### Cipher

```SYMPATRIC ENCRYPTION
L.Tahani Al jehani
Introduction

Definition
 Cryptography,
a word with Greek origins, means
“secret writing”.
 It refers to the science of transferring messages to make
them secure and hard to attacks.
Introduction

Components
 Plaintext
: is the original message before being
transformed.
 Ciphertext: is the message after transformation.
 Cipher : is the encryption and decryption algorithm.
 Key: is an number (or set of numbers) that the cipher, as an
algorithm, operates on.
Terminology





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
Cryptography Categories
Cryptography Categories
In symmetric-key cryptography, the same key is used
by the sender (for encryption) and the receiver (for
decryption).
The key is shared.
Requirements

two requirements for secure use of symmetric
encryption:
a
strong encryption algorithm
 a secret key known only to sender / receiver

mathematically have:
Y = E(K, X)
X = D(K, Y)


assume encryption algorithm is known
implies a secure channel to distribute key
Cryptography Categories
In asymmetric-key cryptography, one key is used by
the sender (for encryption) and other key is used by
The key are different.
Cryptography Categories

Comparison

In the symmetric-key cryptography, both sender and
receiver use the same key for lock and unlock the
message respectively.
 In
the asymmetric-key cryptography, the sender uses a
key to lock the message and the receiver uses another
key to unlock the message.
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
Cryptanalysis


objective to recover key not just message
general approaches:
 cryptanalytic
attack
 brute-force attack

if either succeed all key use compromised
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
computationally secure

Mallory can try all possible key combinations until one works, and
the resulting data or message is understandable.

You might be asking yourself, how many combinations would an
attacker have to try? The answer to that question depends upon the
encryption algorithm or cipher used.



An algorithm is considered computationally secure if the amount
of time needed to compute all possible combinations is so large that
it cannot be done in any reasonable amount of time.
This definition, “in a reasonable amount of time,” is deliberately
vague, because the meaning of computationally secure is everchanging as the speed of a computer is everincreasing. Also, most
data does not need to be protected forever.
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! = 4  1026
2  1026 µs = 6.4  1012 years
26 characters
(permutation)
6.4  106 years
Symmetric-Key Cryptography

There are two main categories of ciphers in Symmetric-key
Cryptography :


Substitution ciphers




Monoalphabetic
Polyalphabetic
Transposition ciphers
Simple Modern Cipher:
XOR Cipher
 Rotation Cipher
 Substitution cipher: S-box
 Transposition cipher: P-box


Moder Round Ciphers:

Data Encryption Standard (DES)
Substitution ciphers

Substitution ciphers replaces one symbol with
another, it can be either:
 Monoalphabetic
 Where
a character (or symbol) in the plaintext is always
changed to the same character (or symbol) in the ciphertext
regardless of its position in the text.
 1:1
 Polyalphabetic
 Where
each occurrence of a character can have a different
substitue.
 1:N
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(k, p) = (p + k) mod (26)
p = D(k, c) = (c – k) mod (26)
Substitution ciphers

Example:
 Use
the shift cipher with key=15 to encrypt the message
“HELLO”
 Is this Cipher Polyalphabetic?
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"
Substitution ciphers

Examples#1 :
Plaintext: HELLO
 Ciphertext: KHOOR
 Is the cipher monoalphabetic?


Examples#2 :
Plaintext: HELLO
 Ciphertext: ABNZF
 Is the cipher monoalphabetic?

Transposition cipher

Transposition cipher, it reorders (permutes) symbols in a
block of symbols.

Key is a mapping between the position of the symbols in the
plaintext and cipher text.
Transposition cipher

Example :


Encrypt the message “HELLO MY DEAR,” using the key
shown in the previous figure.
Solution
 First
remove the spaces in the message.
 Then divide the text into blocks of four characters.
 Add a bogus character Z at the end of the third block.
 The result is HELL OMYD EARZ.
 Ciphertext will be  ELHLMDOYAZER.
Transposition cipher

Using same Example decrypt the message
“ELHLMDOYAZER”.

