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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 receiver (for decryption).
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 ever-changing 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 characters (permutation)
26! = 4 1026 2 1026 µs = 6.4 1012 years 6.4 106 years
Symmetric-Key Cryptography
There are two main categories of ciphers in Symmetric-key Cryptography : Traditional Cipher:
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) Advanced Encryption Standard (AES)
Traditional Cipher
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 partyPHHW 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 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(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 advantages
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.
Advanced Encryption Standard (AES).