Barrons Publishing Dictionary of Computer and Internet Terms 10th

ellipsis

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FIGURE 93. Eliza in action

ellipsis typographic convention of using three dots (. . . ) to indicate the trailing off of a thought. In Windows, the ellipsis is typed by holding down Alt while keying 133 on the numeric keypad. Macintosh users can type an ellipsis by holding down Option and typing : (colon). Note that the three dots are actually one character. The spacing is different than simply typing three consecutive periods (Ellipsis . . . ; Three periods ...).

The ellipsis also has an important function in the menu system of Microsoft Windows. The appearance of ‘. . . ’ after a menu item means that a dialog box will appear when that command is selected.

em dash a long dash (—). See DASH.

EM64T see X64.

Emacs (originally for “editor macros”) a TEXT EDITOR that originated as a set of MACROs written for another text editor by Richard M. Stallman in the 1970s. Today, Emacs is one of the most successful products of the GNU project (see GNU). It is distributed free of charge from www.gnu.org/ software/emacs in versions for numerous operating systems.

Emacs is somewhat complex to use, but very powerful. It can edit files of any size, and they need not be text files, since non-text characters are represented as octal codes. Emacs features an elaborate system of context-sensitive help. A Lisp-like programming language is built in so that users can define new editing operations. See also ELIZA.

embedded font a FONT that is included within a file to ensure faithful reproduction of the formatted document.

embedded Linux any version of Linux that is used in an embedded system.

embedded object an object included in your file that was created in another software package and that still maintains a LINK to the other software. If the object is changed in the original software, it will be updated in the second file. See OLE for more details.

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embedded system a computer that forms part of a larger machine of some other kind. The microprocessor that controls an automobile engine is an example. Embedded systems must usually be extremely reliable. They must also respond to events in real time (i.e., as they happen) without undue delay. See MICROCONTROLLER; REAL-TIME PROGRAMMING.

EMC electromagnetic compatibility, the protection of equipment from electromagnetic interference. See RFI PROTECTION.

emitter one of the three layers of a bipolar transistor. See TRANSISTOR.

emoticon a typewritten symbol for a facial expression, often used in electronic mail. For example, :) denotes a grin (look at it sideways), ;-) means “winking,” and =:-O means scared or surprised.

Some emoticons are written so that they are viewed upright rather than sideways.

ˆ-ˆ smile

0.0surprise

ˆ.ˆ; distress (with drops of sweat)

emulation the process of achieving the same results as if you had a different machine than the one you’re actually using. For example, VT-100 emulation means making a computer act exactly like a VT-100 terminal. Emulation is different from simulation, which involves imitating the internal processes, not just the results, of the thing being simulated.

emulsion the coated surface of photographic film. Normally, a photographic negative looks right (not flipped) if the emulsion of the negative faces away from the person viewing it. Some graphics software offers output choices of “emulsion up” and “emulsion down” to produce flipped images for systems in which a photographic negative is going to be used backward.

en dash a short dash (–). See DASH.

Encapsulated PostScript (EPS) a file format that is widely supported by different computers, printers, and software. Most desktop publishing software supports the importation of Encapsulated PostScript files, thus providing a common denominator for exchanging files. See also POSTSCRIPT.

encoding a way of interpreting binary data as representing characters. The term is used particularly in the Microsoft .NET Framework, which supports several formats of UNICODE and many national variations on ASCII.

encryption the act of converting information into a code or cipher so that people will be unable to read it. A secret key or password is required to decrypt (decode) the information. More and more confidential data is being sent along computer networks, so it is becoming increasingly important to develop ways to send information over computer networks securely.

