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Computer Security
Communications, Networking & Security
Cryptography

• Well established needs for secure communication
– War time communication
– Business transactions
– Illicit Love Affairs
• Requirements of secure communication
1. Secrecy
– Only intended receiver understands the message
2. Authentication
– Sender and receiver need to confirm each others identity
3. Message Integrity
– Ensure that their communication has not been altered, either
maliciously or by accident during transmission
Secure Communication
Needs and Requirements

• Cryptography is the science of secret, or hidden
writing
• It has two main Components:
1. Encryption
– Practice of hiding messages so that they can not be read by anyone
other than the intended recipient
2. Authentication & Integrity
– Ensuring that users of data/resources are the persons they claim
to be and that a message has not been surreptitiously altered
Cryptography
Basics

• Cipher is a method for encrypting messages
• Encryption algorithms are standardized & published
• The key which is an input to the algorithm is secret
– Key is a string of numbers or characters
– If same key is used for encryption & decryption the algorithm is called
symmetric
– If different keys are used for encryption & decryption the algorithm is called
asymmetric
Encryption
Cipher
Plain Text Encryption
Algorithm
Key A Key B
Cipher Text Plain Text
Decryption
Algorithm

• Algorithms in which the key for encryption and
decryption are the same are Symmetric
– Example: Caesar Cipher
• Types:
1. Block Ciphers
– Encrypt data one block at a time (typically 64 bits, or 128 bits)
– Used for a single message
2. Stream Ciphers
– Encrypt data one bit or one byte at a time
– Used if data is a constant stream of information
Encryption
Symmetric Algorithms

• Strength of algorithm is determined by the size of the key
– The longer the key the more difficult it is to crack
• Key length is expressed in bits
– Typical key sizes vary between 48 bits and 448 bits
• Set of possible keys for a cipher is called key space
– For 40-bit key there are 240
possible keys
– For 128-bit key there are 2128
possible keys
– Each additional bit added to the key length doubles the security
• To crack the key the hacker has to use brute-force
(i.e. try all the possible keys till a key that works is found)
– Super Computer can crack a 56-bit key in 24 hours
– It will take 272
times longer to crack a 128-bit key
(Longer than the age of the universe)
Symmetric Encryption
Key Strength

• Caesar Cipher is a method in which each letter in the alphabet
is rotated by three letters as shown
Substitution Ciphers
Caesar Cipher
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
• Let us try to encrypt the message
– Attack at Dawn
Assignment: Each student will exchange a secret message
with his/her closest neighbor about some other person in the
class and the neighbor will decipher it.

Substitution Ciphers
Caesar Cipher
Encryption
Plain Text
Message:
Attack at Dawn
Cipher Text
Message:
Dwwdfn Dw Gdyq
Cipher:
Caesar Cipher
Algorithm
Key (3)
Decryption
Plain Text
Message:
Attack at Dawn
Cipher Text
Message:
Dwwdfn Dw Gdyq
Cipher:
Caesar Cipher
Algorithm
Key (3)
How many different keys are possible?

• Any letter can be substituted for any other letter
– Each letter has to have a unique substitute
• There are 26! pairing of letters (~1026
)
• Brute Force approach would be too time consuming
– Statistical Analysis would make it feasible to crack the key
Substitution Cipher
Monoalphabetic Cipher
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
M N B V C X Z A S D F G H J K L P O I U Y T R E W Q
Encrypted
Message:
Nkn, s gktc wky.
mgsbc
Message:
Bob, I love you.
Alice
Cipher:
Monoalphabetic
Cipher
Key

• Developed by Blaise de Vigenere
– Also called Vigenere cipher
• Uses a sequence of monoalpabetic ciphers in tandem
– e.g. C1, C2, C2, C1, C2
• Example
Substitution Cipher
Polyalphabetic Caesar Cipher
Encrypted
Message:
Gnu, n etox dhz.
tenvj
Message:
Bob, I love you.
Alice
Cipher:
Monoalphabetic
Cipher
Key
Plain Text 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
C1(k=6) F G H I J K L M N O P Q R S T U V W X Y Z A B C D E
C2(k=20) T U V W X Y Z A B C D E F G H I J K L M N O P Q R S

