TIP Sheet Reasoning is the process of using existing knowledge to draw conclusions, make predictions, or construct explanations. Three methods of reasoning are the deductive, inductive, and abductive approaches. Deductive reasoning: conclusion guaranteed If x = 4And if y = 1 Then 2x + y = 9 In this example, it is a logical necessity that 2x + y equals 9; 2x + y must equal 9. As a matter of fact, formal, symbolic logic uses a language that looks rather like the math equality above, complete with its own operators and syntax. But a deductive syllogism (think of it as a plain-English version of a math equality) can be expressed in ordinary language: If entropy (disorder) in a system will increase unless energy is expended,And if my living room is a system, Then disorder will increase in my living room unless I clean it. In the syllogism above, the first two statements, the propositions or premises, lead logically to the third statement, the conclusion. Here is another example: A medical technology ought to be funded if it has been used successfully to treat patients.Adult stem cells are being used to treat patients successfully in more than sixty-five new therapies. Adult stem cell research and technology should be funded. A conclusion is sound (true) or unsound (false), depending on the truth of the original premises (for any premise may be true or false). At the same time, independent of the truth or falsity of the premises, the deductive inference itself (the process of "connecting the dots" from premise to conclusion) is either valid or invalid. The inferential process can be valid even if the premise is false: There is no such thing as drought in the West.California is in the West. California need never make plans to deal with a drought. In the example above, though the inferential process itself is valid, the conclusion is false because the premise, There is no such thing as drought in the West, is false. A syllogism yields a false conclusion if either of its propositions is false. A syllogism like this is particularly insidious because it looks so very logical–it is, in fact, logical. But whether in error or malice, if either of the propositions above is wrong, then a policy decision based upon it (California need never make plans to deal with a drought) probably would fail to serve the public interest. Assuming the propositions are sound, the rather stern logic of deductive reasoning can give you absolutely certain conclusions. However, deductive reasoning cannot really increase human knowledge (it is nonampliative) because the conclusions yielded by deductive reasoning are tautologies-statements that are contained within the premises and virtually self-evident. Therefore, while with deductive reasoning we can make observations and expand implications, we cannot make predictions about future or otherwise non-observed phenomena. Inductive reasoning: conclusion merely likely Conclusions reached by the inductive method are not logical necessities; no amount of inductive evidence guarantees the conclusion. This is because there is no way to know that all the possible evidence has been gathered, and that there exists no further bit of unobserved evidence that might invalidate my hypothesis. Thus, while the newspapers might report the conclusions of scientific research as absolutes, scientific literature itself uses more cautious language, the language of inductively reached, probable conclusions: What we have seen is the ability of these cells to feed the blood vessels of tumors and to heal the blood vessels surrounding wounds. The findings suggest that these adult stem cells may be an ideal source of cells for clinical therapy. For example, we can envision the use of these stem cells for therapies against cancer tumors [...].1 Because inductive conclusions are not logical necessities, inductive arguments are not simply true. Rather, they are cogent: that is, the evidence seems complete, relevant, and generally convincing, and the conclusion is therefore probably true. Nor are inductive arguments simply false; rather, they are not cogent. It is an important difference from deductive reasoning that, while inductive reasoning cannot yield an absolutely certain conclusion, it can actually increase human knowledge (it is ampliative). It can make predictions about future events or as-yet unobserved phenomena. For example, Albert Einstein observed the movement of a pocket compass when he was five years old and became fascinated with the idea that something invisible in the space around the compass needle was causing it to move. This observation, combined with additional observations (of moving trains, for example) and the results of logical and mathematical tools (deduction), resulted in a rule that fit his observations and could predict events that were as yet unobserved. Abductive reasoning: taking your best shot A medical diagnosis is an application of abductive reasoning: given this set of symptoms, what is the diagnosis that would best explain most of them? Likewise, when jurors hear evidence in a criminal case, they must consider whether the prosecution or the defense has the best explanation to cover all the points of evidence. While there may be no certainty about their verdict, since there may exist additional evidence that was not admitted in the case, they make their best guess based on what they know. While cogent inductive reasoning requires that the evidence that might shed light on the subject be fairly complete, whether positive or negative, abductive reasoning is characterized by lack of completeness, either in the evidence, or in the