Chapter 6
Computer & Web Security
Yun Zhang
Computer Security
Reading Sequence:
• 6.1 Security Threats
• 6.2 Security Technologies
• 6.3 Prevention, Detection, and Recovery
Assessments:
• Exercise 6
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6.1 Security Threats
A secure system maintains the following:
• Data confidentiality机密性- data access
is restricted to authorized personnel
• Data integrity完整性- data is not altered
unintentionally
• Data availability可效性- services that
enable data access are operational
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weak computer security for home
• Sensitive information exposed敏感信息暴露- for
example, intruder 入侵者can use your password
and credit card number to steal your money.
• Loss of data 数据丢失- for example, intruders
can delete or corrupt important documents such
as financial statements and business proposals.
• Loss of system functionality系统功能丧失- for
example, intruders can occupy your CPU and
memory using malicious 恶意的 programs
disabling the system to process legitimate正常的
instructions.
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weak computer security for business
• Loss of time and money devoted to resolving
security issues (for example, loss of revenue
opportunity during system downtime)
• Loss of trust in system from customers resulting
in loss of profit, reputation, jobs, and potentially
the entire business (for example, if credit card
information of customers could be obtained by
intruders through an online store, then no one
would use the site, and the business will no
longer be profitable.)
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common attack methods
• three groups:
• identity theft/privacy violation attacks偷窃
行为,
• malicious software programs (for example,
virus, worms, and Trojan horse),
• denial of service attacks.
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Reading Sequence:
•
•
•
•
6.1.1 Intruders: Who, Why, and How?
6.1.2 Identity Theft and Privacy Violation.
6.1.3 Malicious Software.
Textbook, Section 4C. Learning Goal:
Knowledge of computer viruses and how to fight
them.
• Textbook, Subsection "User IDs and Passwords"
in Section 1B.
• 6.1.4 Denial of Service.
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6.1.1 Intruders: Who, Why, and
How?
• People who hack for fun, curiosity好奇心, personal pride,
or just for the sake of breaking into computer systems to
see how far they can get..
• Internal or external personnel who may be seeking
revenge 报仇on the targeted organization.
• People who may want to make a profit or gain other
benefits using confidential data from the targeted system
• Criminals or organizations whose objective is to corrupt
the security of the targeted system for unethical不道德的
purposes including blackmail and industrial espionage 间
谍
• Terrorists who want to promote political aims and
demoralize the victim country
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launch attacks steps
Step 1: Obtain information
• Guess passwords.
• Pretend to be a system administrator
asking for sensitive information.
• Read packets of sensitive information sent
over the Internet or stored on the computer.
• Scan for vulnerabilities.
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launch attacks steps
• Step 2: Analyze Information
• Use the information obtained and look for
weak points in the network to exploit (for
example, open ports, user accounts).
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launch attacks steps
• Step 3: Launch attack
• Alter, delete or corrupt data on system.
• Make system unavailable by creating
excessive amount of traffic on the network
• Slow down a network.
• Deface丑化 a website.
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6.1.2 Identity Theft and Privacy
Violation
•
•
•
•
•
Password Cracking
Packet Sniffing
Social Engineering/Fraud 欺骗
Spoofing哄骗
Port Scanning
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Password Cracking破坏密码
• The easiest password attack mechanism
is called dumpster diving, "rummaging
through trash."
• involves searching for sensitive
information that may be thrown away into
the trash, which can be accessed by an
intruder who can physically retrieve the
trash.
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brute force method
• trying all different alphanumeric
combinations until the password is
cracked.
• dictionary attack, matching every word in
the dictionary against the password to
decrease the search space. There are
many software programs designed to
crack passwords using these methods.
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Prevention:预防
• Do not throw away legible password information
in the trash or leave your passwords at obvious
places
• Destroy or lock up sensitive information
• Use difficult to guess passwords that are
resistant to brute force or dictionary attacks
• Change passwords frequently
• Limit physical access to computer areas,
especially central servers
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Packet sniffing
• A packet sniffer is a software program or a hardware
device that captures data packets as they are
transmitted through the network. Some data packets
captured may contain sensitive information. Installing a
packet sniffer does not require administrator rights. It can
be installed on a laptop and access the network through
an open port.
• Cable modem and wireless users have a higher risk of
exposure to packet sniffers compared to DSL (such as
ADSL, HDSL, and VDSL ) and traditional dial-up users
because entire neighborhoods of cable modem and
wireless users are part of the same LAN. A packet sniffer
installed on any cable modem or wireless user's
computer may be able to capture data packets
transmitted by other computers in the same LAN.
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Prevention:
• Employ data encryption to use encrypted
protocols
• Limit physical access to network
connections
• Monitor network usage and investigate
abnormal or suspicious activities
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Social Engineering/Fraud欺骗
• Social engineering refers to the action of
tricking people into providing information needed
to gain access to systems. Unlike the other
attack methods discussed, this attack does not
involve a software tool.
• manipulating利用 the network administrator or
other authorized user to give you account name
and password information.
• This can be achieved over the phone, via email
or even in person pretending to be someone
important in an organization.
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Prevention:
• Verify identities of people requesting
sensitive information
• Become aware of social engineering
schemes and educate others of security
policies and their importance
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Spoofing 哄骗
• Spoofing is the act of using one machine to
impersonate another. An intruder can launch attacks
using the identity of another machine so that the intruder
would not be held liable for his actions. For instance, an
intruder can mask the identity of a machine with special
access privileges to obtain control of other computers on
the network. Then the intruder can launch a denial of
service attack on a target computer system by sending
numerous data packets to tie up the target system's
network resources, and cause the system to become
unavailable to other users. When the targeted system
tries to find the attacker, the trace results would likely to
point to the machine that was used to mask the identity
of the intruder.