For example, suppose we wish to send this message:

HELLOGOODBYE

encryption

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One way to encrypt it is to replace each letter with the letter that comes 10 places later in the alphabet, so that letter 1 (A) becomes letter 11 (K), letter 2 (B) becomes letter 12 (L), and so forth, starting over at A when we go past Z, like this:

Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ

Encrypted: KLMNOPQRSTUVWXYZABCDEFGHIJ

Mathematically speaking, we change letter n to (n + 10) mod 26. Here mod stands for modulo and refers to the remainder after division by 26. For example, letter 20 (T) is shifted to 30, which becomes 30 mod 26 = 4, which is the letter D. Using this method, our message becomes:

ROVVYQYYNLIO

The recipients can easily decrypt the message as long as they know the algorithm (each letter is shifted by a certain number of places) and the key (in this case, 10).

Unfortunately, this algorithm is so simple that it would be easy for a spy to crack the code. There are only 25 possible keys (a key of 26 would have no effect, and a key of 27 or higher would have the same effect as a lower one). It is easy to check all 25 possibilities:

Trying key: 1 Trying key: 2 Trying key: 3 Trying key: 4 Trying key: 5 Trying key: 6 Trying key: 7 Trying key: 8 Trying key: 9 Trying key: 10

Message decodes as: QNUUXPXXMKHN Message decodes as: PMTTWOWWLJGM Message decodes as: OLSSVNVVKIFL Message decodes as: NKRRUMUUJHEK Message decodes as: MJQQTLTTIGDJ Message decodes as: LIPPSKSSHFCI Message decodes as: KHOORJRRGEBH Message decodes as: JGNNQIQQFDAG Message decodes as: IFMMPHPPECZF Message decodes as: HELLOGOODBYE

In this case the spy can stop after the tenth try.

To make the code harder to crack, we can use a longer key. For example, say that the first letter of the message will be shifted 10, the second will be shifted 8 letters, the third will be shifted 17, and so on. If you use a key with 8 numbers, then you can repeat the pattern after every 8 letters (i.e., the ninth letter will be shifted the same as the first letter, the tenth letter will be shifted the same as the second letter, and so on). The longer the key is, the harder it will be for the spy to try all possibilities. If you can design it so that the time required to check all possibilities exceeds the lifetime of the universe, you’re probably safe from this kind of attack. Even if you can design it so that the expense of cracking the code is greater than the benefit the spy would receive by cracking your code, you’re probably safe.

However, there are other means of attack. Not all letters of the alphabet are used with equal frequency. A spy can program a computer to make a guess for the length of the key; collect all letters encrypted with a particular part of the key; and then check the frequency of encrypted

169	encryption

letters, guessing that the most frequently appearing letter represents E, and so on. That guess may not be right, but guessing with this system will likely proceed much faster than guessing all the possibilities. This kind of attack is easier if the message is longer, but it won’t work for numerical data where the digits are equally likely to appear.

Another likely means of attack would be to attack the key itself. If the spy gets hold of the key, it will be easy to decrypt all the messages. If a lot of people are sending messages to lots of other people, it is hard for them to deliver the keys to the recipients of the messages without letting them fall into the wrong hands.

One way to solve this problem is with public key encryption. In this approach, each person has both a public key (which everyone knows) and a private key (which is kept secret). If Alice is sending a message to Bob, then the message will be encrypted using an algorithm that is based on Bob’s public key. Anyone can use this key to encrypt a message to Bob, but it can only be decrypted using Bob’s private key.

Here is one example of how this can work, using the algorithm developed by Whitfield Diffie and Martin Hellman in 1976. Alice and Bob agree on two numbers: n = 37 and g = 7. (In reality, n and g would be much larger than this.) Each of them has a private key, which we’ll call a and b, respectively. Alice and Bob generate their public keys A and B using the formula:

Public key = g (private key) mod n

Thus:

Alice’s private key a = 8

Alice’s public key A = 78 mod 37

=5,764,801 mod 37

=16

Bob’s private key b = 6

Bob’s public key B = 76 mod 37

=117,649 mod 37

=26

Alice now generates another key K to use for the actual message using this formula:

K= Ba mod n

=268 mod 37

=208,827,064,576 mod 37

=10

This key is known as the session key. Now she can encrypt the message. For example, if she is sending the message HELLOGOODBYE, it will be encrypted as shown at the beginning of this entry.