explanation, or both. A patient may be unconscious or fail to report every symptom, for example, resulting in incomplete evidence, or a doctor may arrive at a diagnosis that fails to explain several of the symptoms. Still, he must reach the best diagnosis he can. The abductive process can be creative, intuitive, even revolutionary.2 Einstein's work, for example, was not just inductive and deductive, but involved a creative leap of imagination and visualization that scarcely seemed warranted by the mere observation of moving trains and falling elevators. In fact, so much of Einstein's work was done as a "thought experiment" (for he never experimentally dropped elevators), that some of his peers discredited it as too fanciful. Nevertheless, he appears to have been right-until now his remarkable conclusions about space-time continue to be verified experientially. References 2. Thagard, Paul and Cameron Shelley. "Abductive reasoning: Logic, visual thinking, and coherence." Waterloo, Ontario: Philosophy Department, Univerisity of Waterloo, 1997. June 2, 2005. < http://cogsci.uwaterloo.ca/Articles/Pages/%7FAbductive.html> Get full access to Practical UNIX and Internet Security, 3rd Edition and 60K+ other titles, with free 10-day trial of O'Reilly. There's also live online events, interactive content, certification prep materials, and more.
Every person who uses a Unix computer should have her own account. An account is identified by a user ID number (UID) that is associated with one or more usernames (also known as account names ). Traditionally, each account also has a secret password associated with it to prevent unauthorized use. You need to know both your username and your password to log into a Unix system. The username is an identifier: it tells the computer who you are. In contrast, a password is an authenticator: you use it to prove to the operating system that you are who you claim to be. A single person can have more than one Unix account on the same computer. In this case, each account would have its own username. Standard Unix usernames may be between one and eight characters long, although many Unix systems today allow usernames that are longer. Within a single Unix computer, usernames must be unique: no two users can have the same one. (If two people did have the same username on a single system, then they would really be sharing the same account.) Traditionally, Unix passwords were also between one and eight characters long, although most Unix systems now allow longer passwords as well. Longer passwords are generally more secure because they are harder to guess. More than one user can theoretically have the same password, although if they do, that usually indicates that both users have picked a bad password. A username can be any sequence of characters you want (with some exceptions), and does not necessarily correspond to a real person’s name. Some versions of Unix have problems with usernames that do not start with a lowercase letter or that contain special characters such as punctuation or control characters. Usernames containing certain unusual characters will also cause problems for various application programs, including some network mail programs. For this reason, many sites allow only usernames that contain lowercase letters and numbers and further require that all usernames start with a letter. Your username identifies you to Unix in the same way that your first name identifies you to your friends. When you log into the Unix system, you tell it your username in the same way that you might say, “Hello, this is Sabrina,” when you pick up the telephone.[31] Most systems use the same identifier for both usernames and email addresses. For this reason, organizations that have more than one computer often require people to use the same username on every machine to minimize confusion. There is considerable flexibility in choosing a username. For example, John Q. Random might have any of the following usernames; they are all potentially valid:
Alternatively, John might have a username that appears totally unrelated to his real name, like avocado or t42. Having a username similar to your own name is merely a matter of convenience. In some cases, having an unrelated name may be a desired feature because it either masks your identity in email and online chat rooms, or projects an image different from your usual one: tall62, fungirl, anonymus, svelte19, and richguy. Of course, as we noted in the last chapter, “handles” that don’t match one’s real name can also be used to hide the true identity of someone doing something unethical or illegal. Be cautious about drawing conclusions about someone based on the email name or account name that they present. Most organizations require that usernames be at least three characters long. Single-character usernames are simply too confusing for most people to deal with, no matter how easy you might think it would be to be user i or x. Usernames that are two characters long are also confusing for some people, because they usually don’t provide enough information to match a name in memory: who was , anyway? In general, names with little intrinsic meaning, such as t42xp96wl, can also cause confusion because they are more difficult for correspondents to remember. Some organizations assign usernames using standardized rules, such as the first initial of a person’s first name and then the first six letters of their last name, optionally followed by a number. Other organizations let users pick their own names. Some organizations and online services assign an apparently random string of characters as the