4.1.3 La educación a Distancia en Estados Unidos, Latino América y los Métodos de Evaluación Utilizados.
4.1.3.1 La Educación a Distancia en Estados Unidos Actualmente Como la UNAD Florida tiene su sede en Estados Unidos, las leyes que le aplican en materia de
Most of prevention mechanisms in practical use focus on access and flow control on a computer and network system. Research efforts are also being undertaken to design secure computers and networks.
2.1.1 Access and flow control
Access and flow control technologies are not covered in detail in this book. Some representative examples of access and flow control technologies, specifically firewalls and authentication/ authorization, are briefly reviewed in this section.
Secure Computer and Network Systems: Modeling, Analysis and Design Nong Ye C
2008 John Wiley & Sons, Ltd
2.1.1.1 Two forms of firewalls: screening routers and application gateways
A firewall is usually installed on a router or an application gateway that controls incoming and outgoing traffic of a protected computer and network system. A firewall on a router, called a screening router, filters traffic data between a protected system and its outside world by defining rules which are applicable to mostly header fields of data packets at the TCP and IP layers of the TCP/IP protocol. The data portion of a network packet may not be readable due to the encrypted application data, and therefore is not usually used to define filtering rules in the firewall. A list of typical TCP/IP header fields is as follows:
r
Timer
Source IP addressr
Source portr
Destination IP addressr
Destination portr
Flags, a combination of TCP control bits: S (SYN), F (FIN), P(PUSH) and R(RST), or asingle ‘.’ for no flags
r
Sequence numberr
Acknowledge numberr
Length of data payloadr
Window size, indicating the buffer space available to receive data to help the flow controlbetween two host computers
r
Urgent, indicating that there is ‘urgent’ datar
Options, indicating TCP options if there are any.A filtering rule can look for specific types of values in one or more header fields, and allow or deny data packets based on these values. TCP/IP headers have information on the source IP address and port, the destination IP address and port, etc. Using the header information, a screening router can deny data packets from a specific source IP address, block data packets targeting specific network ports running vulnerable network services, prevent certain types of data packets such as those containing ICMP Echo Reply messages from going out, and so on. Table 2.1 shows some examples of filtering rules for a screening router.
Table 2.1 Examples of filtering rules for a screening router
Decision Source IP address Destination port Deny In a list of bad host computers Any
Allow Not in a list of bad host computers TCP port 80
Cyber attack prevention 27 A Protected Computer and Network System Proxy Gateway Screening
Router Outside Network
Figure 2.1 An example of a firewall configuration.
A firewall can also be installed on a computer running proxy network applications, called a proxy gateway or application gateway [1]. A proxy gateway transforms network data packets into application data by performing pseudo-application operations. Using information available at the application layer, a proxy gateway can block access to specific services (e.g., certain FTP commands) of an application, and block certain kinds of data (e.g., a file attachment with a detected virus) to an application. For example, a proxy gateway running a pseudo-FTP application can screen FTP commands to allow only acceptable FTP commands. Sophisticated proxy gateways are called guards that carry out sophisticated computation tasks for filtering. For example, a guard may run an email application, perform virus scanning on file attachments to emails, and determine whether to drop or allow file attachments.
Figure 2.1 shows a firewall configuration using both a screening router and a proxy gateway. The screening router performs the initial data filtering based on the header information that is available at the TCP/IP layers. The proxy gateway, which is a host computer inside a protected computer and network system, performs the further data filtering based on information that is available at the Application layer. The firewalls control the entire network perimeter of the protected computer and network system so that all traffic between the system and its outside network must pass through the firewalls. However, a modem on a host computer inside the protected system can be overlooked but be exploited by an attack to have traffic bypassing the firewalls through the modem and thus break the firewall protection of the system. Although limiting user access to given network services with known vulnerabilities through firewalls raises the difficulty of attacks exploiting those vulnerabilities, a computer and network system is not completely free from security threats due to its connection to the outside network through common network services such as emails and the WWW as well as many unknown system vulnerabilities.
2.1.1.2 Authentication and authorization
Authentication and authorization work together to control a user’s access to computer and network assets. Through authentication, a user is verified to be truly what the user claims to be. Through authorization, a user is granted access rights to computer and network assets based on the user’s authenticated identity. Table 2.2 shows examples of READ (R), WRITE (W)
Table 2.2 Examples of users’ access rights to files
Users File 1 File 2 File 3 Directory 1 Printer file
User 1 RWE RW None None W
User 2 None None RW RW W
User Group 1 R R R R W
and EXECUTE (E) rights to given files which are assigned to some users and user groups. In addition to access rights to individual computer and network assets, flow control policies can also be specified to control the information flow between computer and network assets.
