数据通信：网络安全

1 .Chapter 12 Network Security

2 .2 There are many different types of viruses, such as parasitic, boot sector, stealth, polymorphic, and macro. A Trojan Horse virus is a destructive piece of code that hides inside a harmless looking piece of code. Sending an e-mail with a destructive attachment is a form of a Trojan Horse virus. Guarding against viruses Signature-based scanners look for particular virus patterns or signatures and alert the user. Terminate-and-stay-resident programs run in the background constantly watching for viruses and their actions. Multi-level generic scanning is a combination of antivirus techniques including intelligent checksum analysis and expert system analysis Viruses & Worms Update OS (Windows) patches & virus definitions file

3 .3 Standard System Attacks (I) Two leading forms of attacks the last few years: Exploiting known operating system vulnerabilities Exploiting known vulnerabilities in application software For both of these, software company issues a patch Patch may fix it, or introduce even more holes Either way, bad guys find new holes and exploit Very common way to attack vulnerability is via e-mail attachment You open the attachment and launch the virus Second common way to attack is to simply scan your computer ports while you are connected to the Internet (either dial-up or non-dial-up) If you have an open port, hacker will download malicious software to your machine

4 .4 Denial of service attacks or distributed denial of service attacks Bombard computer site with so many messages that site is incapable of answering valid request E-mail bombing User sends an excessive amount of unwanted e-mail to someone Botnets Malicious programs that take over operations on a comprised computer Smurfing Nasty technique in which a program attacks a network by exploiting IP broadcast addressing operations Ping storm Condition in which the Internet ping program is used to send a flood of packets to a server Spoofing When a user creates a packet that appears to be something else or from someone else Standard System Attacks (II)

5 .5 Trojan Horse Malicious piece of code hidden inside a seemingly harmless piece of code Stealing, guessing, and intercepting passwords is also a tried and true form of attack Phishing Hackers create emails which look as if they are coming from a legit source when in reality the hacker is trying to get the user to give up ID and password info Pharming Hacker redirects unknowing user to bogus look-alike website Rootkit A program that has been installed deep within a user’s operating system; defies detection and takes over the user’s computer Keylogger A software system that secretly captures and records keystrokes made at a user’s keyboard Standard System Attacks ( III )

6 .6 Physical Protection Physical Protection: Protection from environmental damage such as floods, earthquakes, and heat Physical security such as locking rooms, locking down computers, keyboards, and other devices Electrical protection from power surges Noise protection from placing computers away from devices that generate electromagnetic interference Surveillance: Proper placement of security cameras can deter theft and vandalism Cameras can also provide a record of activities Intrusion detection is a field of study in which specialists try to prevent intrusion and try to determine if a computer system has been violated Honeypot is an indirect form of surveillance Network personnel create a trap, watching for unscrupulous activity

7 .7 Controlling Access Controlling Access: Deciding who has access to what Limiting time of day access Limiting day of week access Limiting access from a location, such as not allowing a user to use a remote login during certain periods or any time

8 .8

9 .9 Passwords and ID Systems Passwords are the most common form of security and the most abused Simple rules help support safe passwords, including: Change your password often Pick a good, random password (minimum 8 characters, mixed symbols) Don’t share passwords or write them down Don’t select names and familiar objects as passwords Many new forms of “passwords” are emerging (biometrics): Fingerprints Face prints Retina scans and iris scans Voice prints Ear prints

10 .10

11 .11 Access Rights Two basic questions to access right: who and how? Who do you give access right to? No one, group of users, entire set of users? How does a user or group of users have access? Read, write, delete, print, copy, execute? Most network operating systems have a powerful system for assigning access rights

12 .12 Auditing Creating a computer or paper audit can help detect wrongdoing Auditing can also be used as a deterrent Many network operating systems allow the administrator to audit most types of transactions Many types of criminals have been caught because of computer-based audits

13 .13 Cryptography is the study of creating and using encryption and decryption techniques. Plaintext is the data before any encryption has been performed. Ciphertext is the data after encryption has been performed. The key is the unique piece of information that is used to create ciphertext and decrypt the ciphertext back into plaintext. Monoalphabetic Substitution-based Ciphers Polyalphabetic Substitution-based Ciphers Transposition-based Ciphers Encryption and Decryption

14 .14 Very powerful encryption technique in which two keys are used: First key (public key) encrypts message Second key (private key) decrypts message Not possible to deduce one key from the other Not possible to break the code given to the public key If you want someone to send you secure data, give them your public key, you keep the private key Secure sockets layer (SSL) on Internet is a common example of public key cryptography Public Key Cryptography

