Finding a source of entropy in an embedded system?

Question

For a small embedded device (TI MSP430F2274), I am trying to create a Pseudo Random Number Generator (PRNG), but I am having difficulty identifying a potential source of entropy to use as a seed. Unfortunately, the device does not have enough available memory space to include time.h and incorporate the function srand(time(0)) . Has anyone had experience with this family of devices, or has incorporated a PRNG in an embedded device that is constrained by its memory space? Thank you.

Answer 1

Your part (MSP430F2274) does not offer any generic solution or support, but your application may do so. Any unpredictable and asynchronous external event or value that is guaranteed to occur or be available before or exactly when you need can be used.

For example the part has a pair of 16 bit timers, with one of these running, on detection of some asynchronous trigger event such as a user button press, the value of the clock counter at that time may be used as a seed.

Alternatively if you have an analogue input with a continuously varying and asynchronous signal, simply reading that value at any time and perhaps read multiple samples spaced over a suitable time interval to generate a larger seed if necessary.

Even without a specific signal, an otherwise unused ADC input channel is likely to have sufficient noise to make its least significant bit unpredictable - you might concatenate the LSB from a number of independent samples to generate a seed or the required length.

Essentially any unpredictable external event that your application already supports may suffice. Without details of your application it is not possible to advise specifically, but given that this is specifically a mixed-signal microcontroller, there will presumably be some suitable external unpredictability?

Answer 2

If you have multiple clock sources (and the MSP430F2274 seems to have that at a glance), you can use the unpredictable drift between these sources for entropy if you absolutely have nothing better.

The way to do is using two sources, one as a time base, measuring ticks of the other during a period. The count of ticks will vary slightly as the two clock sources are independent. Depending on what options are available for timers, this may be done by timers, otherwise even the watchdog could be an option, configured as an interval timer (if nothing else, it is usually capable to run on a different clock source than the main clock).

This method may requre some time to set up (as the clocks don't deviate a lot from their specified frequency, so you need to wait for relatively long to gather a meaningful amount of random deviance between them, a second or so maybe sufficient).

Otherwise as Clifford mentioned, you could gather entropy from your environment, which is definitely superior, if you have such an environment available. The only good thing in this one (drift between clock sources) is that this is very likely available for just about any setup.

By the way you can not do srand(time(0)) , just from where you are expecting time() to get the count of seconds since epoch on a microcontroller? :)