When Bob receives the message, he will calculate the session key using a very similar formula:

end

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K = Ab mod n

=166 mod 37

=16, 777, 216 mod 37

=10

Notice that this is the same value even though it is calculated from different numbers using a different formula. This works because of the following mathematical identities:

(a × b) mod n = [(a mod n) × (b mod n)] mod n ac mod n = (a mod n)c mod n

abc = (ab)c = (ac)b

abc mod n = (a mod n)bc mod n

=[(a mod n)b]c mod n

=[(a mod n)c]b mod n

abc mod n = (a mod n)bc mod n = (ac mod n)b mod n abc mod n = (a mod n)bc mod n = (ab mod n)c mod n

To calculate the private key (equivalent to c), given the public key and the session key, you need to solve an equation of this general form:

k = j x mod n

If n happens to be a large prime number, it is very difficult to discover the value of x even if you know the values of k, j, and n. Thus, large prime numbers play a crucial role in public-key encryption. In practice, when computers are used for encryption, the calculations are usually carried out directly on the binary digits of the data, using a key given as a binary number. A longer key provides greater security, but the calculation process becomes more complicated.

All this presumes that you can get people’s public keys reliably so that you can be sure you’re really using Bob’s public key when you send messages to Bob. Since public keys are not secret, all you need is a trustworthy database in which you can look up people’s public keys.

Until 2000, the U.S. government regulated the export of strong encryption software in the same way that it regulates the export of weapons. This regulation dated from the 1940s, before general-purpose digital computers existed; encryption machines at that time were considered to be military devices.

See also AES; DES; DIGITAL SIGNATURE; HASH FUNCTION; ONE-WAY FUNCTION; PGP; RSA ENCRYPTION; SSL.

end

1. keyword that marks the end of a particular program structure in several programming languages. In BASIC, the END keyword tells the computer to stop executing the program. In Pascal, END marks the end of blocks of statements that start with BEGIN.

GREEN PC.

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envelope

2. the key on your keyboard that takes your cursor to the end of the current line. Some word processors use Ctrl-End as a keyboard shortcut to take you to the end of the document.

end-of-file mark a symbol that indicates the end of a file. For example, in CP/M, all text files ended with ASCII character 26 (Ctrl-Z) because the computer did not otherwise keep track of the exact length of the file, only the number of disk sectors. In Windows, Ctrl-Z is often used the same way even though the computer knows exactly where the file ends whether or not an end-of-file mark is present. The UNIX end-of-file mark is Ctrl-D (ASCII 4).

end user the person ultimately intended to use a product, as opposed to people involved in developing or marketing it.

Energy Star a set of guidelines proposed by the U.S. Environmental Protection Agency in 1992 to reduce the amount of electricity consumed by personal computers. An Energy Star-compliant computer consumes less than 30 watts of power when idling (i.e., when turned on but not in use) and switches automatically into low-power mode if several minutes elapse without any keyboard activity. See

engine

1. the part of a computer program that implements a special technique; see INFERENCE ENGINE, MONTE CARLO ENGINE, SEARCH ENGINE.

2. the printing mechanism of a laser printer, not including the computer control circuitry. Many laser printers use an engine made by Canon in Japan.

ENIAC (Electronic Numerical Integrator And Calculator) one of the first electronic computers, built at the University of Pennsylvania in the mid1940s. It contained about 18,000 vacuum tubes. Initially, the ENIAC was programmed by plugging cables into circuit boards. Today, one of the Internet nodes at the University of Pennsylvania is named eniac but is, of course, not the same machine.

Enter key the key on a computer keyboard that you press at the end of each line in order to send the contents of that line into the computer. On most keyboards, the Enter key is the same as the Return key. However, IBM 3270-series terminals make a distinction: the Return key starts a new line, but the Enter key sends the contents of the whole screen to the computer.