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IP spoofing
• IP spoofing is a technique used to gain
unauthorized access to computers, whereby the
intruder sends messages to a computer with an
IP address indicating that the message is
coming from a trusted host. To use IP spoofing,
an intruder must first identify an IP address of a
trusted host and then modify the packet headers
so that the packets look like they are coming
from that host. This method is effective because
dialog between machines are automated, which
eliminates the need for username or password.
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email spoofing
• email spoofing: an attacker fakes an email header to
make it appear as if it came from somewhere or someone
other than the actual source.
• Examples of spoofed emails that could lead to an attack
on data integrity:
• Email from a system administrator instructing users to
delete files, or provide sensitive information such as
logins and passwords
• Email claiming to be from a system administrator
requesting users to change their passwords to a specified
string
• Email claiming to be from a person in authority requesting
users to send them a copy of a password file or other
sensitive information
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Prevention:
• Monitor transaction logs of servers such
as email server, Web server, and scan for
unusual behaviors (monitoring should be
done off-line to avoid attacks during the
process)
• Minimize system privileges of servers
• Limit user access to network or
administrator command functions
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Port Scanning端口扫描
• The objective of port scanning is to detect security
weaknesses in a remote or local host. A port scanner is a
program that scans TCP/IP ports and services (for
example, TELNET or FTP) and reports responses from
the target system. An intruder can use a port scanner to
find information about the target host such as which port
is open and whether an anonymous user can log in.
Once an intruder logs in successfully through the open
port, the intruder can try to gain access to more
computers on the network. Port scanning activity is
usually a precursor to an attack on a target system.
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Prevention:
• Close unused ports
• Monitor suspicious network activities (for
example, if a host is scanning through port
activities, the attacker may be preparing
for an attack)
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6.1.3 Malicious Software
•
•
•
•
•
•
Virus 病毒
Trojan Horse 特洛伊木马
Worm 蠕虫
Prevention
Detection
Counter Measures 对策
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Virus
• A virus attaches itself to a file or software application,
and then replicates itself on the host computer,
spreading copies of itself to other files. It can corrupt files,
alter or destroy data, display irritating messages, and/or
disrupt computer operations. When a computer opens an
infected file, it executes the attached virus instructions.
• An undetected virus can hide in a computer for days or
months, while reproducing itself, and attaching itself to
other files. The infected files may be spread when a user
transfers them via the Internet or removable storage
media to other computers.
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Viruses
• Viruses are categorized by the type of files they infect:
• Boot sector- compromises the system at the lowest level.
This leads to difficulties in starting the system securely; it
also causes recurring problems during the computers
operation
• Application software- infects executable files (for
example, .exe files)
• Macro- infects macro files and documents that use
macros such as Microsoft Excel and Microsoft Word; it is
attached to a document or worksheet, and often
distributed as an e-mail attachment. When one opens
the document, the macro virus copies itself into the other
macro files, where it is picked up by other documents.
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Viruses
• Viruses frequently have double extensions such
as .txt.vbs, .mpg.exe, and .gif.scr. The files attached to
the email messages sent by these viruses may appear to
be harmless files of type: text (.txt), movie (.mpg), picture
(.gif) or other file types, when in fact the file is a
malicious script or executable, for
example, .vbs, .exe, .scr. If you are using the Microsoft
Windows operating system, be aware that the operating
system hides extensions for known file types by default.
To make file extensions visible, you can click on My
Computer on the desktop, then from the Tools menu,
select Folder Options... Click on the View tab, and
uncheck Hide file extensions for known file types
option.
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Trojan Horse特洛伊木马
• A Trojan horse is a program that appears to be
performing one task while executing a malicious task in
the background. It may reach your computer as an email
attachment, usually as amusing or seemingly useful
software to entice you to open it. Once you open the
attachment, the Trojan horse program can search for
your user information, steal your login names, and copy
your passwords. Some Trojan horse programs can
delete, modify, or transmit files on your computer. And,
some Trojan horse programs may contain viruses,
worms, or other Trojan horse programs. Trojan horse
programs can use your account privileges to install other
programs such as programs that provide unauthorized
network access. Or, they can use your account to attack
other systems and implicate your site as the source of an
attack.
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Trojan Horse特洛伊木马
• Trojan horse sample: Back Orifice (BO)
• The Back Orifice is a Windows 95/98 backdoor
program. Once installed inadvertently 不注意地,
it permits intruders who know the listening port
number and BO password to control the affected
machine remotely. The intruders can execute
privileged operations such as execute
commands, list files, upload files, and download
files.
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Worm 蠕虫
• A worm is malicious software that can
execute itself on a vulnerable remote
machine. Compared to a viruses, which
infect files and spread through the transfer
of infected files and email messages,
worms can penetrate computer systems
more easily because they do not need a
user to execute them. Worms can perform
trigger events that vary from display of
irritating messages to destruction of data.
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Worm 蠕虫
• Most worms travel within email messages and
TCP/IP packets, replicating from one computer
to another. A worm can arrive as a mass-mailing
worm that sends itself to every address in the
email address book of an infected computer. To
cover its tracks, a mass-mailing worm can set
the "From:" line of the message to be a
randomly selected address from the email
address book.
• Worm sample: 911 Worm
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Worm 蠕虫
• This worm is also known as Chode, Worm.Firkin, and
other names. It searches through a range of IP
addresses of known ISPs to find an accessible computer
that has a non-password-protected shared drive. It uses
the shared drive to copy its files onto the other
computers. Once the infected computer starts Windows,
a .vbs script is launched. On the 19th of the month, this
script deletes files from the following directories:
• C:\windows
C:\windows\system
C:\windows\command
C:\
Then, it displays a message indicating the machine has
been infected.
