Transcript Computer Network 2

Cryptography &
Network Security
MSc. NGUYEN CAO DAT
Dr. TRAN VAN HOAI
Principles of modern ciphers
Implement crypto library
Network Security Applications
System Security
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Course details
Number of credits: 3
 Study time allocation per week:
 2 lecture hours for theory
 2 lecture hours for lab, exercises
 6 hours for self-study
 Website: http://www.cse.hcmut.edu.vn/~dat
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Course outline (1/2)
 Basics of Cryptography
▫ Symmetric key
▫ Public key
▫ Hash function
 Network Security Applications
▫ Authentication applications
▫ E-mail security
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Course outline (2/2)
Network Security Applications (con’t)
▫ Web security
▫ IP security
 System Security
▫ IDS/IPS
▫ Firewalls
▫…
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References
[1] “Cryptography and Network Security
Principles and Practices”, W. Stallings, 4th ed.,
Prentice Hall, 2005
[2] Slides “Cryptography and Network Security”,
Bộ môn Hệ thống và Mạng, Khoa Khoa học và
Kỹ thuật máy tính, ĐHBK Tp.HCM.
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Assessment Scheme
Attending lectures: >80% lecture times
Reading textbooks and references
Self-study and working in group
Lab: 20%
Assignments: 20%
Midterm Exam: 20%, multiple question choice
test – 45’
Final Exam: 40%, multiple question choice test
– 60’
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Chapter 1
Introduction
MSc. NGUYEN CAO DAT
Dr. TRAN VAN HOAI
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Background
Information Security requirements have
changed in recent times.
traditionally provided by physical and
administrative mechanisms.
computer use requires automated tools to
protect files and other stored information.
use of networks and communications links
requires measures to protect data during
transmission.
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Definitions
Computer Security - generic name for the
collection of tools designed to protect data and
to thwart hackers.
Network Security - measures to protect data
during their transmission.
Internet Security - measures to protect data
during their transmission over a collection of
interconnected networks.
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Aim of Course
our focus is on Internet Security
which consists of measures to deter, prevent,
detect, and correct security violations that
involve the transmission & storage of
information
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Security Trends
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OSI Security Architecture
ITU-T X.800 “Security Architecture for OSI”
defines a systematic way of defining and
providing security requirements
for us it provides a useful, if abstract, overview
of concepts we will study
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Aspects of Security
consider 3 aspects of information security:
▫ security attack
▫ security mechanism
▫ security service
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Security Attack
any action that compromises the security of
information owned by an organization
information security is about how to prevent
attacks, or failing that, to detect attacks on
information-based systems
often threat & attack used to mean same thing
have a wide range of attacks
can focus of generic types of attacks
▫ passive
▫ active
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Classify Security Attacks
passive attacks - eavesdropping on, or
monitoring of, transmissions to:
▫ obtain message contents, or
▫ monitor traffic flows
active attacks – modification of data stream to:
▫
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masquerade of one entity as some other
replay previous messages
modify messages in transit
denial of service
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Types of Attacks
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Passive Attacks
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Active Attacks
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Security Service
▫ enhance security of data processing systems
and information transfers of an organization
▫ intended to counter security attacks
▫ using one or more security mechanisms
▫ often replicates functions normally associated
with physical documents
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Security Services
X.800
“a service provided by a protocol layer of
communicating open systems, which ensures
adequate security of the systems or of data
transfers”
RFC 2828
“a processing or communication service provided
by a system to give a specific kind of protection
to system resources”
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Security Services (X.800)
Authentication - assurance that the
communicating entity is the one claimed
Access Control - prevention of the
unauthorized use of a resource
Data Confidentiality –protection of data from
unauthorized disclosure
Data Integrity - assurance that data received is
as sent by an authorized entity
Non-Repudiation - protection against denial by
one of the parties in a communication
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Security Mechanism
feature designed to detect, prevent, or recover
from a security attack
no single mechanism that will support all
services required
however one particular element underlies many
of the security mechanisms in use:
▫ cryptographic techniques
hence our focus on this topic
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Security Mechanisms (X.800)
specific security mechanisms
▫ encipherment, digital signatures, access
controls, data integrity, authentication exchange,
traffic padding, routing control, notarization
pervasive security mechanisms
▫ trusted functionality, security labels, event
detection, security audit trails, security recovery
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Model for Network Security
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Model for Network Security
 using this model requires us to:
1. design a suitable algorithm for the security
transformation
2. generate the secret information (keys) used by
the algorithm
3. develop methods to distribute and share the
secret information
4. specify a protocol enabling the principals to use
the transformation and secret information for a
security service
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Model for Network Access Security
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Model for Network Access Security
 using this model requires us to:
1. select appropriate gatekeeper functions to
identify users
2. implement security controls to ensure only
authorised users access designated information
or resources
 trusted computer systems may be useful to
help implement this model
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Cryptography
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Cryptography
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
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Cryptanalysis
objective to recover key not just
message
general approaches:
▫ cryptanalytic attack
▫ brute-force attack
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Cryptanalytic Attacks
ciphertext only
▫ only know algorithm & ciphertext, is statistical,
know or can identify plaintext
known plaintext
▫ know/suspect plaintext & ciphertext
chosen plaintext
▫ select plaintext and obtain ciphertext
chosen ciphertext
▫ select ciphertext and obtain plaintext
chosen text
▫ select plaintext or ciphertext to en/decrypt
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More Definitions
unconditional security
▫ no matter how much computer power or time is
available, the cipher cannot be broken since the
ciphertext provides insufficient information to
uniquely determine the corresponding plaintext
computational security
▫ given limited computing resources (eg time
needed for calculations is greater than age of
universe), the cipher cannot be broken
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Brute Force Search
always possible to simply try every key
most basic attack, proportional to key size
assume either know / recognise plaintext
Key Size (bits)
Number of Alternative
Keys
Time required at 1
decryption/µs
Time required at 106
decryptions/µs
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232 = 4.3  109
231 µs
= 35.8 minutes
2.15 milliseconds
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256 = 7.2  1016
255 µs
= 1142 years
10.01 hours
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2128 = 3.4  1038
2127 µs
= 5.4  1024 years
5.4  1018 years
168
2168 = 3.7  1050
2167 µs
= 5.9  1036 years
5.9  1030 years
26! = 4  1026
2  1026 µs = 6.4  1012 years
26 characters
(permutation)
6.4  106 years
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Summary
have considered:
▫ definitions for:
 computer, network, internet security
X.800 standard
security attacks, services, mechanisms
models for network (access) securityto
Cryptography, cryptanalysis
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Self study
Symmetric Cipher Model
Classical Substitution Ciphers
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Caesar Cipher
Monoalphabetic Cipher
Playfair Cipher
Polyalphabetic Ciphers
Vigenère Cipher
Cryptanalysis using letter frequencies
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