usernames; although this is generally not popular, it can improve security—especially if these usernames are not used for electronic mail. Although some randomly generated strings can be hard to remember, there are several algorithms that generate easy-to-remember random strings by using a small number of mnemonic rules; typical usernames generated by these systems are xxp44 and acactt. If you design a system that gives users randomly generated usernames, it is a good idea to let people reject a username and ask for another, lest somebody gets stuck with a hard-to-remember username like xp9uu6wi. Unix also has special accounts that are used for administrative purposes and special system functions. These accounts are not normally used by individual users. After you tell Unix who you are, you must prove your identity to a certain degree of confidence (trust). This process is called authentication. Classically, there are three different ways that you can authenticate yourself to a computer system, and you use one or more of them each time:
None of these systems is foolproof. For example, by eavesdropping on your terminal line, somebody can learn your password. By attacking you at gunpoint, somebody can steal your card key. And if your attacker has a knife, you might even lose your finger! In general, the more trustworthy the form of authentication, the more aggressive an attacker must be to compromise it. In the past, the most trustworthy authentication techniques have also been the most difficult to use, although this is slowly changing. Passwords are the simplest form of authentication: they are a secret that you share with the computer. When you log in, you type your password to prove to the computer that you are who you claim to be. The computer ensures that the password you type matches the account that you have specified. If it matches, you are allowed to proceed. Unix does not display your password as you type it. This gives you extra protection if the transcript of your session is being logged or if somebody is watching over your shoulder as you type—a technique that is sometimes referred to as shoulder surfing . Conventional passwords have been part of Unix since its early years. The advantage of this system is that it runs without any special equipment, such as smartcard readers or fingerprint scanners. The disadvantage of conventional passwords is that they are easily captured and reused—especially in a network-based environment. Although passwords can be used securely and effectively, doing so requires constant vigilance to make sure that an unencrypted password is not inadvertently sent over the network, allowing it to be captured with a password sniffer. Passwords can also be stolen if they are typed on a computer that has been compromised with a keystroke recorder. Today, even unsophisticated attackers can use such tools to capture passwords. Indeed, the only way to safely use a Unix computer remotely over a network such as the Internet is to use one-time passwords, encryption, or both (see Section 4.3.3 later in this chapter and also see Chapter 7).[32] Unfortunately, we live in an imperfect world, and most Unix systems continue to depend upon reusable passwords for user authentication. Be careful! When you log in, you tell the computer who you are by typing your username at the login prompt (the identification step). You then type your password (in response to the password prompt) to authenticate that you are who you claim to be. For example: Unix does not display your password when you type it. If the password that you supply with your username corresponds to the password that is on file for the provided username, Unix logs you in and gives you full access to the user’s files, commands, and devices. If the username and the password do not match, Unix does not log you in. On some versions of Unix, if somebody tries to log into an account and supplies an invalid password several times in succession, that account will become locked. A locked account can be unlocked only by the system administrator. Locking has three functions:
If you find yourself locked out of your account, you should contact your system administrator and get your password changed to something new. Don’t change your password back to what it was before you were locked out. The automatic lockout feature can prevent unauthorized use, but it can also be used to conduct denial of service attacks, or by an attacker to lock selected users out of the system so as to prevent discovery of his actions. A practical joker can use it to annoy fellow employees or students. And you can accidentally lock yourself out if you try to log in too many times before you’ve had your morning coffee. In our experience, the disadvantages of indefinite automatic lockouts outweigh the benefits. A much better method is to employ an increasing delay mechanism in the login. After a fixed number of unsuccessful logins, an increasing delay can be inserted between each successive prompt. Implementing such delays in a network environment requires maintaining a record of failed login attempts, so that the delay cannot be circumvented by an attacker who merely disconnects from the target machine and reconnects. You can change your password with the Unix passwd command. You will first be asked to type your old password, then a new one. By asking you to type your old password first, passwd prevents somebody from walking up to a terminal that you left yourself logged into and then changing your password without your knowledge. Unix makes you type the new password twice: % passwd Changing password for sarah. Old