Cns 13f-lec03- Classical Encryption Techniques

Network Security
Classical Encryption
Techniques

2
Outline
• Introduction
• Symmetric Cipher Model
• Substitution Techniques
• Transposition Techniques
• Rotor Machines
• Steganography

Classical encryption techniques
• As opposed to modern cryptography
• Goals:
– to introduce basic concepts & terminology of
encryption
– to prepare us for studying modern
cryptography
3

Basic terminology
• Plaintext: original message to be
encrypted
• Ciphertext: the encrypted message
• Enciphering or encryption: the process of
converting plaintext into ciphertext
• Encryption algorithm: performs encryption
– Two inputs: a plaintext and a secret key
4

• Deciphering or decryption: recovering
plaintext from ciphertext
• Decryption algorithm: performs decryption
– Two inputs: ciphertext and secret key
• Secret key: same key used for encryption
and decryption
– Also referred to as a symmetric key
5
Basic terminology

• Cipher or cryptographic system : a scheme
for encryption and decryption
• Cryptography: science of studying ciphers
• Cryptanalysis: science of studying attacks
against cryptographic systems
• Cryptology: cryptography + cryptanalysis
6
Basic terminology

Ciphers
• Symmetric cipher: same key used for
encryption and decryption
– Block cipher: encrypts a block of plaintext at a
time (typically 64 or 128 bits)
– Stream cipher: encrypts data one bit or one byte
at a time
• Asymmetric cipher: different keys used for
encryption and decryption
7

8
Symmetric Cipher ModelSymmetric Cipher Model
• A symmetric encryption scheme has five
ingredients:
– Plaintext
– Encryption algorithm
– Secret Key
– Ciphertext
– Decryption algorithm
• Security depends on the secrecy of the key, not
the secrecy of the algorithm

Symmetric Encryption
• or conventional / secret-key / single-key
• sender and recipient share a common key
• all classical encryption algorithms are
symmetric
• The only type of ciphers prior to the
invention of asymmetric-key ciphers in
1970’s
• by far most widely used
10

• Mathematically:
Y = EK(X) or Y = E(K, X)
X = DK(Y) or X = D(K, Y)
• X = plaintext
• Y = ciphertext
• K = secret key
• E = encryption algorithm
• D = decryption algorithm
• Both E and D are known to public
11

• two requirements for secure use of
symmetric encryption:
– a strong encryption algorithm
– a secret key known only to sender /
receiver
• assume encryption algorithm is known
• implies a secure channel to distribute
key

13
Model of ConventionalModel of Conventional
CryptosystemCryptosystem

Cryptography
• Cryptographic systems are characterized
along three independent dimensions:
– type of encryption operations used
• substitution
• Transposition
• product
– number of keys used
• single-key or private
• two-key or public

Cryptography
– way in which plaintext is processed
• block
• stream

Cryptanalysis
• Objective: to recover the plaintext of a ciphertext
or, more typically, to recover the secret key.
• Kerkhoff’s principle: the adversary knows all
details about a cryptosystem except the
secret key.
• Two general approaches:
– brute-force attack
– non-brute-force attack (cryptanalytic attack)
16

Brute-Force Attack
• Try every key to decipher the ciphertext.
• On average, need to try half of all possible keys
• Time needed proportional to size of key space
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
17

Cryptanalytic Attacks
Attack Type Knowledge Known to Cryptanalyst
Ciphertext only • Encryption algorithm
• Ciphertext to be decoded
Known Plaintext • Encryption algorithm
• One or more plaintext-ciphertext pairs formed with the same
secret key
Chosen Plaintext • Encryption algorithm
• Plaintext message chosen by cryptanalyst, together with its
corresponding ciphertext generated with the same secret key
Chosen Ciphertext • Encryption algorithm
• Purported ciphertext chosen by cryptanalyst, together with
its corresponding decrypted plaintext generated with the
secret key
Chosen text • Encryption algorithm
• Plaintext message chosen by cryptanalyst, together with its
corresponding ciphertext generated with the secret key
• Purported ciphertext chosen by cryptanalyst, together with
its corresponding decrypted plaintext generated with the
secret key

19
Cryptanalytic Attacks
• May be classified by how much
information needed by the attacker:
– Ciphertext-only attack
– Known-plaintext attack
– Chosen-plaintext attack
– Chosen-ciphertext attack
– Chosen text

20
Ciphertext-only attack
• Given: a ciphertext c
• Q: what is the plaintext m?
• An encryption scheme is completely
insecure if it cannot resist ciphertext-only
attacks.