Under windowed operating systems, pressing the Enter key is usually equivalent to clicking on the currently selected icon or other highlighted item.

enumerator a device driver or operating system component that identifies all hardware devices of a particular type. See BIOS ENUMERATOR.

envelope

1. (in a draw program) the imaginary outline enclosing an object. You can edit the envelope, turning it from a rectangle into a curved shape, and thereby distorting everything inside it.

environment

172

2. (in engineering) the limits imposed by physical or technical constraints (called an “envelope” because they can be envisioned as surrounding an area on a graph). “Pushing the envelope” means working close to, or at, the limits.

FIGURE 94. Envelope manipulation

environment

1.the display and human interface provided by software. On a computer, an environment defines what you can do with the computer. For instance, the operating system, a word processor, and a spreadsheet provide (at least) three different environments that respond to different commands. For example, if you type a word processing command while you are in the operating system environment, or vice versa, the command will not be understood.

2.(in Windows and UNIX) a data area in which you can store information for use by programs. To put information there, use the set command; for example, under Windows,

set prompt=$p$g

tells the computer to display the current disk and directory (e.g., C:\MYDIR>) when it is ready for a command. To see the contents of the environment area, type set by itself.

EPIC see IA-64.

EPROM (Erasable Programmable Read-Only Memory) a type of memory chip that can be programmed electrically and erased by exposure to ultraviolet light. See also ROM; PROM; EEPROM.

EPS see ENCAPSULATED POSTSCRIPT.

Epson a prominent Japanese manufacturer of printers and other computer peripherals, distributed in the United States by Epson America, Inc., of Torrance, California. More information about Epson can be found at www.epson.com.

The Epson MX-80 dot-matrix printer received wide acceptance during the early 1980s and set many standards to which other manufacturers subsequently adhered. Today, Epson makes high-quality color inkjet printers and other computer peripherals.

equalize a paint program filter that adjusts the brightness range of a picture so that all levels of brightness become equally common. If some bright-

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nesses are not used (because the picture is too bright or too dark, or because of a contrast problem), the equalized picture will often look much better. Equalizing can dramatically improve the appearance of objects that are nearly the same brightness as their background.

erase the command that erases a file or set of files from disk in Windows and other operating systems. See also RECOVERING ERASED FILES.

Eraser a paint program tool that removes colors from a picture, leaving the background color in its place (the background can be transparent). The eraser is used by holding down the mouse button (the leftmost if there is more than one) and dragging the eraser tool. You can adjust the size and shape of your eraser to suit your needs. Some programs will even adjust how well the eraser works; it can erase thoroughly or just lighten the color. See NATURAL MEDIA.

FIGURE 95. Eraser

Note: tool erases to transparent background. (Erased material is replaced by the background color.)

ergonomics the science of designing machines and working environments to suit human needs (from the Greek words meaning “the study of work”). An ergonomically designed machine is one whose design is based on the scientific study of human requirements such as vision, posture, and health risks. After all, the most important part of a computer system is the human being who is operating the computer.

Ergonomics goes beyond considering your comfort. Smart workers know that they need to work efficiently. When you work efficiently, you can get more done. Here are some things you can do:

•Desk. Your computer desk should be deep enough to comfortably accommodate all of your equipment. If the system unit keeps threatening to dump the keyboard in your lap, you may not have enough room. Consider putting the system unit on the floor or to the side of the monitor.

Check the height of your desk. Is it too tall for you to type comfortably? You may want to attach a keyboard drawer. This lowers the keyboard to a more comfortable level and gives you a storage place for the keyboard.

•Chair. Your chair is most vital to the health and well-being of your back. You should choose a chair that has adjustments for height and good lumbar support. Try to find a chair that lets you adjust the tilt

error

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of the seat because it helps to periodically change the seat tilt during a long work session.

•Monitor. The monitor is one of the big-ticket items when you purchase your computer system. Ergonomically speaking, you do not want to skimp here. LCD (Liquid Crystal Display) monitors are superior to CRT (Cathode Ray Tube) monitors because they do not flicker. If you are still using an older CRT monitor, set the refresh rate to at least 70 Hz; it may save you a headache. Make sure you are comfortable with the height and tilt of the screen. You may need a special pair of glasses for working at the computer. (See EYE-

GLASSES, COMPUTER.)

