Security
Chapter 9
Panko’s Business Data Networks
and Telecommunications, 6th edition
Copyright 2006 Prentice-Hall
Security Requirements
• Authenticity
• Confidentiality
• Integrity
• Non-repudiation
• Availability
2
Figure 9-2: Malware
• Malware (惡意軟體)
– A general name for evil software
• Viruses
– Pieces of code that attach to other programs
– When infected programs execute, the virus executes
– Infects other programs on the computer
– Spreads to other computers by e-mail attachments, IM,
peer-to-peer file transfers, etc.
– Antivirus programs are needed to scan arriving files
• Also scans for other malware
3
Figure 9-2: Malware
• Worms
– Stand-alone programs that do not need to attach to other
programs
– Can propagate like viruses through e-mail, etc.
• But this require human gullibility, which is slow
– Vulnerability-enabled worms jump to victim hosts directly
漏洞
– Can do this because hosts have vulnerabilities
– Vulnerability-enabled worms can spread with amazing
speed
– Vendors develop patches for vulnerabilities but
companies often fail or are slow to apply them
4
Figure 9-2: Malware
• Payloads
– After propagation, viruses and worms execute their
payloads (damage code)
– Payloads erase hard disks, send users to pornography
sites if they mistype URLs
– Trojan horses: exploitation programs disguise (假裝)
themselves as system files
5
Figure 9-2: Malware
• Attacks on Individuals
– Social engineering—tricking the victim into doing
something against his or her interests
Phishing
– Spam—unsolicited(未被懇求的) commercial e-mail
– Credit card number theft is performed by carders
– Identity theft: collect enough data to impersonate the
victim in large financial transactions
– Fraud: get-rich-quick schemes(一夜致富法), medical
scams(欺詐)
6
Figure 9-2: Malware
• Attacks on Individuals
– Adware pops up advertisements
– Spyware collects sensitive data and sends it to an
attacker
– Phishing: sophisticated social engineering attack in
which an authentic-looking e-mail or website entices the
user to enter his or her username, password, or other
sensitive information
7
Figure 9-3: Human Break-Ins (Hacking)
• Human Break-Ins
– Viruses and worms rely on one main attack method
– Humans can keep trying different approaches until they
succeed
• Hacking
– Breaking into a computer
– Hacking is intentionally using a computer resource
without authorization or in excess of authorization
8
Figure 9-3: Human Break-Ins (Hacking)
• Scanning Phase
– Send attack probes to map the network
and identify possible victim hosts
– Nmap programming is popular (Figure 9-4)
9
Figure 9-4: Nmap
IP Range
to Scan
Type of
Scan
Identified
Host and
Open
Ports
10
Figure 9-3: Human Break-Ins (Hacking)
(開拓)
• The Exploit
– The actual break-in
– Noun: Exploit is the program used to
make the break-in
– Verb: Attackers exploit the computer
11
Figure 9-3: Human Break-Ins (Hacking)
• After the Break-In
– Become invisible by deleting log files
– Create a backdoor (way to get back into the computer)
• Backdoor account—account with a known password
and super user privileges
• Backdoor program—program to allow reentry; usually
Trojanized
• Rootkit—stealthy backdoor that cannot be detected
New
by the operating system
– Do damage at leisure
12
Figure 9-5: Distributed Denial-of-Service Flooding
Attack
TCP Syn Flooding
Attack
Command
Attacker
1.34.150.37
Attack
Command
Handler
Zombie
Attack
Command
Attack Packet
Attack Packet
Victim
60.168.47.47
Attack
Command
Attack
Command
Zombie
Attack Packet
Handler
Zombie
The attacker installs handler and zombie programs on victims
The attacker sends an attack command to handlers.
Handlers send attack commands to zombies.
The zombies overwhelm the victim with attack packets.
13
Figure 9-6: Bots
Human Master
Command
Bot
Software
Update
Bot
Bots are like zombies,
but they can be updated
by the human master
to give new functionality.
