Operating Systems

Unit 11:– Security

Operating Systems

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Security

• prevent unauthorized access to resources and information maintained by computers

• issues: – Guarantee the privacy and integrity of

data– Restricting the use of computer

resources– Providing resilience against malicious

attempts to incapacitate the system

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Security Mechanisms

• Encryption: – Transform data into something that an attacker

cannot understand (confidentiality) – check whether something has been modified

(integrity)

• Authentication: – verify the identity of a subject

• Authorization: – determine if a subject is permitted to request

service

• Auditing: – trace subjects and requests – can help catch an attacker

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Encryption

• Goal: data readable only to intended reader

• Cryptography:– encode and decode data– based on cypher:

• function to encode/decode data• ex: substitution cypher, transposition cypher

– modern cypher has key• parameter to encoding/decoding function

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Cryptography

• issue: key length– 64bit, 128bit, …

• symmetric– 1 key for encoding and decoding– problem: key needs to be known to

both parties

• asymmetric– 2 keys

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Secret-Key Cryptography

• symmetric cryptography• one secret key to encrypt / decrypt

message– Sender

• Encrypts a message using the secret key• Sends encrypted message to the intended

recipient

– Recipient• Decrypts the message using the same secret

key

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Secret-Key Cryptography

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Secret-Key Cryptography: Key distribution

• Problem: parties must agree on secret key

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Public-Key Cryptography

• Asymmetric:– Employs two inversely related keys:

• Public key – Freely distributed

• Private key – Kept secret by its owner

– If the public key encrypts a message, only the corresponding private key can decrypt it

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Public-Key Cryptography

• Scenarios:– sender encrypts message with receivers

public key– receiver decrypts message with private

key

– result: sender has authorized receiver

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Public-Key Cryptography

• Scenarios:– sender encrypts message with private

key– receiver decrypts message with

sender’s public key

– result: receiver has authenticated sender

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Full Public-Key Cryptography

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PK-Infrastructure: Certificates

• Limitation of public-key cryptography – if users share the same set of keys,

it is difficult to establish each party’s identity

• Solution: – certify public key – public key is part of certificate– published by certificate authority– certificate authorization hierarchy

• rooted at Internet Policy Registration Authority

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Authentication

• goal: identify user – unique characteristic of the person

• signature• fingerprint, voiceprint, retina scan

– ownership of an item• key, badge, id-card, smart card

– user knowledge• passwords• personal identification numbers (PINs)• lock combination

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Digital Signatures

• The electronic equivalents of written signatures– Authenticate senders’ identities

• Idea:– run hash function on document to produce hash

value – create signature as encrypted hash value– message is sent with document, signature, hash

function– receiver decrypts signature, runs hash function,

compare hash values

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Authorization Server: Kerberos

• protection against internal security attacks – open-source protocol/server developed at MIT

• employs secret-key cryptography– to authenticate users in a network – to maintain the integrity and privacy of

network communications

• components:– authentication server– Ticket Granting Service

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Kerberos Scenario

1. Client submits username/password to authentication server

2. If valid, the authentication server issues a Ticket-Granting Ticket (TGT) encrypted with the client’s secret key

3. Client sends decrypted TGT to the TGS when requesting a resource. If valid, TGS issues a service ticket encrypted with client’s secret key.

4. Client decrypts service ticket, which it uses to access network resources

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Authentication: Single Sign-On

• Simplifies authentication process – one log in using a single password to

access multiple applications across multiple computers

– Important to secure single sign-on passwords

• examples:– distributed file system– ACM digital library

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Authorization

• Discretionary Access Control (DAC)– File owner controls permissions– ex: read, write, execute, access control

list

• Mandatory Access Control (MAC)– Predefine a central permission scheme

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Access Control Security Models

• role based access control (RBAC)• model elements

– subject has role has privilege to access object

– subject can have multiple roles– role has multiple privileges– privilege defines object access

