Network Security
Tools and Techniques
Mohamed M. Hefeeda
Supervised By: Prof. Reda Ammar
September 2000
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???? Motivations ????
• How does my bank secure my online transactions?
• How does ___.com protect my credit card if I order
online?
• How can an enterprise build a secure private
network to link its distributed sites?
• How can we send e-mails that contain secret or
sensitive data over the insecure Internet?
• How …..?
• They usually say: Don’t Worry, we use
Cryptography, 128-bit encryption, SSL, SET, …!!
• Well, but how???????????
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Outline
• Part I: Applied Cryptography
– The Tools
• Part II: Network Security Techniques
– How to use the Tools to build secure systems
• Part III: Mobil Code Security
– New evolving technology which has many potential
applications but security is a major issue/problem
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Outline (contd.)
• Part I: Applied Cryptography (The Tools)
– Conventional Cryptography (Private- or Symmetric-key)
• Concepts/Principles
• DES, 3-DES, IDEA, CAST, Blowfish, RC2, RC5, ….
– Public-key Cryptography
• Concepts/Principles
• RSA, ElGamal, Diffie-Hellman, …
– Message Digest (One-way Hash algorithms)
• MD5 and SHA
– Digital Signature
• DSS (based on ElGamal and Schnor scheme) and RSA
– Message Authentication Code (MAC)
• HMAC
– Cryptographic Pseudorandom Number Generators
• Blum-Blum-Shub, ANSI X9.17, ...
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Outline (contd.)
• Part II: Network Security (The Techniques)
– Authentication Protocols:
• Kerberos and X.509 Authentication services
– Web Security:
• SSL and SET
– E-mail Security:
• PGP and S/MIME
–
–
–
–
IP Security
Biometrics
Smartcards
Languages which support Security
• Java Security Model
– Case Study (Windows 2000 Security Features)
• Kerberos, Public-Key Infrastructure, Encrypted File System, ...
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Outline (contd.)
• Part III: Mobile Code Security
– Mobile Agents
• Concepts, applications, and Security Issues
– Protecting the host from the visiting agents
• Sandboxing
• Digital-Shrink Wrap
• Proof-Carrying Code
– Protecting the agent during the transfer
• Passive attacks
• Active attacks
– Protecting the agent from the Host
• Limited Backbox
• Computing with encrypted functions
• Cryptographic traces
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Conventional Cryptography: Concepts
• One Key:
– used for both encryption and decryption (symmetric)
– Alice and Bob meet in private and agree on a key
• Principles:
– Confusion
• The ciphertext should depend on the plaintext and the key in a
complicated and involved manner. (done through substitution)
• Objective: hide how the statistics of the ciphertext depend on the
statistics of the plaintext
– Diffusion
• Each plaintext bit should affect every (or as many as possible) of the
ciphertext bits. (done through permutation or transposition)
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Conventional Cryptography: DES
• Data Encryption Standard (DES)
– Late 60’s: IBM developed LUCIFER encryption algorithm
– In 1973: NIST accepts IBM’s proposal as a national cipher
standard
– In 1977: After refinement, DES became the standard
• Description:
– Plaintext X; |X| = 64 bits, X = L0R0
– Key K; |K|= 56 bits
– Three stages:
• IP: Initial Permutation, X0 = IP(L0R0)
• 16 iterations of a certain function, F
• IP-1: Y = IP-1(L16R16)
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DES: Encryption
56-bit key
64-bit plaintext
Initial Permutation
Round 1
Round 2
K1
K2
K16
Round 16
Generate
Sub-keys
32-bit swap
• Decryption: same
procedure but reverse
the order of the sub-keys
Inverse
Initial Permutation
64-bit ciphertext
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DES: Single Round
Li-1
Ri-1
F
Li = Ri-1
Ri = Li-1  F(Ri-1 , Ki)
Ki
+
Li
Ri
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DES: Single Round (contd.)
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DES: S-Boxes
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DES: Security Strength
• The security of DES is based on the design of the Sboxes, which is NOT publicly published! 
• Some suspicious that S-boxes were designed with
some trapdoors that might allow their designers
(NIST) to break the system
• Exhaustive search needs 256  no longer secure
• Hence, other algorithms are proposed:
– 3-DES: with two (112 bits)and three keys (168 bits).
• Encryption: Ek1-> Dk2 ->Ek3
• Decryption: Dk3->Ek2->Dk1
– Blowfish: fast, secure, variable length key (32 to 448)
– IDEA: key is 128-bit
– CAST: key is 128-bit, used in PGP
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Public-key Cryptography: Concepts
• Two keys: (Asymmetric)
—
public: used for encryption
— private: used for decryption
Bob
Alice
M
Plaintext
Encryption
Algorithm
C= EKUB(M)
Ciphertext
M=DKRB(C
)
Decryption
Algorithm
Private Key
Read/Verify
Bob’s public
key
Registration
Public Keys
(Trusted Party)
Certificate Authority,
e.g. VeriSign, CertCo, ...
