Download Unit 11

Operating Systems
Operating Systems
Unit 11:
– Security
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 Systems
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 SignOn
• 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
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