
-
8/7/2019 TCN - Lecture 6
26/34
Rail Fence cipher
write message letters out diagonally over a numberof 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
-
8/7/2019 TCN - Lecture 6
27/34
Row Transposition Ciphers
a more complex transposition
write letters of message out in rows over a specifiednumber of columns
then reorder the columns according to some keybefore reading off the rowsKey: 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 ilt
w o a m x y z
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
-
8/7/2019 TCN - Lecture 6
28/34
Product Ciphers
ciphers using substitutions or transpositions are notsecure 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 newmuch harder cipher
this is bridge from classical to modern ciphers
-
8/7/2019 TCN - Lecture 6
29/34
Rotor Machines
before modern ciphers, rotor machines were mostcommon complex ciphers in use
widely used in WW2 German Enigma, Allied Hagelin, Japanese Purple
implemented a very complex, varyingsubstitution cipher
used a series of cylinders, each giving one
substitution, which rotated and changed aftereach letter was encrypted with 3 cylinders have 263=17576 alphabets
-
8/7/2019 TCN - Lecture 6
30/34
An Example of Rotor Machine
-
8/7/2019 TCN - Lecture 6
31/34
Hagelin Rotor Machine
-
8/7/2019 TCN - Lecture 6
32/34
-
8/7/2019 TCN - Lecture 6
33/34
Summary
have considered:
classical cipher techniques and terminology
monoalphabetic substitution ciphers
cryptanalysis using letter frequencies
Playfair cipher
polyalphabetic ciphers
transposition ciphers product ciphers and rotor machines
stenography
-
8/7/2019 TCN - Lecture 6
34/34
Assignment 20/09/2010
Explain each cipher with examples:
Hill Cipher
Kasiski cipher
Autokey Cipher
Application of Symmetric encryption