tom dunigan
proof-of-concept: generate random bits on LPC1768 using dueling clocks (systick and WDT/RTC)
random bits from dueling clocks
Using dueling clocks to generate random bits is described by Walter Anderson at https://sites.google.com/site/astudyofentropy/project-definition/timer-jitter-entropy-sources/entropy-library
There are implementations for AVR(UNO etc.) and ARM-based (teensy) MCUs.
This mbed LPC1768 implementation uses systick clock versus the RTC crystal (32khz) as a source to the WDT timer. The LPC1768 WDT interrupt cannot be cleared, so this implementation generates the random bits when they are requested. The WDT scales the source clock by /4, so the random bit rate is about 8192 bits/second. If your board doesn't have a 32khz crystal, it is also possible to source the WDT from the 4 MHz IRC oscillator.
I collected several megabytes of random bits and they passed various random-bit testers (rngtest, ent, NIST's STS).
Another mbed random bit generator using ADC noise and mixing with SHA256 is desribed at https://developer.mbed.org/users/Remco/notebook/secure-hardware-random-number-using-the-mbed and an mbed teensy 3.1 generator
One could also just use these generators to create a seed for a hash-based PRNG.
Some ARM chips have builtin hardware TRNG's (DUE, pyboard, Raspberry PI) and Intel Edison.
FYI, RNG data on other MCUs https://github.com/manitou48/DUEZoo/blob/master/RNGperf.txt
and Anderson's spreadsheet
https://docs.google.com/spreadsheet/pub?key=0AukiKiYKrSl9dHNIX19oZ0ZqNDc1RDNMa042SzhZT0E&output=html
Revision 0:808fc29f4d37, committed 2015-07-25
- Comitter:
- manitou
- Date:
- Sat Jul 25 15:27:38 2015 +0000
- Commit message:
- initial
Changed in this revision
| main.cpp | Show annotated file Show diff for this revision Revisions of this file |
| mbed.bld | Show annotated file Show diff for this revision Revisions of this file |
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/main.cpp Sat Jul 25 15:27:38 2015 +0000
@@ -0,0 +1,146 @@
+
+// random bits from systick and WDT
+// based on arduino/teensy Entropy lib
+// https://sites.google.com/site/astudyofentropy/project-definition/timer-jitter-entropy-sources/entropy-library
+// seems WDT ISR will only fire once, so check free-running WDT periodically
+// enable systick as free running 24-bit clock to beat against RTC crystal
+#include "mbed.h"
+
+Timer tmr;
+Serial pc(USBTX, USBRX);
+
+void kick() {
+ // restart WDT countdown
+ __disable_irq();
+ LPC_WDT->WDFEED = 0xAA;
+ LPC_WDT->WDFEED = 0x55;
+ __enable_irq();
+}
+
+void wdt_init() {
+ LPC_WDT->WDCLKSEL = 0x02; // Set CLK src 0 IRC 1 PCLK 2 RTC
+ LPC_WDT->WDTC = 0xfffffff; // max countdown
+ LPC_WDT->WDMOD = 0x01; // enable but no reset
+ kick();
+}
+
+void systick_init() {
+ SysTick->LOAD = 0x123456; // doesn't really matter
+ SysTick->VAL = 0;
+ SysTick->CTRL = 4 | 1; //CLKSOURCE=CPU clock | ENABLE
+}
+
+#if 0
+volatile unsigned int rword;
+void ticker_isr() {
+ uint32_t t;
+ t= LPC_WDT->WDTV;
+ while (t == LPC_WDT->WDTV); // wait til new value
+ rword = SysTick->VAL;
+}
+
+Ticker ticker;
+#endif
+
+// entropy collection
+ const uint8_t gWDT_buffer_SIZE=32;
+ const uint8_t WDT_POOL_SIZE=8;