14
Figure 9-19: Cryptographic Systems
• Cryptographic Systems
– Provide security to multi-message dialogues
• At the Beginning of Each Communication Session
– The two parties authenticate each other
Party A
Party B
Credentials
Credentials
Initial Authentication
15
Figure 9-19: Cryptographic Systems
• Message-by-Message Protection
– After this initial authentication, cryptographic systems
provide protection to every message
– Encrypt each message for confidentiality so that
eavesdroppers cannot read it
Messages Encrypted for Confidentiality
Party A
Party B
16
Figure 9-19: Cryptographic Systems
• Message-by-Message Protection
– Adds an electronic signature to each message
– The electronic signature authenticates the sender
– It also provides message integrity: receiver can tell if a
message has been changed in transit
Party A
Electronic Signature
Party B
17
Symmetric and Public Key Encryption
Symmetric Key Encryption for Confidentiality
Symmetric
Key
Message Encryption
Method &
“Hello”
Key
Encrypted Message
Network
Party A
Party B
Encryption uses a
non-secret encryption method and
a secret key
18
Figure 9-20: Symmetric and Public Key Encryption
Symmetric Key Encryption for Confidentiality
Symmetric
Key
Encrypted Message
Interceptor
Network
Party A
Encrypted Message
Interceptor cannot read
encrypted messages
Party B
19
Figure 9-20: Symmetric and Public Key Encryption
Symmetric Key Encryption for Confidentiality
Symmetric
Key
Message Encryption
Method &
“Hello”
Key
Encrypted Message
Interceptor
Network
Party A
Encrypted Message
Receiver decrypts the message
Using the same encryption message
And the same symmetric key
Same
Symmetric
Key
Decryption Message
Method &
“Hello”
Key
Party B
20
Types of Symmetric Key Encryption
DES
3DES
AES
Key Length (bits)
56
112 or 168
128, 192, or
256
Strength
Weak
Strong
Strong to
Very Strong
Processing
Requirements
Moderate
High
Modest
RAM Requirements
Moderate
High
Modest
21
Figure 9-20: Symmetric and Public Key Encryption
Public Key Encryption for Confidentiality
Encrypt with
Party B’s Public Key
Party A
Encrypted
Message
Decrypt with
Party B’s Private Key
Note:
Four keys are used to encrypt
and decrypt in both directions
Decrypt with
Party A’s Private Key
Encrypted
Message
Party B
Encrypt with
Party A’s Public Key
22
Figure D-7: Digital Signature
Encrypted for Confidentiality
DS
Plaintext
Sender
Receiver
Add Digital Signature to Each Message
Provides Message-by-Message Authentication
23
Figure D-7: Digital Signature: Sender
To Create the Digital Signature:
1. Hash the plaintext to create
a brief message digest; This is
NOT the digital signature
2. Sign (encrypt) the message
digest with the sender’s private
key to create the digital
Signature
Hash algorithms: MD5, SHA-1
http://en.wikipedia.org/wiki/MD5
http://en.wikipedia.org/wiki/SHA-1
Plaintext
Hash
MD
Sign (Encrypt) MD with
Sender’s Private Key
DS
24
Figure D-7: Digital Signature
Send Plaintext Plus Digital Signature
Encrypted with Symmetric Session Key
DS
Plaintext
Transmission
Sender
Encrypts
Receiver Decrypts the Message,
Getting the Plaintext Plus Digital Signature
Receiver
Decrypts
25
Figure D-7: Digital Signature:
Receiver
Received Plaintext
1.
Hash
DS
2.
Decrypt with
True Party’s
Public Key
MD
3.
Are they Equal?
MD
1. Hash the received
plaintext with the same
hashing algorithm the
sender used. This gives
the message digest.
2. Decrypt the digital
signature with the sender’s
public key. This also should
give the message digest.
3. If the two match, the
message is authenticated;
The sender has the true
Party’s private key
26
Figure D-8: Public Key Deception
Impostor
Verifier
“I am the True Party.”
Must authenticate True Party.
“Here is TP’s public key.”
(Sends Impostor’s public key)
Critical
Deception
Believes now has
TP’s public key
“Here is authentication
based on TP’s private key.”
(Really Impostor’s private key)
Believes True Party
is authenticated
based on Impostor’s public key
Decryption of message from Verifier
encrypted with Impostor’s public key,
so Impostor can decrypt it
“True Party,
here is a message encrypted
with your public key.”