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Prevention
• The most effective way to prevent malicious
code attacks is avoid opening unexpected
email messages or attachments. You should
be cautious and use only authorized media for
loading data and software. You should not run
executable programs unless you trust the
sender of the information and you confirmed
with the sender that he intended to send you the
attachment. Also, avoid sending programs from
an unknown source to others.
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Prevention
• You should also be cautious when executing content
such as Java applets, JavaScript, or Active X controls
from web pages. To decrease your computer's
susceptibility to worms, you may wish to configure your
browser to disable the automatic execution of web page
content. You may also want to disable macros in any
product that contains a macro language. For example, to
disable macro execution in Microsoft Word, click on
Tools, select Macro, and then select Security. Choose a
security level from High or Medium. "High" silently
ignores the macro code and "Medium" prompts you to
enable or disable the macro code.
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Detection
• Some notable symptoms of malicious code
infection on your computer system are:
• Unexpected changes in file sizes or date/time
stamps
• Slow starting or slow running because the virus
is exhausting computer's resources
• Unexpected or frequent system failures
• Low computer memory on disks
• Abnormal application behaviors
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Counter Measures
• When you suspect that a virus has entered your
system:
• 1. Try to contain the virus.
• 2. Try to identify the virus.
• 3. Try to recover corrupted data and files.
• 4. Once you have determined the source of
infection, alert others of the virus.
• There are various tools such as anti-virus
scanners that can help you look for known
viruses. You can also use anti-virus monitors to
look for virus-related application behaviors.
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6.1.4 Denial of Service
•
•
•
•
•
Network Connectivity
Network Bandwidth
Other Resource Consumption Attacks
Distributed Denial of Service Attack
Prevention
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Denial of Service
• In a denial of service (DoS) attack, the objective
of the attacker is to exhaust system resources
such as network connectivity and network
bandwidth to prevent legitimate traffic from being
transmitted and processed. This attack is
characterized by the following:
• Disruption中断 of network connectivity and
Internet services
• Disruption of services to specific system(s) or
person(s)
• Consumption of other resources on a computer
system
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Network Connectivity
• To tie up a network connectivity of a target
machine, an intruder can initiate a half-open
connection to the target machine. The target
machine starts the connection process and waits
for intruder’s bogus假的 connection to be
completed. While it is waiting, it is blocking other
legitimate connections. This attack can even be
launched over a dial-up modem connection
against a high-speed network system.
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Network Bandwidth
• One common attack on the target system's
network bandwidth is to generate an
excessive amount of traffic on that network.
This attack is also known as network
flooding. An example of a network
bandwidth attack is the Smurf attack. The
Smurf attack is launched using the Ping
command.
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Smurf attack
• The intruder sends ping commands repeatedly
using the victim's address as the return address.
When the Ping command is broadcast to
multiple hosts on the target system's local
network, all the machines receiving the Ping
request will reply to the innocent, spoofed target
system for each Ping command. This causes the
target system's network to be flooded with ping
replies. If there are enough packets flooding the
network, the spoofed host will no longer be
available to receive or distinguish legitimate
traffic. Its services are rendered unavailable.
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a Smurf attack
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Other Resource Consumption
Attacks
• An intruder may also attempt to stall a
system by attacking other components
including CPU cycles, memory, and disk
space using malicious programs. Recall
that viruses, worms, and Trojan horses are
programs. Executing these programs
consumes CPU cycles, memory, and disk
space. These malicious programs can
copy themselves repeatedly to exhaust
your CPU cycles, memory, and disk space.
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• Another way to consume disk space is by
spamming an email server, generating
excessive numbers of email messages. For
instance, when 80% of your email storage is
filled with spam mails, disk space needed to
store legitimate emails will be limited. An intruder
can also devise ways to cause the system to
generate error messages that need to be written
to disk continuously. When data is continuously
written to disk with no limits on the amount of
data that can be written, the system will
eventually run out of disk space and become
unable to perform other functions.
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Distributed Denial of Service
Attack分布式拒绝服务攻击
• A distributed denial of services (DDoS) attach is
usually launched using multiple computers to
attack one victim computer. In a DDoS attack,
the intruder compromises a few computers to act
as the control centers, also known as "handlers
主控" or "masters." The intruder also installs
"agents代理" (frequently through a Trojan horse
program) that run on other compromised
computers. Each handler computer controls a
group of agent computers.
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DDoS attack
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Prevention
• Disable or block any unused network services.
• Observe your system performance and establish
baselines 基线for ordinary activity. Use the
baseline to gauge 测量unusual levels of disk
activity, CPU usage, or network traffic.
• Routinely examine your physical security with
respect to your current needs (for example,
servers, routers, unattended terminals, network
access points).
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6.2 Security Technologies
• Reading Sequence:
• 6.2.1 Encryption.
• Parsons/Oja, Tech Talk: "Encryption" in
Chapter 6.
• 6.2.2 Applications of Encryption.
• 6.2.3 Authentication.
• Assessments:
• Multiple-Choice Quiz 23
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6.2.1 Encryption
•
•
•
•
•
Substitution Cipher替代密码
Transmitting the Key
Private Key Encryption Scheme
Public Key Encryption Scheme
Hybrid Encryption Schemes
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Encryption
• Sensitive information must be encrypted 加密
prior to transmission, and then decrypted解密 by
the receiver.
