Network Security. Chapter 12. Learning Objectives. Chapter Outline. After reading this chapter, you should be able to:

Transcription

1 Network Security Chapter 12 Learning Objectives After reading this chapter, you should be able to: Recognize the basic forms of system attacks Recognize the concepts underlying physical protection measures Cite the techniques used to control access to computers and networks Cite the strengths and weaknesses of passwords Cite the techniques used to make data secure Explain the difference between a substitution-based cipher and a transposition-based cipher Outline the basic features of public key cryptography, Advanced Encryption Standard, digital signatures, and the public key infrastructure Cite the techniques used to secure communications Recognize the importance of a firewall and be able to describe the two basic types of firewall protection Recognize the techniques used to secure wireless communications List the advantages to a business of having a security policy Chapter Outline 1. Introduction 2. Standard System Attacks 3. Physical Protection 4. Controlling Access a. Passwords and ID systems b. Access rights c. Auditing 5. Securing Data a. Basic encryption and decryption techniques

2 6. Securing Communications a. Spread spectrum technology b. Guarding against viruses c. Firewalls d. Wireless security 7. Security Policy Design Issues 8. Network Security In Action: Making Wireless LANs Secure 9. Summary Lecture Notes Introduction Computer network security has reached a point at which it can best be characterized by two seemingly conflicting statements: Never has network security been better than it is today, and never have computer networks been more vulnerable than they are today. How both these statements can be true is an interesting paradox. Network security, as well as operating system security, has come a long way from the early days of computers. Standard System Attacks Malicious computer users who try to break into a computer system often start with a standard set of system attacks. They hope that the system administrator has not properly secured the system and has left it vulnerable to attack. The two leading methods of attacks have been exploiting known operating system vulnerabilities and exploiting known vulnerabilities in application software. Another category of common system attacks is denial of service. Denial of service attacks bombard a computer site with so many messages that the site is incapable of performing its normal duties. In bombing, a user sends an excessive amount of unwanted to someone. If the has a return address of someone other than the person sending the , then the sender is spoofing. Physical Protection All computer systems need to be physically protected. Whether the system is a simple personal computer in your home or a major computer network such as the Internet, it is necessary to protect the hardware and software from theft, destruction, and malicious acts of vandalism. Surveillance can be used to monitor activity and deter theft.

3 Controlling Access Controlling access to a computer network involves deciding and then limiting who can use the system and when the system can be used. Network administrators can control access rights, enforce password and ID systems, and perform auditing. Securing Data Many times when storing data and when transferring data from one point to another in a computer network, it is necessary to ensure that the transmission is secure from anyone eavesdropping on the line. The term secure means two things. First, it should not be possible for someone to intercept and copy an existing transmission. Second, it should not be possible for someone to insert false information into an existing transmission. Cryptography is the study of creating and using encryption and decryption techniques. Basic cryptography uses substitutionbased ciphers (that replace one or more characters with one or more characters) and transposition-based ciphers (that rearrange the order of the characters). Public key infrastructure (PKI) is the combination of encryption techniques, software, and services that involves all the necessary pieces to support digital certificates, certificate authorities, and public key generation, storage, and management. A company that adheres to the principles of PKI issues digital certificates to legitimate users and network servers, supplies enrollment software to end users, and provides the tools necessary to manage, renew, and revoke certificates. Steganography, the practice of hiding bits of secret messages within other documents, is another approach to making data secure. Securing Communications Along with securing data, it is also necessary to secure the communications transmitted between computers. Using a spread spectrum transmission system, it is possible to transmit either analog or digital data using an analog signal. However, unlike other encoding and modulation techniques, only an intended receiver with the same type of transmission system can accept and decode the transmissions. The idea behind spread spectrum transmission is to bounce the signal around on seemingly random frequencies rather than transmit the signal on one fixed frequency. Anyone trying to eavesdrop will not be able to listen because the transmission frequencies are constantly changing. Most computers and networks support some form of virus detection software in an attempt to identify and capture virus-laden messages.

4 A firewall is a system or combination of systems that supports an access control policy between two networks. The two networks are usually an internal corporate network and an external network, such as the Internet. A firewall can limit users on the Internet from accessing certain portions of a corporate network and can limit internal users from accessing various portions of the Internet. Firewalls come in two basic types: packet filter, or network level, and proxy servers, or application level. Security Policy Design Issues When designing a firewall system and its corresponding security policy, a number of questions should be answered. The first question involves the company s expected level of security. Is the company trying to restrict all access to services not deemed essential to the business? Or does the company wish to allow all or most types of transactions, thus asking the firewall system only to audit transactions and create an orderly request for transactions? A second question stems from the first decision: How much money is the company willing to invest in a firewall system? A third question relates to the company s commitment to security. If the company is serious about restricting access to the corporate network through a link such as the Internet, will the company be equally serious about supporting security on any and all other links into the corporate network environment? Network Security In Action: A Wireless LAN The In Action example for this chapter returns to the example presented in Chapters 7 through 9. Hannah has to decide if she wants to add wireless capability to her local area network. She must consider all the protocols that support the LAN, including security protocols. Quick Quiz 1. What are the different techniques you can use to authenticate a user? Passwords, badges, fingerprints, voiceprints, faceprints, retina scan, and irisprint, to name a few 2. What are the two major forms of cryptography? Substitution-based ciphers and transposition-based ciphers 3. How can a digital certificate be used? It can be assigned to a document so that the owner can later verify ownership. 4. What are the basic ingredients of public key infrastructure? Encryption techniques, digital certificates, certificate authorities, public key generation, storage, and management

