Chapter 8
Network Security
What is network security?
Confidentiality: only sender, intended receiver
should “understand” message contents
sender encrypts message
receiver decrypts message
Authentication: sender, receiver want to confirm
identity of each other
Message Integrity: sender, receiver want to ensure
message not altered (in transit, or afterwards)
without detection
Access and Availability: services must be
accessible and available to users
Security Threats and Attacks
eavesdrop: intercept messages
actively insert messages into connection
impersonation: can fake (spoof) source address
in packet (or any field in packet)
hijacking: “take over” ongoing connection by
removing sender or receiver, inserting himself in
place
denial of service: prevent service from being
used by others (e.g., by overloading resources)
cryptographic Tools
cryptographic algorithms important
element in security services
review various types of elements
symmetric encryption
public-key (asymmetric) encryption
digital signatures and key management
secure hash functions
Symmetric Encryption
or conventional / secret-key / single-key
sender and recipient share a common key
all classical encryption algorithms are private-key
was the only type of cryptography, prior to
invention of public-key in 1970’s
Basic Terminology
plaintext - the original message
ciphertext - the coded message
cipher - algorithm for transforming plaintext to
ciphertext
key - info used in cipher known only to sender/receiver
encipher (encrypt) - converting plaintext to ciphertext
decipher (decrypt) - recovering ciphertext from
plaintext
cryptography - study of encryption principles/methods
cryptanalysis (codebreaking) - the study of principles/
methods of deciphering ciphertext without knowing key
cryptology - the field of both cryptography and
cryptanalysis
Symmetric Cipher Model
Requirements
Two requirements for secure use of
symmetric encryption:
a strong encryption algorithm
a secret key known only to sender / receiver
Y = EK(X)
X = DK(Y)
assume encryption algorithm is known
implies a secure channel to distribute
key
Cryptography
• can be characterized by:
type of encryption operations used
substitution / transposition / product
number of keys used
single-key or secret-key vs two-key or
public-key
way in which plaintext is processed
block / stream
Classical Substitution Ciphers
where letters of plaintext are replaced
by other letters or by numbers or
symbols
or if plaintext is viewed as a sequence
of bits, then substitution involves
replacing plaintext bit patterns with
ciphertext bit patterns
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 party
PHHW PH DIWHU WKH WRJD SDUWB
What’s the key?
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 number
a b c d e f g h i j k l m
0 1 2 3 4 5 6 7 8 9 10 11 12
n o p q r s t u v w x y Z
13 14 15 16 17 18 19 20 21 22 23 24 25
then have Caesar cipher as:
C = E(p) = (p + k) mod (26)
p = D(C) = (C – k) mod (26)
Playfair Cipher
provides security one approach to improving
security was to encrypt multiple letters
the Playfair Cipher is an example
invented by Charles Wheatstone in 1854 but
named after his friend Baron Playfair
Playfair Cipher
Playfair Key Matrix a 5X5 matrix of letters based
on a keyword
fill in letters of keyword (sans duplicates)
fill rest of matrix with other letters
e.g. using the keyword MONARCHY
Encrypting and Decrypting
plaintext is encrypted two letters at a time
1. if a pair is a repeated letter, insert filler like 'X'
2. if both letters fall in the same row, replace each
with letter to right (wrapping back to start from end)
3. if both letters fall in the same column, replace each
with the letter below it (wrapping to top from bottom)
4. otherwise each letter is replaced by the letter in the
same row and in the column of the other letter of the
pair
Vigenère Cipher
simplest polyalphabetic substitution cipher is the Vigenère
Cipher
effectively multiple caesar ciphers
key is multiple letters long K = k1 k2 ... kd
ith letter specifies ith alphabet to use
use each alphabet in turn
repeat from start after d letters in message
decryption simply works in reverse
Example
write the plaintext out
write the keyword repeated above it
use each key letter as a caesar cipher key
encrypt the corresponding plaintext letter
eg using keyword deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
Vigenere Cipher Table
Transposition Ciphers
now consider classical transposition or
permutation ciphers
these hide the message by rearranging
the letter order
without altering the actual letters used
can recognize these since have the
same frequency distribution as the
original text
Rail Fence cipher
write message letters out diagonally over a
number of rows
then read off cipher row by row
Row Transposition Ciphers
is a more complex transposition
write letters of message out in rows over a specified
number of columns then reorder the columns
according to some key before reading off the rows
