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In all the examples discussed so far, we have talked about people authenticating people or people authenticating themselves to computers. In a large distributed system, however, computers are also interacting with other computers. The computers may have to authenticate themselves to each other because all computers cannot be trusted equally. There are many protocols that can be used to allow computer-to-computer authentication, and these protocols will, in general, support three types of authentication: client authentication, server authentication, and mutual authentication. Client authentication involves the server verifying the client s identity, server authentication involves the client verifying the server s identity, and mutual authentication involves the client and server verifying each other s identity. When we discuss protocols, such as Secure Sockets Layer (SSL) in 15, we will discuss the different modes they use to support client, server, and mutual authentication. Whether client, server, or mutual authentication is done often depends upon the nature of the application and the expected threats. Many e-commerce web sites provide server authentication once a user is ready to make a purchase because they do not want the client to submit a credit card number to a spoofed or impostor web site. Spoofed web sites are a significant security threat because they do not cost much to set up. On the other hand, in older cell phone networks, only client authentication was required. Cell phone towers (servers) would only check that a phone (client) that attempted to communicate with it was owned by an authentic customer. The phones did not authenticate the cell phone towers because cell phone towers were costly to set up, and an attacker would require significant capital to spoof a cell phone tower. On the other hand, the cell phones themselves were much cheaper, and hence wireless carriers only required phones to be authenticated. Today, the cost of cell phone base stations is significantly cheaper, and modern-day cell phone networks use mutual authentication. Now that we have completed our discussion of authentication, we are going to explore our next security concept: authorization.
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1.3. Authorization
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Authorization is the act of checking whether a user has permission to conduct some action. Whereas authentication is about verifying identity, authorization is about verifying a user s authority. To give a concrete example, let us examine the case in which Alice authenticates herself at an ATM by putting in her ATM card and entering her PIN. Alice may want to deduct $500, but may only be authorized to deduct a maximum of $300 per day. If Alice enters $500 as the amount that she is requesting to deduct, the system will not authorize her transaction even if she successfully authenticates herself. In the previous example, an authorization check questions whether Alice has the authority to deduct a certain amount of money. Operating systems such as Windows and Linux do authorization checks all the time. For example, when Alice attempts to delete a file, the operating system checks whether Alice is allowed to do so. A general mechanism called an access control list (ACL) is used by many operating systems to determine whether users are authorized to conduct different actions.
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1.3.1. Access Control Lists (ACLs)
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Minimally, an ACL is a set of users and a corresponding set of resources they are allowed to access. For example, Alice may have access to all the files in her home directory,1 but may not have access to Bob s files. Suppose Alice s home directory is /home/Alice, and Bob s home directory is /home/Bob. An ACL that models this would list Alice as the principal,2 and it would also list the set of files in her home directory that she is allowed to access, as shown in Table 1-1. In the table, an asterisk (*) is used as a wildcard to indicate all files and subdirectories within a particular home directory. An ACL may optionally include privileges that are associated with resources. The Privilege column indicates that Alice and Bob are allowed to read, write, and execute files in their respective home directories. Table 1-1. A Simple ACL
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