CS-09 – Data Communications and Networks

 

 

AGENDA (Session 13)   

 

 

 

Problem Solutions

 

Application layer

 

Cryptography

 

Substitution/Transposition/One-time Pad Ciphers

 

Public Key Cryptography

 

 

 

                            

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Application Layer & Cryptography

·        Real Useful functionality for user is here.

·        Security Related, DNS Related, Network Management related protocols.

·        Applications like Email, Browsers, Multimedia Interfaces.

 

Security Concerns:

 

·        Secrecy: Stop Un-authorised listening.

·        Authentication: Open for authorized users

·        Non-repudiation: Source Identity Establishment

·        Integrity: Data is same as it was sent.

 

Cryptography:

 

·        Text : Encryption with encryption key = Cipher.

·        Cipher: Decryption with decryption key = Text.

·        Passive Intruder just listens.

·        Active Intruder alters message.

 

C= Ek (P)  (Encryption with ‘K’ key)

P= Dk (C)  (Decryption with ‘K’ key)

 

P = Dk (Ek (P))

 

 

 

 

 

 

 

 

 

 

 

 

 

Ciphers

Substitution Cipher:

·        Letter or group of letter are replaced by another.

·        Order of letter is preserved.

·        Fixed offset Shifting (Caesar Cipher)

a+3 = D

b+3 = E

z+3 = C               k (key) = offset

 

·        Alphabet Mapping.

a  b  c  d             …………………   z

N O B  E     ………………………. U        k = 26 letter string

 

·        Difficult to break by random testing.

·        Easy to break by knowing frequency of alphabets in English (e,t,o,a,n,I – th, in, er, re, an – the, ing, and, ion etc.).

 

Transposition Cipher:

·        Order of letter is not preserved.

·        Non repeated alphabet word is chosen as a key.

·        Lowest alphabet is given lowest column number.

·        Lowest column written as first letter(s) of cipher.

 

C       A       T (key)

2       1       3  (columns)

u       p       t   (plain text)    

e       c      

 

CIPHER = PCUET

 

·        To break this type of Cipher type of cipher, alphabet letter frequency, suspected letter combination help.

 

 

 

 

Ciphers contd.    -     OTP and Public Key methods

 

One time Pads:

 

·        Random Bit stream chosen as key.

·        Bit pattern of plain text is bit by bit EX-Ored with key bit pattern to get the ciphered pattern.

·        This is impossible to break as bit pattern uniformity or probability of finding regular patterns is low.

·        Since both sender and receiver as supposed to have the key in written form, spying is a big threat.

·        Often CD or other media can be used to store bit patterns that can be used as keys.

 

Public key Cryptography:

·        Different keys for encryption and decryption.

·        Encryption key is made public.

·        Decryption key is private.

·        Encryption with public key of user ‘X’ allows decryption by using private key of ‘X’ only.

·        RSA is a popular public/private key making algorithm.