Cchange random() to use libc srand()/rand()
Uisng libc's implementation is likely to be safer than rolling our own. At least the failure modes are well documented. (glibc's implementation of rand() is actually not bad).master
parent
1bb4377c8c
commit
db5227872a

@ 90,7 +90,6 @@ class MicroBit


private:




void seedRandom();


uint32_t randomValue;




public:





@ 214,8 +213,9 @@ class MicroBit




/**


* Generate a random number in the given range.


* We use the NRF51822 in built random number generator here


* TODO: Determine if we want to, given its relatively high power consumption!


* We use libc's rand() which is sufficient for our applications and much


* more lightweight than the hardware random number generator built into


* the processor, which takes a long time and uses a lot of energy.


*


* @param max the upper range to generate a number for. This number cannot be negative


* @return A random, natural number between 0 and the max1. Or MICROBIT_INVALID_PARAMETER if max is <= 0.





@ 1,4 +1,5 @@


#include "MicroBit.h"


#include <stdlib.h> /* rand(),srand() */




char MICROBIT_BLE_DEVICE_NAME[] = "BBC micro:bit [xxxxx]";





@ 283,13 +284,11 @@ int MicroBit::sleep(int milliseconds)




/**


* Generate a random number in the given range.


* We use a simple Galois LFSR random number generator here,


* as a Galois LFSR is sufficient for our applications, and much more lightweight


* than the hardware random number generator built int the processor, which takes


* a long time and uses a lot of energy.


* We use libc's rand() which is sufficient for our applications and much


* more lightweight than the hardware random number generator built into


* the processor, which takes a long time and uses a lot of energy.


*


* KIDS: You shouldn't use this is the real world to generte cryptographic keys though...


* have a think why not. :)


* KIDS: You shouldn't use this is the real world to generate cryptographic keys though...


*


* @param max the upper range to generate a number for. This number cannot be negative


* @return A random, natural number between 0 and the max1. Or MICROBIT_INVALID_VALUE (defined in ErrorNo.h) if max is <= 0.



@ 301,60 +300,38 @@ int MicroBit::sleep(int milliseconds)


*/


int MicroBit::random(int max)


{


uint32_t m, result;


unsigned long num_bins, num_rand, bin_size, defect;


int result;




//return MICROBIT_INVALID_VALUE if max is <= 0...


if(max <= 0)


return MICROBIT_INVALID_PARAMETER;




// Our maximum return value is actually one less than passed


max;


num_bins = (unsigned long) max;


num_rand = (unsigned long) RAND_MAX + 1;


bin_size = num_rand / num_bins;


defect = num_rand % num_bins;




do {


m = (uint32_t)max;


result = 0;


while(m >>= 1) {


// Cycle the LFSR (Linear Feedback Shift Register).


// We use an optimal sequence with a period of 2^321, as defined by Bruce Schneider here (a true legend in the field!),


// For those interested, it's documented in his paper:


// "PseudoRandom Sequence Generator for 32Bit CPUs: A fast, machineindependent generator for 32bit Microprocessors"


// https://www.schneier.com/paperpseudorandomsequence.html


// Avoid interupts (and hence fibre context switch) while we are doing this




__disable_irq();




randomValue = ((((randomValue >> 31)


^ (randomValue >> 6)


^ (randomValue >> 4)


^ (randomValue >> 2)


^ (randomValue >> 1)


^ randomValue)


& 0x0000001)


<< 31 )


 (randomValue >> 1);




__enable_irq();




result = ((result << 1)  (randomValue & 0x00000001));


}


} while (result > (uint32_t)max);


result = rand();


} while ((num_rand  defect) <= (unsigned long)result); // CARE: avoid overflow






return result;


return result/bin_size;


}






/**


* Seed our a random number generator (RNG).


* We use the NRF51822 in built cryptographic random number generator to seed a Galois LFSR.


* We use the NRF51822 in built cryptographic random number generator to seed rand().


* We do this as the hardware RNG is relatively high power, and use the the BLE stack internally,


* with a less than optimal application interface. A Galois LFSR is sufficient for our


* with a less than optimal application interface. rand() is sufficient for our


* applications, and much more lightweight.


*/


void MicroBit::seedRandom()


{


randomValue = 0;




unsigned int seed = 0;




// Start the Random number generator. No need to leave it running... I hope. :)


NRF_RNG>TASKS_START = 1;





@ 366,11 +343,13 @@ void MicroBit::seedRandom()


// Wait for a number ot be generated.


while ( NRF_RNG>EVENTS_VALRDY == 0);




randomValue = (randomValue << 8)  ((int) NRF_RNG>VALUE);


seed = (seed << 8)  ((int) NRF_RNG>VALUE);


}




// Disable the generator to save power.


NRF_RNG>TASKS_STOP = 1;




srand(seed);


}








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