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View Full Version : How to get a good random number seed?




jrdoner
Dec 4, 2009, 09:14 PM
I need a random number generator in an IPhone app, and I want it to seed each time from a new seed, not known to or controlled by the user. In another language development system, the time could be recalled as a floating point number, the fractional part of which could represent the time down to microseconds.

This was ideal, because if you grab the microseconds, convert to an integer, and use it as a seed, it pretty well assured a good, unpredictable result.

Well, I am trying to find this same feature in IPhone XCode, but NSDate (the closest thing I could think of) doesn't seem to offer time below the seconds level.

So is there some other high-resolution clock that I could take advantage of?

Thanks in advance for any help.

John Doner



lloyddean
Dec 4, 2009, 09:58 PM
I believe the following might work as you've described.


#include <sys/time.h>

struct timeval tv;
gettimeofday(&tv, NULL);
srand(tv.tv_usec);

admanimal
Dec 4, 2009, 10:02 PM
Depending on how often you need to reseed the generator and exactly how unpredictable you need your results to be, just using the seconds since 1970 or whatever should be more than adequate.

lloyddean
Dec 5, 2009, 12:15 AM
Depending on how often you need to reseed the generator and exactly how unpredictable you need your results to be, just using the seconds since 1970 or whatever should be more than adequate.

Substitute 'tv_sec' (in my previous code fragment above) for 'tv_usec' and that's what you'll get - seconds since 1970.

firewood
Dec 5, 2009, 02:30 AM
There are web sites that provide high-entropy random numbers from physically random sources (radioactive decay, turbulent motion, etc.)

For on-device entropy, you could try turning on audio recording from the iPhone mic, and hash a few seconds of background noise samples. Same with the accelerometers if the device is moving.

bredell
Dec 5, 2009, 11:22 AM
Well, I am trying to find this same feature in IPhone XCode, but NSDate (the closest thing I could think of) doesn't seem to offer time below the seconds level.

So is there some other high-resolution clock that I could take advantage of?

You can use NSDate. It's true that it counts seconds but it's not an integer. NSDate uses a double precision floating point value with microsecond precision.