• Obtain a key to for the algorithm and then shift the alphabets
– For instance if the key is word we will shift all the letters by four and remove the
letters w, o, r, & d from the encryption
• We have to ensure that the mapping is one-to-one
– no single letter in plain text can map to two different letters in cipher text
– no single letter in cipher text can map to two different letters in plain text
Substitution Cipher
Using a key to shift alphabet
Encrypted
Message:
??
Message:
Bob, I love you.
Alice
Cipher:
WORD
Plain Text 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
C1(k=6) W O R D A B C E F G H I J K L M N P Q S T U V X Y
Z

• This involves rearrangement of characters on the plain text into columns
• The following example shows how letters are transformed
– If the letters are not exact multiples of the transposition size there may be a few
short letters in the last column which can be padded with an infrequent letter such
as x or z
Transposition Cipher
Columnar Transposition
T H I S I
S A M E S
S A G E T
O S H O W
H O W A C
O L U M N
A R T R A
N S P O S
I T I O N
W O R K S
T S S O H
O A N I W
H A A S O
L R S T O
I M G H W
U T P I R
S E E O A
M R O O K
I S T W C
N A S N S
Plain Text Cipher Text

• The amount of secrecy needed should determine the
amount of labor appropriate for the encryption and
decryption.
• The set of keys and the enciphering algorithm
should be free from complexity.
• The implementation of the process should be as
simple as possible.
• Errors in ciphering should not propagate and cause
corruption of further information in the message.
• The size of the enciphered text should be no larger
than the text of the original message.
Ciphers
Shannon’s Characteristics of “Good” Ciphers

• It is based on sound mathematics.
– Good cryptographic algorithms are are derived from
solid principles.
• It has been analyzed by competent experts and
found to be sound.
– Since it is hard for the writer to envisage all possible
attacks on the algorithm
• It has stood the “test of time.”
– Over time people continue to review both mathematical
foundations of an algorithm and the way it builds upon
those foundations.
– The flaws in most algorithms are discovered soon after
their release.
Encryption Systems
Properties of Trustworthy Systems

• Cryptanalysis is the process of breaking an encryption code
– Tedious and difficult process
• Several techniques can be used to deduce the algorithm
– Attempt to recognize patterns in encrypted messages, to be able to
break subsequent ones by applying a straightforward decryption
algorithm
– Attempt to infer some meaning without even breaking the
encryption, such as noticing an unusual frequency of communication
or determining something by whether the communication was short
or long
– Attempt to deduce the key, in order to break subsequent messages
easily
– Attempt to find weaknesses in the implementation or environment
of use of encryption
– Attempt to find general weaknesses in an encryption algorithm,
without necessarily having intercepted any messages
Cryptanalysis
Techniques

• Goal of DES is to completely scramble the data and
key so that every bit of cipher text depends on every
bit of data and ever bit of key
• DES is a block Cipher Algorithm
– Encodes plaintext in 64 bit chunks
– One parity bit for each of the 8 bytes thus it reduces to
56 bits
• It is the most used algorithm
– Standard approved by US National Bureau of Standards
for Commercial and nonclassified US government use in
1993
Data Encryption Standard (DES)
Basics

• DES run in reverse to
decrypt
• Cracking DES
– 1997: 140 days
– 1999: 14 hours
• TripleDES uses DES 3
times in tandem
– Output from 1 DES is
input to next DES
Data Encryption Standard (DES)
Basics
64-bit input
L1 R1
F(L1, R1, K1)
L2 R2
L3 R3
L17 R17
56-bit key
48-bit k1
48-bit k2
48-bit k3
48-bit k16
F(L2, R2, K2)
F(L16, R16, K16)