+ uint8_t gWDT_buffer[gWDT_buffer_SIZE];
+ uint8_t gWDT_buffer_position;
+ uint8_t gWDT_loop_counter;
+ volatile uint8_t gWDT_pool_start;
+ volatile uint8_t gWDT_pool_end;
+ volatile uint8_t gWDT_pool_count;
+ volatile uint32_t gWDT_entropy_pool[WDT_POOL_SIZE];
+
+static void collect() {
+ uint32_t t;
+ t= LPC_WDT->WDTV;
+ while (t == LPC_WDT->WDTV); // wait til new value
+ // TODO kick WDT to avoid reset when counter < 100000 or schedule
+ gWDT_buffer[gWDT_buffer_position] = SysTick->VAL;
+ gWDT_buffer_position++;
+ if (gWDT_buffer_position >= gWDT_buffer_SIZE)
+ {
+ gWDT_pool_end = (gWDT_pool_start + gWDT_pool_count) % WDT_POOL_SIZE;
+ // The following code is an implementation of Jenkin's one at a time hash
+ // This hash function has had preliminary testing to verify that it
+ // produces reasonably uniform random results when using WDT jitter
+ // on a variety of Arduino platforms
+ for(gWDT_loop_counter = 0; gWDT_loop_counter < gWDT_buffer_SIZE; ++gWDT_loop_counter)
+ {
+ gWDT_entropy_pool[gWDT_pool_end] += gWDT_buffer[gWDT_loop_counter];
+ gWDT_entropy_pool[gWDT_pool_end] += (gWDT_entropy_pool[gWDT_pool_end] << 10);
+ gWDT_entropy_pool[gWDT_pool_end] ^= (gWDT_entropy_pool[gWDT_pool_end] >> 6);
+ }
+ gWDT_entropy_pool[gWDT_pool_end] += (gWDT_entropy_pool[gWDT_pool_end] << 3);
+ gWDT_entropy_pool[gWDT_pool_end] ^= (gWDT_entropy_pool[gWDT_pool_end] >> 11);
+ gWDT_entropy_pool[gWDT_pool_end] += (gWDT_entropy_pool[gWDT_pool_end] << 15);
+ gWDT_entropy_pool[gWDT_pool_end] = gWDT_entropy_pool[gWDT_pool_end];
+ gWDT_buffer_position = 0; // Start collecting the next 32 bytes of Timer 1 counts
+ if (gWDT_pool_count == WDT_POOL_SIZE) // The entropy pool is full
+ gWDT_pool_start = (gWDT_pool_start + 1) % WDT_POOL_SIZE;
+ else // Add another unsigned long (32 bits) to the entropy pool
+ ++gWDT_pool_count;
+ }
+}
+
+
+uint32_t random() {
+ uint32_t retVal;
+
+ while (gWDT_pool_count < 1) collect(); // gather entropy
+
+ __disable_irq(); // crtical section
+ retVal = gWDT_entropy_pool[gWDT_pool_start];
+ gWDT_pool_start = (gWDT_pool_start + 1) % WDT_POOL_SIZE;
+ --gWDT_pool_count;
+ __enable_irq();
+ return(retVal);
+}
+
+#define REPS 50
+void display() {
+ uint32_t r,t;
+ int i;
+ float bps;
+
+ t=tmr.read_us();
+ for (i=0;i<REPS;i++)r=random();
+ t= tmr.read_us() -t;
+ bps = REPS*32.e6/t;
+ printf("%f bps %0x\n",bps,r);
+ wait(3.0);
+}
+
+void logger() {
+ // await start byte from host then start sending random numbers
+ // ./logger 5000000 for diehard tests
+ unsigned int rng;
+ char *bytes = (char *) &rng;
+
+ pc.getc(); // await byte from host
+ while(1) {
+ rng = random();
+ for (int i=0; i<4;i++) pc.putc(bytes[i]);
+ }
+}
+
+int main() {
+
+ gWDT_buffer_position=0;
+ gWDT_pool_start = 0;
+ gWDT_pool_end = 0;
+ gWDT_pool_count = 0;
+ tmr.start();
+ systick_init();
+ wdt_init();
+
+ // ticker.attach_us(&ticker_isr,1000); // 1/8192 122 us
+
+ while(1) {
+ display();
+ // logger();
+ // printf("rword %x\n",rword); wait(3.0);
+ }
+}
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/mbed.bld Sat Jul 25 15:27:38 2015 +0000 @@ -0,0 +1,1 @@ +http://mbed.org/users/mbed_official/code/mbed/builds/bad568076d81 \ No newline at end of file