password:tuna4fis New password: nosSMi32 Retype new password: nosSMi32 %If the two passwords you type don’t match, your password remains unchanged. This is a safety precaution: if you made a mistake typing the new password and Unix only asked you once, then your password could be changed to some new value and you would have no way of knowing that value. On systems that use Sun Microsystems NIS or NIS+, you may need to use the command yppasswd or nispasswd to change your password. Except for having different names, these programs work in the same way as passwd. However, when they run, they update your password in the network database with NIS or NIS+. When this happens, your password will be immediately available on other clients on the network. With NIS, your password will be distributed during the next regular update. The -r option to the passwd command can also be used under Solaris. To change NIS or NIS+ passwords, the format would be passwd -r nis or passwd -r nisplus, respectively. It is possible to have a local machine password that is different from the one in the network database, and that would be changed with passwd -r files. Even though passwords are not echoed when they are printed, the Backspace or Delete key (or whatever key you have bound to the “erase” function) will still delete the last character typed, so if you make a mistake, you can correct it. Once you have changed your password, your old password will no longer work. Do not forget your new password! If you forget your new password, you will need to have the system administrator set it to something you can use to log in and try again.[33] If your system administrator gives you a new password, immediately change it to something else that only you know! Otherwise, if your system administrator is in the habit of setting the same password for forgetful users, your account may be compromised by someone else who has had a temporary lapse of memory; see Password: ChangeMe for an example. If you are a system manager and you need to change a user’s password, do not change the user’s password to something like changeme or password, and then rely on the user to change their password to something else. Many users will not take the time to change their passwords but will, instead, continue to use the password that you have inadvertently “assigned” to them. Give the user a good password, and give that user a different password from every other user whose password you have reset. After you have changed your password, try logging into your account with the new password to make sure that you’ve entered the new password properly. Ideally, you should do this without logging out, so you will have some recourse if you did not change your password properly. This is especially crucial if you are logged in as root and you have just changed the root password! One way to try out your new password is to use the su command. Normally, the su command is used to switch to another account. But as the command requires that you type the password of the account to which you are switching, you can effectively use the su command to test the password of your own account. % /bin/su nosmis password: mypassword %(Of course, instead of typing nosmis and mypassword , use your own account name and password.) If you’re using a machine that is on a network, you can use the telnet, rlogin, or ssh programs to loop back through the network to log in a second time by typing: % ssh -l dawn localhost dawn@loaclhost's password: w3kfsc! Last login: Sun Feb 3 11:48:45 on ttyb %You can replace localhost in the above example with the name of your computer. This method is also useful when testing a change in the root password, as the su command does not prompt for a password when run by root. If you try one of the earlier methods and discover that your password is not what you thought it was, you have a definite problem. To change the password to something you do know, you will need the current password. However, you don’t know that password! You will need the help of the system administrator to fix the situation. (That’s why you shouldn’t log out—if the time is 2:00 a.m. on Saturday, you might not be able to reach the administrator until Monday morning, and you might want to get some work done before then.) The superuser (user root) can’t decode the password of any user. However, the system administrator can help you when you don’t know what you’ve set your password to by using the superuser account to set your password to something known. If you get email from your system manager advising you that there are system problems and that you should immediately change your password to tunafish (or some other value), disregard the message and report it to your system management. These kinds of email messages are frequently sent by computer criminals to novice users. The hope is that the novice user will comply with the request and change his password to the one that is suggested—often with devastating results. If you are running as the superuser (or the network administrator, in the case of NIS+), you can set the password of any user, including yourself, without supplying the old password. You do this by supplying the username to the passwd command when you invoke it: # passwd cindy New password: NewR-pas Retype new password: NewR-pas #Get Practical UNIX and Internet Security, 3rd Edition now with the O’Reilly learning platform. O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers. |
The process of proving to the computer that you are who you say you are is called ____________.
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