• very flexible

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Auditing

• log all system and network activity• host protection

– tripwire.org• Intrusion Detection Systems

– commercial and open source systems• snort.org, dshield.org, lids.org

– detection strategies• statistical anomaly • pattern-matching

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Security Attacks

• Crypt-analytic attacks• Viruses and worms• Denial-of-service attacks

– Domain name system (DNS) attack

• Software exploitation– Buffer overflow

• System penetration– Web defacing

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Cryptanalytic Attack

• Attempt to decrypt encrypted text • Goal is to determine the key • Encryption algorithm is analyzed to find

relations between bits of the encryption key and bits of the encrypted text

• Weak statistical trends between encrypted text and keys can be exploited to gain knowledge about the key

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Viruses

• executable code– attachment to an e-mail message– hidden within as audio clips, video clips and

games• if code is execute, mischief occurs:

– file corruption– change application behavior– send emails– erase hard drive, …

• replicates itself– via send email

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Virus Types

• boot sector virus– controls OS

• transient virus– runs with another program

• resident virus– operates while OS is active

• logic bomb– executes its payload at given time

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Worms

• Executable code that spreads by infecting files over a network– Rarely requires any user action to

propagate– Does not need to be attached to another

program or file to spread• Once a virus or worm is released, it can

spread rapidly, often infecting millions of computers worldwide within minutes or hours

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Denial-of-Service (DoS) Attacks• prevent system from servicing

legitimate requests • unauthorized traffic saturates a network’s

resources, restricting access for legitimate users

• typical: flood servers with data packets

• attach is carried out by network of computers

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Buffer overflow attacks

– occurs when an application sends more data to a buffer than it can hold

– can push the additional data into adjacent buffers, corrupting or overwriting existing data

– can replace executable code in an application’s stack to alter its behavior• malicious code that will then be able to execute

with the same access rights as the application it attacked

– depending on the user and application, the attacker may gain access to the entire system

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Attack Prevention and Security Solutions

• Firewalls• Antivirus software• Security patches • Secure file systems

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Firewall

• Protect against intruders outside the network– Police inbound and outbound traffic for the LAN

• Types of firewalls– Packet-filtering firewall

• allows only defined traffic • Inspects packets for inconsistencies such as

incorrect source address

– Application-level gateways• Inspect packets for malicious payloads

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Antivirus Software

• Attempts to protect against virus attack– monitor file access– identify and remove viruses

• many commercial and open source packages– www.clamav.net

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Detection techniques

• Signature scanning– Uses a known virus list

• Relies on knowledge about the structure of the computer virus’s code

• Can be ineffective against variants and polymorphic viruses

• Heuristic scanning– looks for virus-like behavior:

• Replication, residence in memory and/or destructive code

– can detect viruses that have not yet been identified

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Security Patches

• Code releases that address security flaws– Simply releasing a patch for a security flaw is

insufficient to improve security

• Developers should address security flaws by:– Notifying their users quickly– Providing software that facilitates the

process of applying security patches• Example: Hotfixes

– Microsoft Automatic Updates

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Secure File Systems

• Protect sensitive data regardless of how the data is accessed

• Encrypting File System (EFS)– Uses cryptography to protect files and

folders in an NTFS file system– Uses secret-key and public-key

encryption to secure files

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Secure Communication Protocols

• Developed to provide security in several layers of the traditional TCP/IP stack

• Secure Sockets Layer (SSL)– common extension to http

• Internet Protocol Security (IPSec)– needed for IPv4

• Virtual Private Network (VPN)– uses IPSec channel to virtually extend LAN

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Wireless Security

• Wired Equivalent Privacy (WEP) protocol – encrypts transmitted data– prevents unauthorized access to the

wireless network– shared single key

• Wi-Fi Protected Access (WPA) – provides improved data encryption– uses authentication server

• enables user authentication• session key for each user

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Steganography

• The practice of hiding information within other information– For example: a message or image, within

another image, message or other form of multimedia

• Digital watermarks – Used to protect intellectual property– Exploit unused portions of files to store

hidden messages, while the digital files maintain their intended semantics

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Agenda for next week:

– Project Presentations

– Final Exam