15 .15 Data Encryption Standard (DES) Take a 64-bit block of data and subjected it to 16 levels of encryption The choice of encryption performed at each of the 16 levels depends on the 56-bit key applied Triple-DES Data encrypted using DES three times: the first time by the first key, the second time by a second key, and the third time by the first key again. (Can also have 3 unique keys.) CPU intensive Advanced Encryption Standard (AES) Selected by the U.S. government to replace DES National Institute of Standards and Technology selected the algorithm Rijndael (pronounced rain-doll) in October 2000 as the basis for AES More elegant mathematical formulas, requires only one pass, and was designed to be fast, unbreakable, and able to support even the smallest computing device Key size of AES: 128, 192, or 256 bits Estimated time to crack (assuming a machine could crack a DES key in 1 second) : 149 trillion years Very fast execution with very good use of resources Encryption Standards

16 .16 Document to be signed is sent through a complex mathematical computation that generates a hash Hash is encoded with the owner’s private key To prove future ownership, hash is: Decoded using owner’s public key Compared with a current hash of the document If the two hashes agree, the document belongs to the owner U.S. approved legislation to accept digitally signed documents as legal proof in 2000 Digital Signatures

17 .Pretty Good Privacy (PGP) Encryption software created by Philip Zimmermann Can be used to secure email and other data files Employs public key cryptography and digital signatures Available to anyone in the U.S. for free 17 17

18 .Kerberos An authentication protocol designed to work on client/server networks Employs private key cryptography (one key both encrypts and decrypts) Another free software for use in the U.S. Many operating systems provide Kerberos for authentication of users and services 18 18

19 .19 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 digital certificate is an electronic document that establishes your credentials when you are performing transactions A digital certificate contains your name, serial number, expiration dates, copy of your public key, and digital signature of certificate-issuing authority Certificates are usually kept in a registry so other users may check them for authenticity Certificates are issued by a certificate authority (CA). A CA is either specialized software on a company network or a trusted third party Public Key Infrastructure (I)

20 .20 Public Key Infrastructure (II) Ordering over the Internet The web server requests your browser to sign the order with your private key (obtained from your certificate) The web server then requests your certificate from the third party CA, validates that certificate by verifying third party’s signature, then uses that certificate to validate the signature on your order The user can do the same procedure to make sure the web server is not a bogus operation A certificate revocation list is used to “deactivate” a user’s certificate Applications that could benefit from PKI: World Wide Web transactions Virtual private networks Electronic mail Client-server applications Banking transactions

21 .21 Steganography The art and science of hiding information inside other, seemingly ordinary messages or documents Unlike sending an encrypted message, you do not know when steganography is hiding a secret message within a document Examples include creating a watermark over an image or taking “random” pixels from an image and replacing them with the hidden data

22 .22 Securing Communications One way to secure the transfer of data is to scramble the signal as it is being transmitted Spread spectrum technology: A secure encoding technique that uses multiple frequencies or codes to transmit data. Two basic spread spectrum technologies: Frequency hopping spread spectrum Direct sequence spread spectrum

23 .23 A system or combination of systems that supports an access control policy between two networks A firewall can limit the types of transactions that enter a system, as well as the types of transactions that leave a system Firewalls can be programmed to stop certain types or ranges of IP addresses, as well as certain types of TCP port numbers (applications ) Firewalls (I)

24 .24 Packet filter : firewall that is essentially a router and has been programmed to filter out or allow to pass certain IP addresses or TCP port numbers Proxy server : more advanced firewall that acts as a doorman into a corporate network Any external transaction that requests something from the corporate network must enter through the proxy server More advanced but make external access slower Application layer – inspects all packets coming into or leaving a connection using the application layer of the TCP/IP protocol suite Goes beyond IP addresses and TCP port numbers and inspects packet to see to which application it belongs Firewalls (II)

25 .25 Wireless Security WEP (Wired Equivalency Protocol) was the first security protocol used with wireless LANs It had weak 40-bit static keys and was too easy to break WPA (Wi-Fi Protected Access) replaced WEP. Major improvement including dynamic key encryption and mutual authentication for wireless clients Both of these should eventually give way to a new protocol created by the IEEE - IEEE 802.11i (WPA2) 802.11i allows the keys, the encryption algorithms, and negotiation to be dynamically assigned Also, AES encryption based on the Rijndael algorithm with 128-, 192-, or 256-bit keys is incorporated

26 .26 What is the company’s desired level of security? How much money is the company willing to invest in security? If the company is serious about restricting access through an Internet link, what about restricting access through all other entry ways? Company must have a well-designed security policy Security Policy Design Issues