5 Discussion Topics 1. What parts of the body can be used for identification? Are any of these an infringement on privacy? 2. What are some examples of video camera surveillance? Are any of these pushing the limits of privacy? 3. The hackers that break into systems and disrupt Web site services: are they criminals, or are they heroes helping computer specialists discover faults within computer networks and systems? 4. Can the U.S. government really stop advanced encryption techniques from falling into the hands of criminals? 5. List several uses of steganography. Is this technology virtually unstoppable? Teaching Tips 1. When discussing viruses, show students a Web site (such as that discusses virus hoaxes. Solutions to Review Questions 1. How do hackers exploit operating system vulnerabilities? By launching a virus that attacks something about the operating system 2. What is a Trojan horse? A malicious piece of code that is hidden in a normal piece of code 3. How does a denial of service attack work? Bombards a selected site with an overwhelming number of messages 4. What is spoofing, and how does it apply to a denial of service attack? They substitute a fake IP address in the place of their IP address in the Source IP Address field of the IP header. 5. What is a ping storm, and how does it apply to a denial of service attack? A ping storm is when a user uses the TCP/IP ping command to constantly bombard a site.

6 6. List three forms of physical protection. Protection from fire, heat, flooding, and theft 7. How can surveillance be used to improve network security? It can be used to deter crime and to catch a criminal after the fact. 8. How does an intrusion detection system work? It watches for someone trying to attack a system and either alerts an administrator and/or begins to close out portions of the system. 9. What is the major weakness of a password? What is its major strength? Someone else can discover it. It s easy to pick a difficult one, and it can be changed easily and frequently. 10. What are the most common types of access rights? Who and how. Who: user or owner, group, system, world. How: read, write, execute, print, delete, copy, rename, append. 11. How can auditing be used to protect a computer system from fraudulent use? It can be used to deter crime, and it can catch a criminal by tracing his or her transactions. 12. Describe a simple example of a substitution-based cipher. Something in which one or more characters are replaced with one or more characters 13. Describe a simple example of a transposition-based cipher. Anything that reassembles the text into a new position 14. How can public key cryptography make systems safer? You don t have to give out your decryption key to allow someone to send you encrypted data. 15. Give a common example of an application that uses a secure sockets layer. Sending your credit card information over the Internet is very common. 16. What is the Data Encryption Standard? A standard that applies a 56-bit key to 16 levels of encryption

7 17. How is the Advanced Encryption Standard different from the Data Encryption Standard? Uses a vastly superior encryption algorithm and a much larger key 18. What is a digital signature? A digital signature is a hash of a document that has been encrypted with a private key. 19. What kind of applications can benefit from Pretty Good Privacy? Basically anything, such as transfers and storage of documents. 20. Is Kerberos a public key encryption technique or a private key? Explain. Private key. There is only one key used to both encode and decode. Thus, you have to keep the one key secret, or private. 21. List the basic elements of public key infrastructure. Encryption techniques, digital certificates, certificate authorities, public key generation, storage, and management 22. What kind of applications can benefit from Public Key Infrastructure? Any transaction that requires a secure transfer of information 23. What kind of entity issues a certificate? A certificate authority 24. Under what circumstances might a certificate be revoked? Normal expiration, nonpayment of fees, security breech 25. How is steganography used to hide secret messages? By taking a little bit of the secret message and hiding it somehow within another document or file 26. What are the two basic techniques used to create a spread spectrum signal? Direct sequence and frequency hopping

8 27. What is a computer virus, and what are the major types of computer viruses? Parasitic, boot sector, stealth, polymorphic, and macro 28. What are the different techniques used to locate and stop viruses? Signature-based scanner, terminate-and-stay-resident antivirus software, multi-level generic software 29. What is the primary responsibility of a firewall? To keep out malicious attacks and to keep internal users from accessing certain outside services 30. What are the two basic types of firewalls? Packet filter and proxy server 31. What are the advantages of having a security policy in place? Everyone employees, management, external users knows the score. Suggested Solutions to Exercises 1. A major university in Illinois used to place the computer output from student jobs on a table in the computer room. This room is the same computer room that housed all the campus mainframe computers and supporting devices. Students would enter the room, pick up their jobs, and leave. What kinds of security problems might computer services encounter with a system such as this? Dirt, dust, moisture, smoke, theft 2. You have forgotten your password, so you call the help desk and ask them to retrieve your password. After a few moments, they tell you your forgotten password. What has just happened and what is its significance? Normally passwords are stored in the computer in an undecipherable form. Apparently in this system they were not, which means anyone might be able to find the password file and dump its contents. 3. Create (on paper) a simple example of a substitution-based cipher. 4. Create (on paper) a simple example of a transposition-based cipher.