27
Digital Certificates
• Digital certificates are electronic documents
that give the true party’s name and public key
• Applicants claiming to be the true party have
their authentication methods tested by this
public key
• If they are not the true party, they cannot use
the true party’s private key and so will not be
authenticated
• Digital certificates follow the X.509 Standard
28
Figure D-10: Roles of Digital Certificates
and Digital Signatures in Authentication
• Public key authentication requires both a
digital signature and a digital certificate to give
the public key needed to test the digital
signature
Certificate Authority
Digital
Certificate:
True Party’s
Public Key
Applicant
DS
Plaintext
Verifier
29
Figure D-10: Roles of Digital Certificates
and Digital Signatures in Authentication
Applicant
Certificate Authority
Digital
Signature
Digital
Certificate:
True Party’s
Name and
Public Key
Must be Tested with
True Party’s
Digital Certificate
Authentication
Verifier
30
Figure D-9: Public Key Infrastructure
(PKI)
1.
Create
Public Key/
Private Key
Pair
Certificate Authority
PKI Server
Verifier
(Cheng)
2.
Distribute
Private
Key
Verifier
(Brown)
Applicant (Lee)
31
Figure D-9: Public Key Infrastructure
(PKI)
Certificate Authority
PKI Server
3. Request
Certificate
for Lee
Verifier
(Cheng)
4.
Certificate
for Lee
Verifier
(Brown)
Applicant (Lee)
32
Figure D-9: Public Key Infrastructure
(PKI)
Certificate Authority
PKI Server
6. Request Certificate
Revocation List (CRL)
7. CRL
Verifier
(Cheng)
Verifier
(Brown)
5.
Certificate
for Lee
Applicant (Lee)
33
Figure 9-9: Access Control
• Companies must then develop an access control
plan for each asset
– The plan includes the AAA protections
– Authentication—proving the identity of the person
wishing access
– Authorization—determining what the person may do if
they are authenticated
– Auditing*—logging data on user actions for later
appraisal
*3rd A: Accounting http://en.wikipedia.org/wiki/AAA_protocol
34
Figure 9-10: Authentication
1. 憑據
Credentials
(Password, etc.)
Authentication
Verifier
Server
2. OK?
3. OK and
Authorizations
4. Welcome
Applicant
Verifier
Applicant
The applicant is the person who wishes to prove his or her identity.
The verifier is the person who wants to authenticate the applicant.
The applicant sends credentials (passwords, etc.).
Usually a central authentication server judges the credentials.
This provides consistency in authentication.
35
Figure 9-11: Password Authentication
• Passwords should be complex
– Mix case (A and a), digits (6), and other keyboard
characters ($, #, etc.)
– Can only be cracked with brute force attacks (trying all
possibilities)
• Passwords should be long
– Eight characters minimum
– Each added character increases the brute force search
time by a factor of about 70
36
Figure 9-12: Digital Certificate
Authentication
• Public and Private Keys
– Each party will have both a public key and a private key
– Each party makes its public key available to everybody
– Each party keeps the private key secret
• Digital Certificate
– Tamper-proof file giving a party’s public key
37
Figure 9-12: Digital Certificate
Authentication
• Operation
– Applicant performs a computation with his or her private
key, which only he or she should know
– Verifier tests the calculation with the public key in the
digital certificate of the party the applicant claims to be
– If the test is successful, the applicant is authenticated as
knowing the claimed party’s secret public key
– If the test fails, the applicant is rejected
38
Figure 9-12: Digital Certificate
Authentication
Calculation
Digital Certificate
Calculation
created with the
private key of
applicant
Public key of
the person
the applicant
claims to be
Authentication
Verifier tests the calculation with the public key of the
claimed party. If the test succeeds, the applicant must
know the secret private key of the claimed party, which
only the claimed party should know.
39
Figure 9-12: Digital Certificate
Authentication
• Appraisal (評價)
– Digital signature authentication gives extremely strong
authentication
– Very expensive: must set up infrastructure for distributing
public-private key pairs
– Must do the labor of creating, distributing, and installing
private keys.