• Encryption加密 is the process of transforming
information so it is unintelligible to eavesdroppers
偷听者.
• Decryption解密is the process of transforming
encrypted information so that it is intelligible 可理
解的 to the intended recipient.
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Encryption
• A message to be encrypted is known as
plaintext.明码文本
• The encrypted version is called ciphertext. 密文
• In order to encrypt a message in a way that the
receiver can decrypt it again, there must be a
key密钥, or password, that determines how the
encryption is done.
• Cryptography algorithms加密算法 are
mathematical functions used for encryption or
decryption.
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Substitution Cipher置换密码
• The most basic type of private key encryption is
the substitution cipher. A substitution cipher
changes the plaintext to ciphertext by replacing
each element of the plaintext with its encrypted
substitute. The elements might be individual
letters, but they could also be words or phrases.
• This coding scheme therefore has a very small
keyspace.
• simple codes can be broken quickly by brute force.
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Substitution encryption results
Message
Encryption
Technique
Encrypted
Message
HELLO
WORLD
shift right by
one character
IFMMP
XPSME
IBM
shift left by
one character
HAL
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Passphrase key encryption
Plaintext
Key
Copied Key
Key Converted to Numbers
Plaintext
Ciphertext
HELLO WORLD
CAT
CATCATCATC
3 1 20 3 1 20 3 1 20 3
HELLOWORLD
KFFOPQRSFG
• Using a passphrase密码短语to control the
substitution cipher greatly expands the
keyspace.
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Transmitting the Key
• Suppose you want to send messages to a friend
who will be traveling to a hostile location. You
could pick a passphrase and give it to your
friend before she departs on her trip.
• What happens if the passphrase leaks out after
your friend has left? Your communication will no
longer be secure, and you will have no way to
get a new passphrase to her without it being
intercepted截取. Another problem is that if you
stick with the same passphrase for too long, an
eavesdropper might be able to crack your code
by statistical analysis of the encrypted messages.
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Transmitting the Key
• It is safer to choose a new key for each message. But,
you will need a way of securely transmitting that key to
your friend. One solution to this problem that has been
used over the centuries is to pick a line in a book to use
as a passphrase, assuming that both you and your friend
have copies of the book. Instead of transmitting the
actual key, you just indicate the line number to look up to
find the passphrase.
• For instance, H.II.ii.24 might refer to Hamlet Act II, Scene
ii, Line 24. Even if an eavesdropper realized that this
was a reference to Shakespeare's play, the
eavesdropper would need to know the edition that was
used in order to get an accurate key.
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Passphrase key transmission and
encryption
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Private Key Encryption Scheme
• In a private key encryption私钥加密 scheme, the same
key that was used to encrypt a message can also be
used to decrypt it. This is also known as the symmetric
encryption. In this scheme, the sender and receiver
must keep the key to themselves, hence the term,
“private” key encryption scheme. Private key encryption
schemes are used by modern encryption algorithms
such as DES and RC4.对称式加密就是加密和解密使用
同一个密钥，通常称之为“Session Key”。如DES和RC4
加密标准就是典型的“对称式”加密法。
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Public Key Encryption Scheme
• Public key encryption公钥加密, uses one
key is to encrypt a message and another
key to decrypt it. It is also known as
asymmetric encryption scheme. In this
encryption scheme, only the decryption
key need be kept secret, and the
encryption key can be freely published.
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Public Key encryption
In a public key encryption scheme, each public
key has a corresponding private key. An
eavesdropper cannot decrypt a message even if
he knows the public key that was used to
encrypt it. Only the person holding the private
key can decrypt the message.使用公开密钥的系
统叫做公共密钥基础结构（PKI）。
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number theory数论
• Public key encryption is based on a branch of
mathematics called number theory, which says that
certain types of problems, such as factoring large
numbers, appear to be intractably hard. In other words, if
a number is many digits long, it can take an extremely
long time to find a smaller number (other than 1) by
which it divides evenly. Because intractable 难处理的
problems are so hard, a code based on these types of
problems should be safe from brute force attack. The
calculations required to break such a code would take
way too long—perhaps trillions of years, even on a very
fast computer.
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PGP
• One common way for people to
communicate via public key encryption is a
software package called PGP, which
stands for Pretty Good Privacy. PGP was
developed by an American programmer,
Phil Zimmermann, because he wanted to
ensure that private citizens, not just
governments, would be able to
communicate without fear of
eavesdropping.
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Hybrid混合 Encryption Schemes
• Although public key encryption is very secure, it does have some
drawbacks. This method for encrypting and decrypting messages
requires a lot of computation, so it is slower than other types of
codes. Also, the encrypted message may be quite a bit longer than
the original plaintext, so it will take longer to transmit over a network.
These considerations led people to develop hybrid schemes in
which public key encryption is used to send a freshly-created key,
called the session key, and then the actual message is encrypted
using a symmetric encryption scheme like RC4 or DES, based on
that session key. Because session keys are generated randomly
and thrown away after one use, even if an eavesdropper manages
to decrypt one message by brute force, the eavesdropper will be
unable to decrypt any other messages between the same parties.
So, the hybrid scheme is reasonably secure, and it takes advantage
of the efficiency of the simpler symmetric encryption schemes.
• Most public-key encryption schemes are actually hybrids of this sort.
SSL (the Secure Socket Layer protocol used by web browsers) is a
hybrid scheme.
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6.2.2 Applications of Encryption
•
•
•
•
Email
Hard Drives
DVD Movies
Cellular Phones
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Email
• Email may travel through a dozen
computers on its way from the sender to
the intended recipient, so there is ample
opportunity for eavesdropping. By default,
email is not encrypted at present, but
many users are opting to use PGP to
secure their email communications. Some
mail clients now include built-in PGP
support.