Encryption Algorithm
Summary
Algorithm Type Key Size Features
DES Block
Cipher
56 bits Most Common, Not
strong enough
TripleDES Block
Cipher
168 bits
(112 effective)
Modification of DES,
Adequate Security
Blowfish Block
Cipher
Variable
(Up to 448 bits)
Excellent Security
AES Block
Cipher
Variable
(128, 192, or
256 bits)
Replacement for DES,
Excellent Security
RC4 Stream
Cipher
Variable
(40 or 128 bits)
Fast Stream Cipher,
Used in most SSL
implementations

• Any exposure to the secret key compromises secrecy
of ciphertext
• A key needs to be delivered to the recipient of the
coded message for it to be deciphered
– Potential for eavesdropping attack during transmission of
key
Symmetric Encryption
Limitations

• Uses a pair of keys for encryption
– Public key for encryption
– Private key for decryption
• Messages encoded using public key can only be decoded by the
private key
– Secret transmission of key for decryption is not required
– Every entity can generate a key pair and release its public key
Asymmetric Encryption
Basics
Plain Text
Cipher
Public Key Private Key
Cipher Text Plain Text
Cipher

• Two most popular algorithms are RSA & El Gamal
– RSA
• Developed by Ron Rivest, Adi Shamir, Len Adelman
• Both public and private key are interchangable
• Variable Key Size (512, 1024, or 2048 buts)
• Most popular public key algorithm
– El Gamal
• Developed by Taher ElGamal
• Variable key size (512 or 1024 bits)
• Less common than RSA, used in protocols like PGP
Types

• Choose two large prime numbers p & q
• Compute n=pq and z=(p-1)(q-1)
• Choose number e, less than n, which has no common factor (other
than 1) with z
• Find number d, such that ed – 1 is exactly divisible by z
• Keys are generated using n, d, e
– Public key is (n,e)
– Private key is (n, d)
• Encryption: c = me mod n
– m is plain text
– c is cipher text
• Decryption: m = cd
mod n
• Public key is shared and the private key is hidden
RSA

• P=5 & q=7
• n=5*7=35 and z=(4)*(6) = 24
• e = 5
• d = 29 , (29x5 –1) is exactly divisible by 24
• Keys generated are
– Public key: (35,5)
– Private key is (35, 29)
• Encrypt the word love using (c = me
mod n)
– Assume that the alphabets are between 1 & 26
RSA
Plain Text Numeric Representation me
Cipher Text (c = me
mod n)
l 12 248832 17
o 15 759375 15
v 22 5153632 22
e 5 3125 10

• Decrypt the word love using (m = cd
mod n)
– n = 35, c=29
RSA
Cipher
Text
cd
(m = me
mod n) Plain
Text
17 481968572106750915091411825223072000 17 l
15 12783403948858939111232757568359400 15 o
22 85264331908653770195619449972111000000
0
22 v
10 100000000000000000000000000000 10 e

• Efficiency is lower than Symmetric Algorithms
– A 1024-bit asymmetric key is equivalent to 128-bit
symmetric key
• Potential for man-in-the middle attack
• It is problematic to get the key pair generated for the
encryption
Weaknesses

• Hacker could generate a key pair, give the public key away and
tell everybody, that it belongs to somebody else. Now,
everyone believing it will use this key for encryption, resulting
in the hacker being able to read the messages. If he encrypts
the messages again with the public key of the real recipient, he
will not be recognized easily.
Man-in-the-middle Attack
Bob
Attacker
David
Bob’s
Message
+ Public key
Cipher
David’s
Public Key
Trudeau
(Middle-man)
Trudeau’s
Message
+ public key
Cipher
Trudeau’s
Public Key
Bob’s
Encrypted
Message
Trudeau’s
Encrypted
Message
David’s
Message
+ public key
Cipher
Trudeau’s
Encrypted
Message
Bob’s
Public Key
Trudeau’s
New Message
+ public key
Cipher
Trudeau’s
Encrypted
Message
David’s
Public Key

