The document discusses cryptography and its classification based on encryption operations, key types, and plaintext processing methods. It explains various cryptanalytic attacks and the concept of computational security in encryption schemes. Additionally, it covers monoalphabetic and transposition ciphers, their implementations, and vulnerabilities.

1. Information Security
Lecture 3
By
Usman Zia

2. Cryptography
 can characterize cryptographic system by:
 type of encryption operations used
• substitution
• transposition
• product
 number of keys used
• single-key or private
• two-key or public
 way in which plaintext is processed
• block
• stream

3. Cryptanalysis
 objective to recover key not just message
 general approaches:
 cryptanalytic attack
 brute-force attack
 if either succeed all key use compromised

4. Cryptanalytic Attacks
 ciphertext only
 only know algorithm & ciphertext, is statistical,
know or can identify plaintext
 known plaintext
 know/suspect plaintext & cipher text
 chosen plaintext
 select plaintext and obtain ciphertext
 chosen ciphertext
 select ciphertext and obtain plaintext
 chosen text
 select plaintext or ciphertext to en/decrypt

5.  An encryption scheme: 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

6.  A generalization of the Caesar cipher, called a simple
substitution cipher or monoalphabetic substitution
cipher, maps plaintext letters to ciphertext letters
according to a fixed mapping (the key).
 Example:
 p : ABCDEFGHIJKLMNOPQRSTUVWXYZ
 Fk(p) : QWERTYUIOPASDFGHJKLZXCVBNM
 Both the sender and receiver secretly share the key,
representing the plaintext-ciphertext letter mapping,
which is also called the substitution alphabet.
Simple substitution cipher

7.  Rather than just shifting the alphabet, 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
Monoalphabetic Cipher

8.  Now a total of 26! = 4 x 1026 keys
 With so many keys, might think its secure
 But it would be !!!WRONG!!!
 Problem is language characteristics
Monoalphabetic Cipher Security

9.  A transposition cipher rearranges the plaintext letters according to a
secret transformation defined by the key.
 The simplest example of this is the railfence cipher, in which the
plaintext is written in rows of n letter blocks (the number of columns n is
the key) and then the ciphertext is read in columns
 Example:
 Plaintext: TRANSPOSITIONCIPHERX
 In this example, the key is: 5
 Re-write as rows of 5-letter blocks:
 TRANS
 POSIT
 IONCI
 PHERX
 Ciphertext: TPIPROOHASNENICRSTIX
Transposition cipher

10.  Problems with the railfence cipher:
 The first and last letters of the plaintext do not move
 The key is a number that divides the total message length
 In a single columnar transposition cipher, the key is a word or phrase whose
letters, in alphabetic order, indicate the order of the columns as they are read
 Example:
 Plaintext: TRANSPOSITIONCIPHER
 Key is “SECRET”, so re-write as rows of 6-letter blocks:
 SECRET CEERST
 521436 123456
 TRANSP ARSNTP
 OSITIO ISITOO
 NCIPHE ICHPNE
 R R
 Ciphertext: AII$RSC$SIH$NTP$TONRPOE$
Transposition cipher