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|  |  | To access the contents, click the chapter and section titles. Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Code in C (cloth) (Publisher: John Wiley & Sons, Inc.) Author(s): Bruce Schneier ISBN: 0471128457 Publication Date: 01/01/96 Search this book:
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The book goes on to discuss the results of various randomness tests on the data. It also suggests how to use the book to find a random number: The lines of the digit table are numbered from 00000 to 19999. In any use of the table, one should first find a random starting position. A common procedure for doing this is to open the book to an unselected page of the digit table and blindly choose a five-digit number; this number with the first digit reduced modulo 2 determines the starting line; the two digits to the right of the initially selected five-digit number are reduced modulo 50 to determine the starting column in the starting line. To guard against the tendency of books to open repeatedly at the same page and the natural tendency of a person to choose a number toward the center of the page: every five-digit number used to determine a starting position should be marked and not used a second time for this purpose. The meat of the book is the “Table of Random Digits.” It lists them in 5-digit groups—“10097 32533 76520 13586...”—50 on a line and 50 lines on a page. The table goes on for 400 pages and, except for a particularly racy section on page 283 which reads “69696,” makes for a boring read. The book also includes a table of 100,000 normal deviates. The interesting thing about the RAND book is not its million random digits, but that they were created before the computer revolution. Many cryptographic algorithms use arbitrary constants—so-called “magic numbers.” Choosing magic numbers from the RAND tables ensures that they haven’t been specially chosen for some nefarious reason. Khafre does this, for example. Using Random Noise The best way to collect a large number of random bits is to tap the natural randomness of the real world. Often this method requires specialized hardware, but you can play tricks with computers. Find an event that happens regularly but randomly: atmospheric noise peaking at a certain threshold, a toddler falling while learning to walk, or some such. Measure the time interval between one event and the next event. Record it. Measure the time interval between the second event and the third event. Record it as well. If the first time interval is greater than the second, output 1 as the bit. If the second time interval is greater than the first, output 0 as the event. Do it again for the next event. Throw a dart at the New York Stock Exchange closing prices in your local newspaper. Compare the closing price of the stock you hit with the closing price of the stock directly above it. If the one you hit is more, output 0; if it less, output 1. Hook a Geiger counter up to your computer, count emissions over a fixed time interval, and keep the least significant bit. Or measure the time between successive ticks. (Since the radioactive source is decaying, the average time between successive ticks is continuously getting longer. You want to choose a source with the half life long enough to make this negligible—like plutonium. Or, if you’re worried about your health, you can apply appropriate statistical corrections.) G. B. Agnew proposed a real random-bit generator, suitable for integration into a VLSI device [21]. It is a metal insulator semiconduction capacitor (MISC). Two of them are placed in close proximity, and the random bit is a function of the difference in charge between the two. Another random-number generator generates a random-bit stream based on the frequency instability in a free-running oscillator [535]. A commercial chip from AT&T generates random numbers from the same phenomenon [67]. M. Gude built a random-number generator that collected random bits from physical phenomena, such as radioactive decay [668,669]. Manfield Richter developed a random-number generator based on thermal noise from a semiconductor diode [1309]. Supposedly the time intervals between successive 2e4 light emissions from a trapped mercury atom are random. Use that. Better yet, find a semiconductor company that makes random-number-generation chips; they are out there. There is also a random-number generator that uses the computer’s disk drive [439]. It measures the time required to read a disk block and uses the variation in that time as a random number source. It filters the timing data to remove structure that comes from quantization, then applies a fast Fourier transform to vectors of the numbers. This removes bias and correlation. Finally, it uses the spectral angles for frequencies in (0, π), normalized to the unit interval, as the random bits. A large part of the variation in disk rotation speed is caused by air turbulence, so there is randomness in the system. There are caveats, though. If you keep too many bits of the output, you are using the fast Fourier transform as a random-number generator and risk predictability. And it’s best to read the same disk block over and over, so that your filtering doesn’t have to remove structure that comes from the disk-scheduler. An implementation of this system was able to collect about 100 bits per minute [439]. Using the Computer’s Clock If you want a single random bit (or even a few), take the least significant bit from any clock register. This might not be terribly random in a UNIX system because of various potential synchronizations, but it works on some personal computers. Beware of getting too many bits this way. Executing the same subroutine several times in succession could easily skew bits generated in this manner. For example, if each bit generation subroutine takes an even number of clock ticks to execute, you will get an endless stream of the same bit out of the generator. If each subroutine takes an odd number of clock ticks to execute, you will get an endless stream of alternating bits out of the generator. Even if the resonance isn’t this obvious, the resultant bit stream will be far from random. One random-number generator works this way [918]: Our truly random number generator...works by setting an alarm and then incrementing a counter register rapidly in the CPU until an interrupt occurs. The contents of the register are then XORed with the contents of an output buffer byte (truncating the register’s data to 8 bits). After each byte of the output buffer is filled, the buffer is further processed by doing a right, circular shift of each character by 2 bits. This has the effect of moving the most active (and random) least significant bits into the most significant positions. The entire process is then repeated 3 times. Finally each character of the buffer has been touched by the two most random bits of the counter register after interrupts. That is 4n interrupts have occurred where n is the number of desired random bytes. This method is very sensitive to the randomness of system interrupts and the granularity of the clock. The output looked pretty good when tested on real UNIX machines. Measuring Keyboard Latency People’s typing patterns are both random and nonrandom. They are nonrandom enough that they can be used as a means of identification, but they are random enough that they can be used to generate random bits. Measure the time between successive keystrokes, then take the least significant bits of those measurements. These bits are going to be pretty random. This technique may not work on a UNIX terminal, since the keystrokes pass through filters and other mechanisms before they get to your program, but it will work on most personal computers.
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