21
Known-plaintext attack
• Given: (m1,c1), (m2,c2), …, (mk,ck) and a new
ciphertext c.
• Q: what is the plaintext of c?
• Q: what is the secret key in use?

22
Chosen-plaintext attack
• Given: (m1,c1), (m2,c2), …, (mk,ck), where
m1,m2, …, mk are chosen by the adversary;
and a new ciphertext c.
• Q: what is the plaintext of c, or what is the
secret key?

Computational Security
• An encryption scheme is computationally
secure if
– The cost of breaking the cipher exceeds the
value of information
– The time required to break the cipher exceeds
the lifetime of information

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
• All the ciphers we have examined are not
unconditionally secure.

Classical Ciphers
• Plaintext is viewed as a sequence of
elements (e.g., bits or characters)
• Substitution cipher: replacing each element
of the plaintext with another element.
• Transposition (or permutation) cipher:
rearranging the order of the elements of the
plaintext.
• Product cipher: using multiple stages of
substitutions and transpositions
25

26
Substitution Techniques
• Caeser Cipher
• Monoalphabetic Ciphers
• Playfair Cipher
• Polyalphabetic Ciphers
• One-Time PAD

Caesar Cipher
• Earliest known substitution cipher
• Invented by Julius Caesar
• Each letter is replaced by the letter three
positions further down the alphabet.
• Plain: 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
Cipher: 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
• Example: ohio state  RKLR VWDWH
27

Caesar Cipher
• Mathematically, map letters to numbers:
a, b, c, ..., x, y, z
0, 1, 2, ..., 23, 24, 25
• Then the general Caesar cipher is:
c = EK(p) = (p + k) mod 26
p = DK(c) = (c – k) mod 26
• Can be generalized with any alphabet.
28

Cryptanalysis of Caesar Cipher
• Key space: {0, 1, ..., 25}
• Vulnerable to brute-force attacks.
• E.g., break ciphertext "UNOU YZGZK“
• Need to recognize it when have the
plaintext
29

Monoalphabetic Substitution Cipher
• Shuffle the letters and map each plaintext letter to a
different random ciphertext letter:
Plain letters: abcdefghijklmnopqrstuvwxyz
Cipher letters: DKVQFIBJWPESCXHTMYAUOLRGZN
Plaintext: ifwewishtoreplaceletters
Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA
• What does a key look like?
30

Monoalphabetic Cipher Security
• Now we have a total of 26! keys.
• With so many keys, it is secure against
brute-force attacks.
• But not secure against some cryptanalytic
attacks.
• Problem is language characteristics.
31

Language Statistics and Cryptanalysis
• Human languages are not random.
• Letters are not equally frequently 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.
• There are tables of single, double & triple letter
frequencies for various languages
32

Statistics for double & triple letters
• Double letters:
th he an in er re es on, …
• Triple letters:
the and ent ion tio for nde, …
34

Use in Cryptanalysis
• Key concept: monoalphabetic substitution does
not change relative letter frequencies
• To attack, we
– calculate letter frequencies for ciphertext
– compare this distribution against the known
one
35

Example Cryptanalysis
• Given ciphertext:
UZQSOVUOHXMOPVGPOZPEVSGZWSZOPFPESXUDBMETSXAIZ
VUEPHZHMDZSHZOWSFPAPPDTSVPQUZWYMXUZUHSX
EPYEPOPDZSZUFPOMBZWPFUPZHMDJUDTMOHMQ
• Count relative letter frequencies (see next page)
• Guess {P, Z} = {e, t}
• Of double letters, ZW has highest frequency, so
guess ZW = th and hence ZWP = 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
36

Letter frequencies 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
37

Playfair Cipher
• Not even the large number of keys in a
monoalphabetic cipher provides security.•
• One approach to improving security is to
encrypt multiple letters at a time.
• The Playfair Cipher is the best known
such cipher.
• Invented by Charles Wheatstone in 1854,
but named after his friend Baron Playfair.
38

Playfair Key Matrix
• Use a 5 x 5 matrix.
• Fill in letters of the key (w/o duplicates).
• Fill the rest of matrix with other letters.
• E.g., key = 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
39