Solution

The result is HELL OMYD EARZ. After removing the bogus
character and combining the characters, we get the original
message “HELLO MY DEAR.”
Simple Modern Ciphers Vs Traditional Ciphers

The traditional ciphers are character-oriented; however,
Simple Modern Ciphers are bit-oriented.
Bit-oriented ciphers are needed because:
 Information to be encrypted is not just text, it can be
graphics, audio, and video data.
 It is provide more security.


Modern ciphers is made up of simple ciphers.
Type of Cipher




Stream Cipher
Block cipher
Both do :Combination of input plaintext and key to
produce cipher text
They differ in : How the plaintext and key and
combined
Stream cipher



Characterized by operating on one symbol at a
time
The alphabetic substitutions we have seen so far
have been stream ciphers
If the algorithm is XOR, this is a stream cipher
Stream cipher



It provides integrity. If any of the cipher text bits are
changed, it will be obvious to Bob when he decrypts the
message
There are some stream ciphers that do not propagate
errors through the entire message. that means if an
error occurs while the message is being sent from Alice
to Bob, it will only prevent that section of the message
from being decrypted properly.
It is very fast algorithm using in mobile encryption



Stream ciphers:
–They operate relatively fast since they work on
only one character at a time
–Lower error propagation since each symbol is
affected only by itself
Block Ciphers



Characterized by operating on more than one
symbol at a time
A block cipher takes a group of symbols as input,
combines them without output, and outputs a block
of cipher text
if the algorithm is XOR, this is a block cipher:
Block Ciphers





each block is processed independently, and there is
no correlation between the encrypting of one
message block and another.
repeated blocks (problem )
order of blocks ( problem )
Although the encryption method provides
confidentiality, integrity can be broken.
Solution : block ciphers use different encryption
modes.
Encryption modes.





Electronic code book (ECB)
Cipher-block chaining (CBC)
Propagating cipher-block chaining (PCBC)
Cipher feedback (CFB)
Output feedback (OFB)
XOR Cipher
An XOR operation needs two data inputs: plaintext
and a key.
 The size of the plaintext, key, and ciphertext are the
same.

Rotation Cipher
In rotation cipher, the input bits are rotated to the left
or right.
 The rotation can be either keyed or keyless.



In Keyed , the value of the key in = # of rotations.
In the keyless, the number of rotation is fixed.
NOTES in Rotation Cipher


If the length of the original stream is N, after N rotation,
we get the original message.
The decryption algorithm for the rotation cipher uses the
same key and the opposite rotation direction.
Substitution Cipher: S-box


The S-box is normally keyless.
The function that matches the input to the output is
defined either mathematically or by a table.
Transposition Cipher: P-box






Sharing the keys:
The key is shared before communication begins.
If Alice wants to communicate with Bob, but she
has never met Bob before. How can Alice and Bob
communicate securely?
They could create keys and encrypt them so no one
knows the keys.
Solution 1:
Use a trusted third party.
Sharing the keys




Sharing the keys:
Alice create a key to be used to communicate with
Bob. She will encrypt this key using a pre-shared
key that she has with Trent.
Trent will decrypt and encrypt again with a shared
key with Bob.
Problem: what if Trent is not really Trent but
another person?
Sharing the keys









Sharing the keys:
Solution 2: key agreement protocol
Alice and Bob agree to use a specific prime number (p)
and a base number (g).
Alice and Bob each choose a secret integer.
Suppose that the secrete integer choosen by Alice is a
is the secrete integer choosen by Bob is b.
Alice sends : g a mod p
Bob sends: g b mod p
Alice calculates the key = (Message bob) a mod p
Bob calculates the key = (Message Alice) b mod p
Modern Round Ciphers



The ciphers of today are called round ciphers
because they involve multiple rounds.
Each round is a complex cipher made up of simple
ciphers.
They key used in each round is a subset or variation of
the general key called the round key.
Modern Round Ciphers

There are three modern symmetric key ciphers:

Data Encryption Standard (DES).
 Triple
DES.