• Used to improve efficiency
– Symmetric key is used for encrypting data
– Asymmetric key is used for encrypting the symmetric key
Session-Key Encryption
Plain Text Cipher
(DES)
Session Key
Recipient’s Public Key
Cipher Text
Encrypted
Key
Cipher
(RSA)
Send to Recipient

• Pretty Good Privacy (PGP)
– Used to encrypt e-mail using session key encryption
– Combines RSA, TripleDES, and other algorithms
• Secure/Multipurpose Internet Mail Extension (S/MIME)
– Newer algorithm for securing e-mail
– Backed by Microsoft, RSA, AOL
• Secure Socket Layer(SSL) and Transport Layer Socket(TLS)
– Used for securing TCP/IP Traffic
– Mainly designed for web use
– Can be used for any kind of internet traffic
Encryption Protocols

• Key agreement is a method to create secret key by exchanging only public
keys.
• Example
– Bob sends Alice his public key
– Alice sends Bob her public key
– Bob uses Alice’s public key and his private key to generate a session key
– Alice uses Bob’s public key and her private key to generate a session key
– Using a key agreement algorithm both will generate same key
– Bob and Alice do not need to transfer any key
Key Agreement
Cipher
(DES)
Session Key
Cipher
(DES)
Bob’s
Public Key
Alice’s
Public Key
Bob’s
Private Key
Alice’s
Private Key
Alice and Bob
Generate Same
Session Key!

Key Diffie-Hellman Mathematical Analysis
Bob & Alice
agree on non-secret
prime p and value a
Generate Secret
Random Number x
Compute Public Key
ax
mod p
Compute Session Key
(ay
)x
mod p
Generate Secret
Random Number y
Compute Public Key
ay
mod p
Compute Session Key
(ax
)y
mod p
Bob Alice
Identical Secret Key
Bob & Alice
exchange
public keys

• Diffie-Hellman is the first key agreement algorithm
– Invented by Whitfield Diffie & Martin Hellman
– Provided ability for messages to be exchanged securely
without having to have shared some secret information
previously
– Inception of public key cryptography which allowed keys
to be exchanged in the open
• No exchange of secret keys
– Man-in-the middle attack avoided
Key Agreement con’t.

• Authentication is the process of validating the identity
of a user or the integrity of a piece of data.
• There are three technologies that provide
authentication
– Message Digests / Message Authentication Codes
– Digital Signatures
– Public Key Infrastructure
• There are two types of user authentication:
– Identity presented by a remote or application participating
in a session
– Sender’s identity is presented along with a message.
Authentication
Basics

• A message digest is a fingerprint for a document
• Purpose of the message digest is to provide proof that data has
not altered
• Process of generating a message digest from data is called
hashing
• Hash functions are one way functions with following
properties
– Infeasible to reverse the function
– Infeasible to construct two messages which hash to same digest
• Commonly used hash algorithms are
– MD5 – 128 bit hashing algorithm by Ron Rivest of RSA
– SHA & SHA-1 – 162 bit hashing algorithm developed by NIST
Authentication
Message Digests
Message
Message
Digest
Algorithm
Digest

• A message digest created with a key
• Creates security by requiring a secret key to be
possesses by both parties in order to retrieve the
message
Message Authentication Codes
Basics
Message
Message
Digest
Algorithm
Digest
Secret Key

• Password is secret character string only known to user and
server
• Message Digests commonly used for password authentication
• Stored hash of the password is a lesser risk
– Hacker can not reverse the hash except by brute force attack
• Problems with password based authentication
– Attacker learns password by social engineering
– Attacker cracks password by brute-force and/or guesswork
– Eavesdrops password if it is communicated unprotected over the
network
– Replays an encrypted password back to the authentication server
Password Authentication
Basics

