information security Lectures -03,04.ppt

information security lec 3 and 4

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
ciphertextonly
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 ciphertextand obtain plaintext
chosen text
select plaintext or ciphertextto 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 monoalphabeticsubstitution
cipher, maps plaintext letters to ciphertextletters
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-ciphertextletter 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
ciphertextletter
Hence, key is 26 letters long
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Plaintext: ifwewishtoreplaceletters
Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA
MonoalphabeticCipher

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
MonoalphabeticCipher 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 railfencecipher, in which the
plaintext is written in rows of n letter blocks (the number of columns n is
the key) and then the ciphertextis 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 railfencecipher:
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