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Hard Drives
• Some users are choosing to encrypt their entire hard drive, in order
to protect against loss of secrets in the event that their computer is
stolen or seized. An old laptop full of trade secrets or classified
military information might be worth much more to a thief than a new
laptop with an empty hard drive. While user directories can be
protected with passwords, an intruder with physical access to the
machine can use his own copy of the operating system to boot the
computer, and thereby gain access to all the data on the computer.
Or, the intruder can make a copy of the hard drive, a process known
as mirroring, and thus steal the data it contained.
• Encrypting the hard drive prevents others from reading your files,
even if they steal or copy the drive. Each time your computer boots,
you would be required to enter a pass phrase to unlock the drive.
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DVD Movies
• Movies on DVD (Digital Video Disk) are encrypted using
a scheme called CSS (Content Scrambling System), in
order to prevent people from making unauthorized
copies. However, the encryption scheme was not secure.
In 1999, several groups of computer programmers broke
the encryption by reverse-engineering a software DVD
player. Although the motion picture studios filed several
lawsuits to prevent the distribution of software for
decrypting DVD movies, their efforts have been
unsuccessful. Programmers claimed that computer code
is a type of speech, and therefore should enjoy the same
legal protections as books and music.
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Cellular Phones
• Every cellular phone has an electronic serial number to provide
unique identification of the phone. When a call is made, the cellular
phone transmits its identification information. This allows the phone
system to provide continuous service to the caller as the caller
travels from one place to another, and it also allows the call to be
billed accordingly. Someone who captured this information could
"clone" the cellular phone by copying its ID into another phone. As a
result, its calls would then be billed to the original owner. In order to
prevent this, digital cellular phones use encryption to protect the
phone's identity. (Analog cellular phones, based on older technology,
do not use encryption.) Because people often use the pushbuttons
on a phone to enter private data such as bank account numbers and
credit card numbers, all digits dialed on a digital phone are also
encrypted.
• Cellular phones use relatively weak encryption. Recently several
cellular phone encryption schemes have been cracked by
independent cryptography experts, but even this weak encryption is
sufficient to deter casual eavesdroppers.
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6.2.3 Authentication
•
•
•
•
•
Strong Passwords
Smart Cards
Biometrics
Digital Signatures
Digital Certificates and Certificate
Authorities
• SSL Protocol
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Authentication 证明, 鉴定
• Authentication is the process of
confirming an identity, determining
whether you are who you claim to be. This
is usually done by verifying that your login
name and password match. However,
passwords can often be stolen,
accidentally revealed, or forgotten.
•
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Strong Passwords
• Characteristics of a good password:
• Difficult to guess
• At least 8 characters long, the longer the better (if
you can remember it)
• Contains a mix of uppercase letters, lowercase
letters, numbers, symbols, and punctuation marks
• Characters are arranged in an unpredictable order
• Can be typed in quickly by you to prevent someone
from obtaining your password by looking at your
key strokes
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Strong Passwords
• Characteristics of a bad password:
• Based on personal information such as all or part of your name,
nickname, birth date, company name, and relative's name
• Based on surrounding objects, such as "computer", "desk“
• Words from a dictionary
• Names of fictional characters from movies or books
• Words spelled in a particular patter (e.g. with the last letter
omitted, backwards)
• Character sequence that is easy to type, such as "asdf" and
"qwer"
• Characters that follow a certain pattern such as
"abcabcdabcde" and "1122334455"
• Passwords you have seen or used previously
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Strong Passwords
• Generating a good password:
• Use a password generator application
• Use the third letter of each word (more than 2 characters
long) from a randomly selected sentence
Example:
– Sentence: "AUTHENTICATION is the process of confirming an
identity, determining whether someone is who he claims to be."
– Password: "Teonetemoa."
• Insert symbols randomly (e.g. "Te*netem$a.")
• Mix of uppercase letters, lowercase letterers, numbers,
symbols, and punctuation marks (e.g. "T1e*netEm$a.")
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Smart Cards
• A smart card is a credit card-sized plastic
card with an embedded integrated circuit
chip. It serves as a secure medium for
storing important personal identification
information such as picture identifications,
voiceprints, fingerprints, signatures, and
account information.
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Smart Cards
• Smart card chips can be embedded on phone cards, banking
cards or health cards. Using the phone card as an example,
the smart card chip is embedded on the phone card, which
contains your phone account number and other related
information. To use the card, you would slide the card into the
card reader usually embedded as part of the public phone.
The card reader would prompt for a PIN number, and then it
would read off the information from the smart card to
authenticate you.
• Similarly, a smart card that stores login and password
information can be used to log onto computer systems. To
access a computer system, you would insert the smart card
into a card reader connected to the computer system. After
providing your PIN number, the system will authenticate you
based on the information on the smart card and the PIN
number you provided.
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Biometrics 生物测定学
• Biometrics refers to the automatic identification of a
person based on his/her physiological or behavioral
characteristics. Among the features measured are face,
fingerprints, handwriting, iris 虹膜, and voice. Biometric
technologies are being developed to enhance identity
verification solutions. This method of identification is
more secure compared to traditional methods involving
passwords and PIN numbers because the person to be
identified is required to be physically present at the pointof-identification. Also, it is more convenient because it
eliminates the need to remember passwords or PINs, or
carry identification cards.