• Set of rules that governs the communication of data related to authentication
between the server and the user
• Techniques used to build a protocol are
– Transformed password
• Password transformed using one way function before transmission
• Prevents eavesdropping but not replay
– Challenge-response
• Server sends a random value (challenge) to the client along with the authentication
request. This must be included in the response
• Protects against replay
– Time Stamp
• The authentication from the client to server must have time-stamp embedded
• Server checks if the time is reasonable
• Protects against replay
• Depends on synchronization of clocks on computers
– One-time password
• New password obtained by passing user-password through one-way function n times
which keeps incrementing
• Protects against replay as well as eavesdropping
Authentication Protocols
Basics

• Kerberos is an authentication service that uses symmetric key
encryption and a key distribution center.
• Kerberos Authentication server contains symmetric keys of all
users and also contains information on which user has access
privilege to which services on the network
Authentication Protocols
Kerberos

• Personal Tokens are hardware devices that generate unique
strings that are usually used in conjunction with passwords for
authentication
• Different types of tokens exist
– Storage Token: A secret value that is stored on a token and is available
after the token has been unlocked using a PIN
– Synchronous one-time password generator: Generate a new password
periodically (e.g. each minute) based on time and a secret code stored
in the token
– Challenge-response: Token computes a number based on a challenge
value sent by the server
– Digital Signature Token: Contains the digital signature private key and
computes a computes a digital signature on a supplied data value
• A variety of different physical forms of tokens exist
– e.g. hand-held devices, Smart Cards, PCMCIA cards, USB tokens
Authentication
Personal Tokens

• Uses certain biological characteristics for
authentication
– Biometric reader measures physiological indicia and
compares them to specified values
– It is not capable of securing information over the network
• Different techniques exist
– Fingerprint Recognition
– Voice Recognition
– Handwriting Recognition
– Face Recognition
– Retinal Scan
– Hand Geometry Recognition
Authentication
Biometrics

• Probability of two irises producing exactly the same
code: 1 in 10 to the 78th power
• Independent variables (degrees of freedom)
extracted: 266
• IrisCode record size: 512 bytes
• Operating systems compatibility: DOS and
Windows (NT/95)
• Average identification speed (database of 100,000
IrisCode records): one to two seconds
Authentication
Iris Recognition
The scanning process takes advantage of the
natural patterns in people's irises, digitizing them
for identification purposes
Facts

• A digital signature is a data item which accompanies or is
logically associated with a digitally encoded message.
• It has two goals
– A guarantee of the source of the data
– Proof that the data has not been tampered with
Authentication
Digital Signatures
Message
Sent to
Receiver
Digest
Algorithm
Digital
Signature
Sent to
Receiver
Message
Digest
Sender’s
Private Key
Sender’s
Public Key
Message
Digest
Signature
Algorithm
Signature
Algorithm
Digest
Algorithm
Message
Digest
Sender Receiver
Same?

• A digital certificate is a signed statement by a trusted party that another
party’s public key belongs to them.
– This allows one certificate authority to be authorized by a different authority
(root CA)
• Top level certificate must be self signed
• Any one can start a certificate authority
– Name recognition is key to some one recognizing a certificate authority
– Verisign is industry standard certificate authority
Authentication
Digital Cerftificates
Identity
Information
Certificate
Authority’s
Private Key
Sender’s
Public Key
Signature
Algorithm
Certificate

• Chaining is the practice of signing a certificate with another private key
that has a certificate for its public key
– Similar to the passport having the seal of the government
• It is essentially a person’s public key & some identifying information signed
by an authority’s private key verifying the person’s identity
• The authorities public key can be used to decipher the certificate
• The trusted party is called the certificate authority
Authentication
Cerftificates Chaining
Certificate
Authority’s
Private Key
Signature
Algorithm
New Certificate
Certificate

• Practice of analyzing and breaking cryptography
• Resistance to crypt analysis is directly proportional to the key
size
– With each extra byte strength of key doubles
• Cracking Pseudo Random Number Generators
– A lot of the encryption algorithms use PRNGs to generate keys which
can also be cracked leading to cracking of algorithms
• Variety of methods for safe guarding keys (Key Management)
– Encryption & computer access protection
– Smart Cards
Cryptanalysis
Basics

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