Clock drift refers to several related phenomena where a clock does not run in the exact right speed compared to another clock. That is, after some time the clock "drifts apart" from the other clock. An analog wall clock A clock (from the Latin cloca, bell) is an instrument for measuring time and for measuring time intervals of less than a day—as opposed to a calendar. ...

This phenomenon is also used for instance in computers to build random number generators. A Lego RCX Computer is an example of an embedded computer used to control mechanical devices. ... A random number generator is a computational or physical device designed to generate a sequence of elements (usually numbers), such that the sequence can be used as a random one. ...

Clock drift in normal clocks

Normal clocks such as clocks at home and wristwatches usually drift compared to the actual time. That is why one must reset their time periodically. Clocks often drift differently depending on their quality, the exact power they get from the battery, the surrounding temperature and so on. Thus the same clock can have different clock drift rates at different occasions. A wrist watch A watch is a small portable clock that displays the time and sometimes the day, date, month and year. ...

More advanced clocks and old mechanical clocks often have some kind of speed trimmer where one can adjust the speed of the clock and thus reduce the clock drift. For instance, in pendulum clocks the clock drift can be manipulated by slightly changing the length of the pendulum. A pendulum clock uses a pendulum as its time base. ... For other uses, see Pendulum (disambiguation). ...

Atomic clocks, the Earth and relativity

Atomic clocks are very precise and have nearly no clock drift at all. The rotation of the Earth itself actually has more clock drift (less accuracy) than modern atomic clocks. Thus to keep the Coordinated Universal Time (UTC) in line with the Earth's rotation some years a leap second has to be added. Atomic clock Chip-Scale Atomic Clock Unveiled by NIST An atomic clock is a type of clock that uses an atomic resonance frequency standard as its counter. ... It has been suggested that leap second be merged into this article or section. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ...

As Einstein predicted clock drift can also occur in any clock when they are moved to new locations or travel at high speed due to relativistic effects. For instance the atomic clocks in GPS satellites experience this effect. Albert Einstein, photographed in 1947 by Oren J. Turner. ... The special theory of relativity was proposed in 1905 by Albert Einstein in his article On the Electrodynamics of Moving Bodies. ... GPS satellite in orbit, image courtesy NASA GPS redirects here. ...

Random number generators

Computer programs often need good random numbers, especially when doing cryptography. There are several similar ways clock drift can be used to build random number generators (RNGs). The terms computer program, software program, applications program, system software, or just program are used to refer to either an executable program by both lay people and computer programmers or the collection of source code from which an executable program is created (eg, compiled). ... It has been suggested that this article or section be merged into randomness. ... The German Lorenz cipher machine, used in World War II for encryption of very high-level general staff messages. ... A random number generator is a computational or physical device designed to generate a sequence of elements (usually numbers), such that the sequence can be used as a random one. ...

One way to build a hardware random number generator is to use two independent clock crystals, one that for instance ticks 100 times per second and one that ticks 1 million times per second. On average the faster crystal will then tick 10000 times for each time the slower one ticks. But since clock crystals are not precise the exact number of ticks will vary. That variation can be used to create random bits. For instance if the number of fast ticks are an even number a 0 is chosen and if the number of ticks are uneven a 1 is chosen. Thus such a 100/1000000 RNG circuit can produce 100 truly random bits per second. Note that care has to be taken so the RNG does not become biased, otherwise it might for instance produce more 0s than 1s. In computing, a hardware random number generator is an apparatus that generates random numbers from a physical process. ... A crystal oscillator (sometimes abbreviated to XTAL on schematic diagrams) is an electronic circuit that uses the mechanical resonance of a physical crystal of piezoelectric material along with an amplifier and feedback to create an electrical signal with a very precise frequency. ...

There is also a similar way to build a kind of "software random number generator". This involves comparing the timer tick of the computer (the tick that usually is 100 times per second) and the speed of the CPU. If the computer timer and the CPU run on two independent clock crystals the situation is ideal and more or less the same as the previous example. But even if they both use the same clock crystal the process/program that does the clock drift measurement is "disturbed" by many more or less unpredictable events in the CPU such as interrupts and other processes and programs that runs at the same time. Thus the measurement will still produce fairly good random numbers. (Some argue they are then not true random numbers but they seem to be good enough for most needs.) CPU redirects here. ... In computing, a process is a running instance of a program, including all variables and other states. ... In computing, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ...

The pseudocode below shows a "software random number generator" that produces 256 random bits and that loads the CPU fully for about 1/3 of a second. This assumes the system tick is 100 times per second and a fairly fast modern computer where the number of loops per tick will be high so each loop will vary with at least 8 bits worth of randomness. The measurement is done 32 times (during 32 separate ticks), each time producing 8 bits of randomness.

 /* Code for a software random number generator. */ char RNGarray[32] /* An array of 32 bytes. */ /* Do 32 clock drift measurements. */ for x = 0 to 31 do { tick = clock() /* Count loops until the next tick. */ loop = 0 while( clock() == tick) { loop = loop + 1 } /* Use the lower 8 bits since those are the ones that vary wildly. */ RNGarray[x] = loop modulo 256 } /* The RNGarray now contains 256 random bits. */ 

Note that most hardware random number generators such as the ones described above are fairly slow. Therefore most programs only use them to create a good seed that they then feed to a pseudorandom number generator or a cryptographically secure pseudorandom number generator to produce many random numbers fast. A pseudorandom number generator (PRNG) is an algorithm that generates a sequence of numbers which are not truly random. ... A cryptographically secure pseudo-random number generator (CSPRNG) is a pseudo-random number generator (PRNG) with properties that make it suitable for use in cryptography. ...