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Digital Signatures数字签名
• Just as you would sign a document to indicate to the
receiver of your message that the message came from
you, you can attach a digital signature to an electronic
document to indicate that the message did indeed
originate from you. A digital signature must be both
unique to the sender and unique to the particular
message so that it can be verified but not reused. For
instance, if you were to send an email request to have
your friend deposit $2000 to your account, you may
attach a digital signature to your email request so that
your friend can trust that the request did come from you
and the account information included in the email is not
altered.
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Digital Signatures数字签名
• Sender:
• 1. Transform the entire message using a
hashing algorithm to generate a hash of
the message.
• 2. Generate the digital signature by
encrypting the message hashed using the
sender's private key.
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Digital Signatures数字签名
• Receiver:
• 1. Transform the entire message using a hashing
algorithm to generate a hash of the message.
• 2. Decrypt the digital signature using the sender's public
key.
• 3. Compare the message hashed, and the decrypted
digital signature, which should be the same as the
sender's message hashed. If these two hashes are the
same, then the receiver can trust that the message was
sent from the sender and that the message was not
altered during transmission. Otherwise, the message
may have been altered.
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Digital Certificates数字证书
• Just as a driver‘s license or a passport
is used to identify a person, a digital
certificate is used to identify an
individual, a server, a company, or
some other entity. A digital certificate
is an electronic identity document
whose purpose is to help prevent
impersonation.
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Certificate Authority
• To extend this analogy, just as you would go to a
government agency to obtain a passport, you would go
to a certificate authority (CA) to obtain a digital
certificate. A CA is a trusted third-party organization or
company that validates identities and issues certificates.
The certificates are used to associate public keys with
entities (e.g. organizations, people). The role of the CA in
data security is important, as electronic exchange of data
becomes a necessity for communications and commerce.
For example, electronic-commerce sites such as
Amazon.com and BestBuy.com have digital certificates
so that users can trust that they are using legitimate sites
instead of a fraudulent site created by hackers.
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Digital
certificate
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Certification Path
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Certificate warning
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PKI
• The set of standards and services that
govern the use of public-key cryptography
and the system of certificates is called
Public Key Infrastructure (PKI).
• Issuance 发行of digital certificates to
individual users and organizations
• Integration with corporate certificate
directories; tools for managing, renewing,
and revoking certificates
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SSL Protocol
• SSL (Secure Socket Layer) is a protocol layer
that operates on top of TCP/IP to provide
encrypted communications. The protocol is a set
of rules governing server authentication, client
authentication, and encrypted communication
between servers and clients. SSL uses public
key cryptography to transmit a unique session
key for each connection. It then uses a faster,
symmetric encryption algorithm to encrypt
whatever information the application needs to
transmit.
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SSL Protocol
• When you communicate with a Web server using the
prefix https instead of http, your browser establishes an
SSL connection. Many websites such as Amazon.com
use this technique to keep your password and personal
data secure from others. You will also see a lock icon
appear at the bottom right of the web page window
indicating the data on the page came to you encrypted
via a SSL protocol. If do not see a lock icon when
transmitting sensitive information, you should be aware
that your data was not encrypted, and may be seen by
eavesdroppers. The image below highlights the URL that
begins with "https" and the lock icon, indicating that the
data on the page will be transmitted securely.
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6.3 Prevention, Detection, and
Recovery
•
•
•
•
•
Reading Sequence:
6.3.1 Firewall.
6.3.2 Intrusion Detection Tools.
6.3.3 Data Recovery.
Textbook, Section 4D. Learning Goal: Learn
about disk backup procedures that provide for
the recovery of lost or corrupted files.
• 6.3.4 Summary of Security Tips.
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6.3.1 Firewall
•
•
•
•
•
•
Application Gateway
Packet Filter
Application Gateway versus Packet Filter
Hybrid
Intruder Attacks Prevented by Firewall
Setting up a Firewall
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Firewall
• A firewall is a program or hardware device that
protects your network by filtering incoming
packets from the Internet entering your
protected network or computer system. It can
be used as a cost-effective method to protect a
computer at home, or within an organization. It
provides a level of separation between your
protected system and the Internet.
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Application Gateway应用网关
• The first type is an application gateway.
Instead of allowing corporate hosts to
communicate directly with external hosts,
communication has to go through an application,
called a proxy代理服务器, running on or directly
behind the firewall.
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Packet Filter包过滤
• It uses information in the header of every packet
to decide whether a packet is acceptable to pass
the firewall. A packet contains the address of its
sender, the address of the destination, and data.
If a packet's source or destination is not
acceptable, the packet is blocked, and a record
is entered in a log that can be inspected by the
network manager.
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6.3.2 Intrusion Detection Tools
• Intrusion Detection Systems
• Network Monitoring Tools
• Anti-Virus Software
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Intrusion Detection Systems
• An intrusion detection system (IDS) for a computer is like
a security system for a home. When an intrusion is
detected, the IDS alerts the network administrators, just
as the alarm of the home security system would go off to
alert the homeowners. An IDS gathers and analyzes
information within a computer or a network to identify
possible security breaches. In case of a security breach,
an IDS can provide traces of events to help track down
the intruder. However, an IDS does not block potentially
malicious traffic. When used with a firewall, an IDS can
verify the firewall configurations and serve as an added
layer of security that alerts network administrators of
suspicious data that passed through the firewall.
Compared to firewall, IDS provides more in-depth traffic
monitoring.
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Intrusion Detection Systems
• An IDS works by matching incoming traffic against an
intrusion detection (ID) signature database of known
attacks or suspicious activity, and alerts administrators
when a match occurs. An ID signature can be a special
TCP state, special bytes in the IP header, or a special
byte stream in a packet. IDS can also send automatic
notifications to alert system administrators of potential
security breaches via a variety of channels, including
email and mobile phones. These notifications would help
network administrators identify subsequent steps for
resolving the security breach and they can help
strengthen security policies.