A username and a password are commonly used for user authentication. In addition to information keys such as passwords, there are also physical keys such as magnetic cards and security calculators, and biometric keys such as voice print, finger print, and retinal print.
Just like a credit card, a magnetic card contains the identity of the card holder. In addition to a magnetic card, a user may have to enter a personal identification number. One disadvantage of using a magnetic card as a key is that a card reader needs to be attached to each computer. A security calculator is not uncommon in practice. To use a security calculator, a user first presents a username to a computer and network system. The system responds with a challenge value. The user enters a personal identification number along with the challenge value into the security calculator. The security calculator computes a response value. The user presents the response value as the key to the system. If the response value matches the expected response value computed by the system, the user is successfully authenticated. Hence, the system must store the personal identification number for each username. An advantage of using a security calculator is that the response value as the key to pass the authentication process changes with the challenge value. Since different challenge values are usually generated at different times, response values as keys change at different times when the system is used. This makes it difficult to guess the key each time when the computer is used. Even if a break-in is successful using a key, the key cannot be used the next time for break-in. Moreover, a user must have the right personalized security calculator and the correct personal identification number to compute the correct response value as the key.
A voice print, a finger print, and a retinal print, which is a blood vessel pattern on the back of an eye, and a hand geometry are examples of biometric keys. Like a magnetic card, those biometric keys need a special device attached to a computer and a network system to read and recognize these biometric keys.
A digital signature has become an increasingly popular method of authenticating the sender of a digital document. For example, using a public key cryptographic algorithm such as the Rivest-Shamir-Adelman (RSA) algorithm [2], the sender of a digital document has a pair of a private key and a public key which is known by others. The sender first uses the private key to encrypt the document as a way of signing the document, and then sends the encrypted document to a receiver. If the receiver of the document can use the sender’s public key to decrypt the document, this proves that the document is truly signed by the sender since only the sender knows the private key which matches the public key.
A public key cryptographic algorithm can also be used to encrypt data in transmission or data in storage to protect the confidentiality or the integrity of those computer and network assets because encrypted data cannot easily be read or modified by others. Take the example of protecting data in transmission over a network. The sender of data can encrypt the data using the receiver’s public key. The encrypted data can be decrypted using only the private key which is paired with the public key and is known by the receiver only. Hence, only the receiver can use the private key to decrypt the data. Details of cryptographic algorithms for data encryption and decryption can be found in [2].
User authentication is a part of an authorization process which determines which access rights are granted to which computer and network assets for an authenticated user. Hence, authorization controls a user’s access to computer and network assets, and can also limit information flow between computer and network assets. Authentication/authorization aims at
Cyber attack detection 29 access and flow control by limiting each user to the user’s own work space on a computer and network system.
Unfortunately, many assets on a computer and network system are shared by multiple users, thereby creating a common work environment for multiple users. Such shared assets include the processor, the main memory, the hard disk, the network, and so on. As discussed in Chapter 1, an attacker’s mischief to the common environment shared by multiple users can produce vulnerabilities in security violations. There are also possibilities of bypassing as discussed in Chapter 1. Hence, like firewalls, access and flow control through authentication/authorization increases attack difficulty, but cannot completely prevent attacks.
2.1.2 Secure computer and network design
Instead of restricting asset access and information flow to only authorized users and/or activi- ties, efforts on secure computers and networks aim to remove computer and network vulner- abilities by reducing or eliminating specification/design, coding, configuration and operation management faults. Secure computer and network design has been addressed from many per- spectives, such as improving software engineering practice to reduce system design and coding faults, developing fault tolerance technologies to enable a computer and network system to sustain its operation under an attack, introducing automated network management tools to reduce or eliminate system configuration faults [3, 4], developing new service models [5–10] to address system design faults and vulnerabilities introduced by the best effort service model, or designing secure system architectures and policy-based security protection, and so on.
Research on an asset protection-driven security architecture and policy-based security pro- tection is described in Chapter 3. The policy-based security protection in an asset protection- driven security architecture is developed using the asset risk framework (see Chapter 1) to provide the advantages of threat coverage, adaptability and robustness in security protection. Chapters 4-6 describe admission control, job scheduling and the job reservation components of a new service model which has been developed to guarantee the end-to-end delay of a com- puter and network job over a global network such as the Internet and ensure service stability of local-level computer and network resources. Hence, Part II presents some specific examples of how to design secure computer and network systems.