Question

Kleptography

Describe substitution, Mon alphabets, and other classes, and attacks

Answer 1

Substitution technique in which letters of plaintext is replaced by letter, number or symbols.

Substitution techniques:

1) Monoalphabetic

2) polyalphabetic cipher

1) vigenere cipher

2) playfair cipher

3) hill cipher

4) one time pad

1) Monoalphabetic cipher

Monoalphabetic cipher is substitution cipher in which each character or symbol in plaintext is replaced by same character in ciphertext regardless their position.

There is one to one relationship between character in plaintext and ciphertext.

Means if letter A is replaced with C ,every time when letter A comes in plaintext is replaced with C.

Example plaintext :hello

Ciphertext : jgnnq

So monoalphabetic cipher easier to break because of the frequency of they reflect frequency of letters of plaintext.

2) polyalphabetic cipher

In polyaphabetic substitution cipher each occurrence of character has different substitute.

Relationship between character in plaintext and character in ciphertext is one to many.

Means letter a is encrypted as c in starting and at the end when letter a comes encrypted as n.

One Time pad :

In which length of the key is same as length of the plaintext.

Key is used only once.

Example plaintext :hello

Key : abcda

Ciphertext : igopp

Types of cryptography attack

1)known plaintext attack

In known plaintext attack attacker know plaintext for some part of ciphertext.

Then attacker need to decrypt rest of ciphertext using that information either by using key or some other methods.

2)choosen plaintext attack

Chosen plaintext attack is similar to known plaintext attack but here attacker choose plaintext with ciphertext and find out key.

Once attacker find out key attacker know encryption process and attacker able to decrypt message using this key information.

3)choosen ciphertext attack

In choosen ciphertext attack attacker choose decrypted portion of ciphertext.

Then compare decrypted ciphertext with plaintext in order to finding a key.

4)choosen key

In choosen key attack attacker have some knowledge about various keys used in cipher.

He knows exactly what keys have been used.

5)ciphertext only

In ciphertext only attack attacker know ciphertext of message that can be encrypted using some encryption algorithm.

Attacker aim is to find out key that can be used to decrypt other messages.