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Intrusion Detection Systems
Strengths of IDS:
• Can trace跟踪 each step of an attack
• Cannot be easily circumvented包围
Weaknesses of IDS:
• Cannot block intruding traffic
• Only as strong as its signature database
• Possibility of false alarms
• Set up may require some level of configuration
and security knowledge
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Setting up an IDS
• Typically, an IDS is
set up behind the
firewall to examine
more thoroughly
packets that have
filtered through the
firewall.
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Network Monitoring Tools
• Network monitoring should be conducted continuously to
maintain confidence in the security of a protected
network and data resources. Network monitors may be
installed at strategic locations to collect and examine
information continuously that may indicate suspicious
activity. Some systems can react to suspicious network
activity by blocking suspect connections, limiting or
disabling affected services, isolating affected systems,
and collecting evidence for subsequent analysis.
Additionally, monitoring can help determine whether
security countermeasures are effective.
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Anti-Virus Software
• In general, an anti-virus software program
searches through files (or memory) for
indications of malicious code using a virus
profile (sometimes called "signatures")
provided by the vendor. Anti-virus software
programs can also remove viruses from
files, isolate files that cannot be cleaned,
and discover Trojan horse programs and
worms. Some anti-virus software can
recover data deleted by viruses.
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Anti-Virus Software
• Because new viruses are discovered daily, the
effectiveness of anti-virus software is dependent upon
having the latest virus profiles installed on your computer.
Some virus tool vendors provide subscription services or
other distribution facilities to help customers keep up to
date with the latest viral signatures.
Strengths of anti-virus software:
• Protect users from email borne malicious code that can
go unnoticed by IDSs and firewalls
• Can detect a network-propagated virus infection
Weaknesses of anti-virus software:
• Must be updated frequently
• Does not protect against all network-based exploits
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6.3.3 Data Recovery
• In case of a security breach where your data is
compromised, tampered with, corrupted, or deleted, you
need to be sure you have a backup, or a copy of your
data to restore your computer system. Data that can be
compromised include files, application programs, and the
operating system. Backing up your data is one of the
most important actions in securing your computer
system. Having a backup of your data not only mitigates
data loss caused by intruder attacks, backups also help
recover from losses caused by unintentional deletion of
data, software malfunction, hardware malfunction, and
natural disasters.
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Backup
• In general, you should back up your system:
• Periodically (for example, monthly, weekly, every
other day, everyday) depending on how
frequently your data changes
• Before you make potentially 潜在地 irreversible
不能撤回的changes to your data such as
changing file structures, deleting large amounts
of data
• Before installing hardware or software
components that may alter your data
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Backup
• Backups should be kept on a removable
medium or stored at a remote site in case
of natural disasters or physical destruction
of computer equipment. When performing
virus scans or data integrity checks on
your system, be sure to perform it on your
back ups too. It is important that the
backed up data is not compromised or
corrupted.
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Backup
• Differential backup差分备份:A copy of all the
data that changed since the last full backup
• Incremental backup增量备份: A copy of all
the data that changed since the last full,
differential, or incremental back up. Note
that the first incremental backup is also
a differential backup from the full
backup.
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Comparing backup methods
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Example 1
• organization that does a full backup every Friday
at 5 P.M. and a differential backup every
Wednesday at 5 P.M.
• Incidence: data failure on Thursday at 6 P.M.
• Data recovery method: restore data from last
Friday's full backup and then Wednesday's
differential backup.
• Data loss: data after Wednesday's differential
backup and before data failure Thursday- about
25-hours worth of work on the machine
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Example 2
• organization that does a full backup every Friday
at 5 P.M., a differential backup every Wednesday
at 5 P.M., and an incremental backup every
other day at 5 P.M.
• Incidence: data failure on Thursday at 6 P.M.
• Data recovery method: restore data from last
Friday's full backup, then Wednesday's
differential backup, and finally Thursday's
incremental backup.
• Data loss: data after Thursday's incremental
backup- about one hour worth of work on the
machine
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• Notice that it may take more time to recover data if many
incremental backups were done, but the amount of data
lost would be less. When determining how often you
should backup your data, ask yourself how many hours
worth of work on the computer are you willing to risk
losing? For example, if your answer is 24 hours, then
you should back up everyday. If your answer is 48 hours,
you should back up every other day. When choosing a
backup method, keep in mind the relative amount of time
each backup method takes to complete when you would
conduct the backup. For example, a full backup would
take the longest amount of time; therefore, they are
usually performed during hours when the computer
system is not being used as much. At the minimum, full
backups should be performed on a regular basis.
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6.3.4 Summary of Security Tips
• There are many ways to violate your data integrity, breach your data
confidentiality, and interrupt your system availability through
malicious acts, user errors, and disasters. The following is a list of
security practices that can help you protect your computer system:
• Keep applications and operating system updated with the latest
security patches (vendors such as Microsoft are continually fixing
vulnerabilities and posting updates to their customers)
• Turn off your computer or disconnect from the network when not in
use
• Close all unused Internet ports
• Install firewalls to protect your network boundary in order to keep
unwanted traffic and users out of the network
• Use security software such as firewall and intruder detection
systems
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Security Tips
• Practices against privacy violation and
identity theft:
• Use strong passwords that are 8 or more
characters long, consisting of a mix of
upper and lower case letters, numbers,
and punctuation marks
• Encrypt sensitive data such as passwords
and credit card numbers
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Practices against malicious code:
• Avoid opening unexpected email attachments. If you must, be sure
your virus definitions are up-to-date, save the file to your hard disk,
scan the file using your anti-virus software, and then open the file
• Expose hidden filename extensions
• Exercise caution when downloading unauthenticated software. If
you choose to install software that has not been signed by a trusted
source, you may wish to wait for a period of time before installing it
in order to see if a Trojan horse is discovered
• Disable Java, JavaScript, and ActiveX if possible- a malicious web
developer may attach a script to the content sent from a web site.