9 5. Using the Vigenére Cipher and the key NETWORK, encode the phrase this is an interesting class. GLBOW JKAMG PSIOF XBJUT VNWL 6. Using the transposition-based cipher from this chapter and the same key, COMPUTER, encode the phrase birthdays should only come once a year. BSNN ADEA RHYE ISLC TOCA YOOR DLME HUOY 7. You are using a Web browser and want to purchase a music CD from an electronic retailer. The retailer asks for your credit card number. Before you transfer your credit card number, the browser enters a secure connection. What sequence of events created the secure connection? The server sends your browser a certificate, your browser selects an algorithm and creates a private key, the browser encrypts its private key with the server s public key, and the browser sends an encrypted private key back to server. 8. You want to write a song and apply a digital signature to it so that you can later prove it is your song. How do you apply the signature, and later on, how do you prove the song is yours? You take the song, convert it to a digital form, take the hash of the form, and apply a private key to the hash. Then you save the encrypted hash. If someone questions ownership at a later date, you decrypt the hash and rehash the song, comparing the hashes. 9. List three examples (other than those listed in the chapter) of everyday actions that might benefit from applying PKI. Many possible answers here, including banking, stock markets, insurance applications, school registrations, other financial transactions, major purchases. 10. Can a firewall filter out requests to a particular IP address, a port address, or both? What is the difference? Both. The IP address would be the address of a device connected to the Internet, while a port address would be the address of a particular application on a machine. You might want to restrict all access to a particular machine or just restrict access to particular applications on a machine. 11. One feature of a firewall is its ability to stop an outgoing IP packet, remove the real IP address, insert a fake IP address, and send the packet on its way. How does this feature work? Do you think it would be effective? Firewall keeps a table of fake IP addresses, pulls out real address and inserts a fake one. This is usually an effective technique.

10 12. How does the size of a key affect the strengths and weaknesses of an encryption technique? Consider both a friendly use of the key and an unfriendly use of the key. Clearly, the bigger the key, the harder it is (more possible combinations) to crack. From an unfriendly point of view, large keys make it virtually impossible to guess. From a friendly point of view, larger keys are harder to remember, especially since you don t want to place a key on paper. 13. Assume a key is 56 bits. If it takes a computer seconds to try each key, how long will it take to try all possible keys? What if 10,000 computers are working together to try all keys? 256 equals x combinations, times seconds per combination, equals x seconds. That equals 548,383.5 years. If 10,000 computers are working together, that comes down to 54.8 years. 14. What are the answers to the questions in Exercise 13 if the key is 128 bits in length? equals x 1038 combinations. At seconds per combination, that equals x seconds. That equals 2.59 x years. With 10,000 computers, that is still 2.59 x years! 15. You want to hide a secret message inside an image file using steganography. You have decided to place one bit at a time from the message into the image s pixels. How are you going to select the pixels? Will they be random or all in a row? And once a pixel is chosen, which bit are you going to replace with the bit from the secret message? Why? Random would be the hardest for anyone to find, including the one that is supposed to find the message. So you would probably have to use a pseudo-random sequence one that appears to be random to an intruder, but isn t. If you select the right-most bit of a pixel (the least significant bit), you should cause the least effect to the image. 16. Why can t a truly random sequence be used in a frequency hopping spread spectrum system? Because if it was truly random, nobody would be able to follow it, including the good guys. Thinking Outside the Box 3. You are working for a company that allows its employees to access computing resources from remote locations and allows suppliers to send and receive order transactions online. Your company is considering incorporating PKI. How would you recommend that PKI be implemented to support these two application areas? I would recommend hiring a third-party company to support your PKI.

11 4. You have a computer at home with a wireless NIC and wireless router. List all the security measures that should be employed so that your home network is secure. Set up a firewall to block illegal port access. Turn on and use the best encryption available on router. Install anti-spyware, anti-spam, antivirus software. 5. Your supervisor has asked you to explore the concept of ID management for the company. What is involved? How does it pertain to the topic of security? Is it a reasonable concept or a concept too new for a functioning business? It is reasonable to consider. ID management involves the decision of password versus ID card versus biometric requirement, etc. Then once the form of ID is decided, how are they managed?