40
Figure 9-13: Biometric Authentication
• Biometric Authentication
– Authentication based on bodily measurements
– Promises to eliminate passwords
• Fingerprint Scanning
– Dominates biometrics use today
– Simple and inexpensive
– Substantial error rate (misidentification)
– Often can be fooled fairly easily by determined impostors
– Not a problem for low-risk situations like home computers
41
Figure 9-13: Biometric Authentication
• Iris Scanners
– Scan the iris (colored part of the eye)
– Irises are complex, so very strong
authentication
– Expensive
• Face Recognition
– Camera allows analysis of facial structure
– Can be done surreptitiously—without the knowledge or
consent of person being scanned
– Very high error rate and easy to fool
42
Figure 9-13: Biometric Authentication
• Error Rates and Deception(欺騙)
– Error rates are higher than vendors claim
– Deception is easier than vendors claim
– The usefulness of biometrics is uncertain
43
Figure 9-14: Firewall Operation
Allowed
Legitimate Packet
Hardened Server
Attack
Packet
Internet
Firewall
Internet
Attacker
Hardened
Client PC
Denied
Attack
Packet
Log File
Ingress Filtering
Legitimate
Egress Filtering Packet
Internal
Corporate
Network
Legitimate
Host
Firewalls inspect each packet.
Legitimate packets are allowed through.
Provable attack packets are dropped and logged.
44
Figure 9-15: Stateful Firewall Filtering
• Stateful Firewall Filtering
– There are several types of firewall filtering
– Stateful inspection is the dominant methodology today
– Stateful firewalls often use other filtering mechanisms as
secondary mechanisms
45
Figure 9-15: Stateful Firewall Filtering
• Connection Initiation
– Some Packets Attempt to Open a Connection
• Example: packets with TCP segments whose SYN bits are set
• Stateful firewalls have default rules for connection-opening
attempts
Internally Initiated Connections
Are Allowed by default
Site
Stateful
Border
Firewall
Externally
Initiated
Connections are
Rejected
By Default
46
Stateful Firewalls
All Packets
Connection-Opening
Attempts
Default Behavior
for Connections
Other Packets
Not Part of
Previously
Permitted
Connection
Part of
Previously
Permitted
Connection
Drop Packet
Accept Packet
ACL Exceptions
47
Figure 9-15: Stateful Firewall Filtering
• Perspective
– Simple operation leads to inexpensive stateful firewall
operation
– However, stateful inspection firewall operation is highly
secure
48
Figure 9-17: Ingress Access Control List
(ACL) for a Stateful Inspection Firewall
• 1. If packet’s source and destination sockets are in
the connection table, PASS.
– If the packet is part of an previously-established
connection, pass it without further filtering.
• 2. If the packet’s source and destination sockets
are not in the connection table and the packet is
not a connection-opening attempt, DROP and LOG.
– Drop any packet that is not a connection-opening
attempt and that is not part of an established connection.
49
Figure 9-17: Ingress Access Control List
(ACL) for a Stateful Inspection Firewall
• 3. If protocol = TCP AND destination port number =
25, PASS and add connection to connection table
– This rule permits external access to all internal mail
servers.
• 4. If IP address = 10.47.122.79 AND protocol =
TCP AND destination port number = 80, PASS and
add connection to connection table.
– This rule permits access to a particular webserver,
10.47.122.79
50
Figure 9-17: Ingress Access Control List
(ACL) for a Stateful Inspection Firewall
• 5. Deny All AND LOG
– If earlier rules do not result in a pass or deny decision,
this last rule enforces the default rule of banning all
externally-initiated connection-opening attempts.
51
Figure 9-18: Firewalls, Intrusion Detection
Systems, and Intrusion Prevention Systems
• Firewalls
– Drop provable attack packets
• Intrusion Detection Systems (IDSs)
– Very sophisticated filtering—better than firewalls
– Identify suspicious packets
– Cannot drop--suspicious packets may be legitimate
• Intrusion Prevention Systems (IPSs)
– Use IDS filtering mechanisms
– Drop suspicious packets highly likely to be attacks
– Ignore other suspicious packets
52
Figure 9-18: Firewalls, Intrusion Detection
Systems, and Intrusion Prevention Systems
• IDS and IPS filtering
– Stream Analysis
• Analyze streams of packets to identify suspicious
patterns
– Deep packet inspection
• Inspect headers and messages at the internet,
transport, and application layers
53
Figure 9-18: Firewalls, Intrusion Detection
Systems, and Intrusion Prevention Systems
Firewalls
IDSs
IPSs
Processing
Power Needed
Modest
Heavy
Heavy
Maturity
Fairly Mature
Still immature.
Too many false
positives
New.
Tuning reduces
false positives
but is laborintensive
Only used to stop
attacks that can
be identified fairly
accurately.
54