The malicious script is transferred to your browser, where it can
execute and compromise your machine. However, disabling Java,
JavaScript, and ActiveX may impair your browsing experience
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Practices against malicious code:
• Disable scripting features in email programs if possibleemail programs use the same code as web browsers to
display HTML, therefore vulnerabilities that affect
ActiveX, Java, and JavaScript are often applicable to
email as well as web pages
• Turn off macros in applications
• Install anti-virus software and keep it up-to-date on user
machines and on email servers
• Consult your system support personnel if you work from
home to conform with employer's policies or procedures
relating to the security of your home network
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Practices against malicious code:
• Practices against data failure, corruption, or deletion:
• Develop a recovery plan in case your system is compromised, or
destroyed
• Perform and verify backups
• Make a boot disk before your computer is damaged or compromised
• If your computer is compromised:
• Disconnect it from the network and restore from backups
• Verify the integrity of the entire computer (software, configuration
files, logs, data, etc.)
• If integrity cannot be verified, reinstall operating system and
applications
• Apply all security patches and workarounds before reconnecting to
network
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Practices against malicious code:
• It is also possible to safeguard your system so that traffic
other than that which is necessary cannot pass through your
system. But, keep in mind, you may be limiting your system's
ability to interact with other systems or users (for example,
filtering out email attachments). Or, you can go to the
extremes to avoid security threats and limit your use of the
computer by disconnecting from the network. That, however,
would severely limit the functionality of your computer. When
setting up a computer system, there is a balance that you will
need to establish between risk and reward. You should
estimate how much risk you are willing to take in order to
obtain the functionality that you desire for your computer
system. In general, when safeguarding your computer system,
make sure you follow a reasonable security policy advised by
security experts.
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Exercise 6
• Question 1. Malicious Email
• Items a-g below are email scenarios where you are the
receiver of the email. Answer the questions in each item
and justify your answers.
• a. You received an unexpected email with an
attachment named, "SecurityPatches4You.exe", from an
unknown sender. What type(s) of malicious code (virus,
worm, or Trojan horse) may be included in the email?
How did the email reach you? Would you open the
attachment?
• b. You received an unexpected email with an
attachment named, "SecurityPatches4You.exe", from
your friend Joe. What type(s) of malicious code (virus,
worm, or Trojan horse) may be included in the email?
How did the email reach you from Joe? Would you open
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Exercise 6
• c. You received an unexpected email with an
attachment named, "finalDraft.doc", from your
friend Joe. What type(s) of malicious code
(virus, worm, or Trojan horse) may be included
in the email? How did the email reach you from
Joe? Would you open the attachment?
• d. You received an expected email with .doc
attachment from your friend Joe (assume
hidden extensions are exposed). What type(s) of
malicious code (virus, worm, or Trojan horse)
may be included in the email? How did the email
reach you? What would you do and why?
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Exercise 6
• e. You received an unexpected email with no
attachment from your friend Joe. The email instructs
you to click on the enclosed URL. What type(s) of
malicious code (virus, worm, Trojan horse, or mobile
code) may be included in the email? How did the email
reach you fro Joe? What would you do and why?
• f. You received an unexpected, digitally signed email
with no attachment from your friend Joe. The email
instructs you to deposit $10,000 to Joe’s bank account.
How did the email reach you? What would you do and
why?
• g. You received an expected, digitally signed email
without an attachment from your friend Joe. Joe also
confirmed that he sent the email. The email instructs you
to deposit $20,000 to Joe’s bank account. How did the
email reach you? What would you do and why?
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Exercise 6
• Question 2. Cryptography
• In this section, you will encode/decode
simple messages using different
cryptography methods.
• a. Consider the alphabet shift cipher.
Complete the following table:
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Exercise 6
Plaintext
Encryption
Technique
Software
Shift right by 4
characters
Shift right by 3
characters
Ciphertext
Vbvwhpv
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Exercise 6
• b. What is the maximum number of ways you can
replace a character using the above alphabet shift cipher?
What is the size of the key space? What is a weakness
of the alphabet shift cipher above?
• c. How does using a secret passphrase as the
encryption key compare to using the alphabet shift
cipher?
• Let us try using a passphrase to encrypt the message
"Meet at noon." with a larger key space. The key is
"CBA". In this case, only encrypt alphabet letters.
• d. What is the encrypted message?
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Exercise 6
• Now let us explore how to use private/public
keys and digital signature.
Suppose Jean is sending her friend Erin a secret
note with a digital signature.
• e. How does Jean include a digital signature?
What key does Jean use to encrypt the entire
message including the digital signature?
• f. How does Erin decrypt the message, and
verify that the message was from Jean?
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Exercise 6
• Question 4. Data Backups
• Consider the process of restoring data to a server from
backup media. Assume the server failed Friday,
November 14, at 3:00 P.M.
• For each backup procedure below, indicate what data is
lost, and describe the procedures for restoring data.
• a. Full backup every Friday at 6:00 P.M.
• b. Full backup every Friday at 6:00 P.M. and differential
backup every Wednesday at 6:00 P.M.
• c. Full backup every Friday at 6:00 P.M. and incremental
backup every Monday, Tuesday, Wednesday, and
Thursday at 6:00 P.M.
•
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