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Public-key Cryptography: RSA
• Based on the “Factoring is Hard” assumption
• Key generation:
–
–
–
–
select p, q large primes, let n= p*q
select e where: gcd ((n), e) = 1. ( (n) = (p-1)(q-1) )
compute the private key, d = e-1 mod (n)
public key: {e, n}
• Encryption
– C = Me (mod n)
• Decryption:
– M = Cd (mod n)
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Network Security:Authentication Techniques
• Authentication: prove your identity usually by:
– knowing a shared secret (conventional cryptography) or
– signing a message using your private key (public-key
cryptography)
• Kerberos Authentication Protocol (MIT)
– Two versions: ver 4 (still widely used) and ver 5 (future)
– Uses DES
– Four entities:
• Client (C): requesting for a service
• Server (V): providing the service
• Authentication Server (AS): knows all passwords. Shares a unique secret
key with each server.
• Ticket-Granting Server (TGS): issues tickets to users who have been
authenticated by AS to access a certain service on a certain server V.
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Kerberos: overview
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Kerberos: messages exchange
Table 1. Summary of Kerberos Ver 4 Messages Exchange
(a) Authentication service Exchange: to obtain ticket granting ticket
(1) C  AS:
IDc || IDtgs || TS1
(2) AS  C:
Ekc[Kc,tgs || IDtgs || TS2 || Lifetime2 || Tickettgs]
Tickettgs = Ektgs[Kc,tgs || IDc || ADc ||IDtgs || TS2 || Lifetime2]
(b) Ticket-Granting Service Exchange: to obtain service-granting ticket
(3) C  TGS:
IDv || Tickettgs || Authenticatorc
(4) TGS  C:
Ekc,tgs[Kc,v || IDv || TS4 || Ticketv]
Tickettgs = Ektgs[Kc,tgs || IDc || ADc ||IDtgs || TS2 || Lifetime2]
Ticketv = Ekv[Kc,tgs || IDc || ADc ||IDv || TS4 || Lifetime4]
Authenticatorc = Ekc,tgs[IDc || ADc || TS3]
(c) Client/Server Authentication Exchange: to obtain service
(5) C  V:
Ticketv || Authenticatorc
(6) V  C:
Ekc,v[TS5+1]
Ticketv = Ekv[Kc,v || IDc || ADc ||IDv || TS4 || Lifetime4]
Authenticatorc = Ekc,v[IDc || ADc || TS5]
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Network Security: Web Security
• Security threats:
– Passive attack (eavesdropping)
– Active attacks (modify data)
• Security approaches:
– Secure Socket Layer. (just above the Transport Layer)
– Secure Electronic Transaction (in the Application Layer)
– Using the IP Security (Network Layer)
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Network Security: SSL
• Secure Socket Layer (SSL)
– Originated by Netscape
– Supports security over the Internet and Intranets
– Provides:
• Authentication
– using certificates (public key)
• Confidentiality (encryption)
– DES40, DES, 3DES, IDEA, RC2, RC4, and Fortezza
• Integrity (MAC uses one of the following)
– MD5 (128-bit hash)
– SHA (160-bit hash)
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SSL: overview
SSL Protocol Stack
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SSL: Handshake Protocol
• Used for Authentication and Key exchange
SSL Handshake Protocol Summary
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SSL: Record Protocol
• Provides confidentiality and message integrity
SSL Record Protocol Operations
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Network Security: Biometrics
• Authentication/identification approaches based on:
– Something you know, e.g. password, PIN number, ...
– Something you have, e.g. ATM card, physical key, …
– Something you are, e,g. biometrics: fingerprint, …
• Biometrics: authenticate individuals based on their
biological and/or behavioral characteristics
–
–
–
–
–
–
–
fingerprint matching
Iris scanning
Hand geometry matching
Retina scanning
Face recognition
voice recognition
DNA, …..
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Biometrics: system architecture
Database
Enrollment Phase
User
Biometric
Reader
Feature
Extractor
Store
Identification Phase
User
Biometric
Reader
Feature
Extractor
Feature
Matcher
Restore
A typical biometric system
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Case Study: Windows 2000
• Security Features:
– Kerberos ver 5 authentication protocol
• Faster, more efficient, and more secure than NTLM
– Encrypted File System (EFS)
•
•
•
•
transparent to the user
public- and private- key encryption
encrypts the file using DES
encrypts the key of the DES using the user’s private key
– Public-key based protocols
• SSL/TLS
• Certificate Server
– IP Security (IPSec)
• to build a Virtual Private LAN
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