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 the letter to its right (circularly).
3. If both letters fall in the same column, replace
each with the the letter below it (circularly).
4. Otherwise, each letter is replaced by the letter
in the same row but in the column of the other
letter of the pair.
40

Security of Playfair Cipher
• Equivalent to a monoalphabetic cipher with an
alphabet of 26 x 26 = 676 characters.
• Security is much improved over the simple
monoalphabetic cipher.
• Was widely used for many decades
– eg. by US & British military in WW1 and early WW2
• Once thought to be unbreakable.
• Actually, it can be broken, because it still leaves
some structure of plaintext intact.
41

Polyalphabetic Substitution Ciphers
• A sequence of monoalphabetic ciphers (M1, M2,
M3, ..., Mk) is used in turn to encrypt letters.
• A key determines which sequence of ciphers to
use.
• Each plaintext letter has multiple corresponding
ciphertext letters.
• This makes cryptanalysis harder since the letter
frequency distribution will be flatter.
42

Vigenère Cipher
• Simplest polyalphabetic substitution cipher
• Consider the set of all Caesar ciphers:
{ Ca, Cb, Cc, ..., Cz }
• Key: e.g. security
• Encrypt each letter using Cs, Ce, Cc, Cu,Cr,
Ci, Ct, Cy in turn.
• Repeat from start after Cy.
• Decryption simply works in reverse.
43

Example of Vigenère Cipher
• Keyword: deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext: ZICVTWQNGRZGVTWAVZHCQYGLMGJ
44

Security of Vigenère Ciphers
• There are multiple (how many?) ciphertext letters
corresponding to each plaintext letter.
• So, letter frequencies are obscured but not totally lost.
• To break Vigenere cipher:
1. Try to guess the key length. How?
2. If key length is N, the cipher consists of N Caesar
ciphers. Plaintext letters at positions k, N+k, 2N+k,
3N+k, etc., are encoded by the same cipher.
3. Attack each individual cipher as before.
45

Guessing the Key Length
• Main idea: Plaintext words separated by multiples
of the key length are encoded in the same way.
• In our example, if plaintext = “…thexxxxxxthe…”
then “the” will be encrypted to the same ciphertext
words.
• So look at the ciphertext for repeated patterns.
• E.g. repeated “VTW” in the previous example
suggests a key length of 3 or 9:
ciphertext: ZICVTWQNGRZGVTWAVZHCQYGLMGJ
• Of course, the repetition could be a random fluke.
46

Transposition Ciphers
• Also called permutation ciphers.
• Shuffle the plaintext, without altering the
actual letters used.
• Example: Row Transposition Ciphers
48

Row Transposition Ciphers
• Plaintext is written row by row in a rectangle.
• Ciphertext: write out the columns in an order
specified by a key.
Key: 3 4 2 1 5 6 7
Plaintext:
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
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
49

Product Ciphers
• Uses a sequence of substitutions and
transpositions
– Harder to break than just substitutions or
transpositions
• This is a bridge from classical to modern ciphers.
50

Rotor Cipher Machines
• Before modern ciphers, rotor machines were most common
complex ciphers in use.
• Widely used in WW2.
• Used a series of rotating cylinders.
• Implemented a polyalphabetic substitution cipher of period K.
• With 3 cylinders, K = 263
=17,576.
• With 5 cylinders, K = 265
=12 x 106
.
• What is a key?
– If the adversary has a machine
– If the adversary doesn’t have a machine
52

Steganography
• Hide a message in another message.
• E.g., hide your plaintext in a graphic image
– Each pixel has 3 bytes specifying the RGB color
– The least significant bits of pixels can be
changed w/o greatly affecting the image quality
– So can hide messages in these LSBs
• Advantage: hiding existence of messages
• Drawback: high overhead
57

58
• Take a 640x480 (=30,7200) pixel image.
• Using only 1 LSB, can hide 115,200 characters
• Using 4 LSBs, can hide 460,800 characters.

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
59

Cns 13f-lec03- Classical Encryption Techniques

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Cns 13f-lec03- Classical Encryption Techniques (20)

More from babak danyal (20)

Recently uploaded (20)

Cns 13f-lec03- Classical Encryption Techniques

Editor's Notes