•Mouse and keyboard. The big risk is CARPAL TUNNEL SYNDROME, a condition that creates numbness or a buzzing feeling in your hands. Prevention is the key. You should keep your wrists straight when typing; don’t allow them to bend. Some people enjoy a cushioned wrist rest for their keyboard. If using the mouse gives you any discomfort, try using another pointing device such as a TRACKBALL.

•Lighting. To prevent glare on the screen, do not place your computer opposite a window. Overhead lighting should be soft (not as bright as it would be for reading).

•Posture. Good posture is important. Try to imagine that an invisible string is pulling your head up and back in line with your spine. Be relaxed rather than stiff. Sit with your feet in front of you; if they don’t reach the floor, your chair is too high or you need a footrest. Take frequent stretching breaks.

error a malfunction; a situation in which a computer cannot follow its instructions, or in which recorded data cannot be retrieved correctly. In computing, error does not necessarily denote a mistake made by a human being. See ERROR MESSAGE; HARD ERROR; SOFT ERROR.

error-correcting code any method of encoding data that allows small errors to be corrected. Contrast CHECKSUM, CYCLICAL REDUNDANCY CHECK, and PARITY, which are techniques for detecting errors but not correcting them.

A simple error-correcting code would be to send each message three times, and if some part of the message does not come out the same in all three copies, let the majority rule. In order to be uncorrectable, an error would have to corrupt two of the three copies, not just one. Even then, you would know that an error had been made.

Practical error-correcting codes are more concise and are based on binary matrix arithmetic.

error message a message that indicates that a computer cannot do what is requested or that some part of the software or hardware is defective. Error messages range from “You can’t divide by zero” to “The disk drive isn’t working.” They do not necessarily mean that the user of the computer has made a mistake. See also ERROR.

error trapping see TRAPPING.

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ESRB

eSATA (external SATA) a hardware implementation of the ATA disk drive protocol for connecting external disk drives to computers. It is similar to SATA but uses different connectors. See SATA.

escalate to transfer a customer’s help request from the person who originally handled it to someone more highly trained, and/or to mark it as more urgent.

escape code a code that indicates that the following character is to be handled specially (e.g., as a printer control code), or a code that stands for a character that cannot otherwise be typed. For example, in HTML, the characters < > mark the beginning and end of a command, so if you want them to appear on the screen, you have to type them as the escape codes < and >, respectively.

Escape key a key on a computer keyboard that has a special meaning depending on what software is being used. In many programs and under Microsoft Windows, the Escape key means “get out of where you are now and get back to where you were before” (e.g., back out of a menu without making any of the choices on it). The Escape key transmits ASCII character code 27, which is a character originally used to send special messages to devices. See also ESCAPE SEQUENCE.

escape sequence a special sequence of character codes that cause a screen or printer to perform some action (e.g., changing type style) rather than displaying the characters.

ESDI (Enhanced Small Device Interface) a standard introduced by Maxtor in 1983 as an interface for hard disks. It has largely been superseded by IDE and SCSI. See IDE; SCSI.

ESRB Entertainment Software Rating Board (www.esrb.org) a non-profit, independent organization established in 1994 that reviews entertainment software and web sites and assigns standardized ratings based on suitability for children, as well as descriptions indicating the amount of violent and/or sexual content. ERSB labels can be found on most commercial software titles. The ratings are:

EC	“Early childhood”	Age 3 and up
E	“Everyone”	Age 6 and older; comic violence,
		no sex
K–A	“Kids to adults”	Equivalent to E, obsolete
T	“Teen”	Age 13 and older;
		limited violence, suggestive themes
M	“Mature”	Age 17 and older;
		violence, sexual themes
AO	“Adults only”	Graphic sex and/or violence
RP	“Rating pending”	Not yet rated

Contrast ICRA, which uses ratings self-assigned by the authors; see

PARENTAL CONTROLS.