osd-contiki/platform/avr-ravenusb/rng.c

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// ADC-based strong RNG
// Very slow, but who cares---if you need fast random numbers, use a PRNG.
#include "rng.h"
#include <avr/interrupt.h>
#include <avr/io.h>
#include <util/delay.h>
#include "contiki.h"
#ifndef RNG_CONF_USE_ADC
#define RNG_CONF_USE_ADC (!RNG_CONF_USE_RADIO_CLOCK && defined(ADMUX) && defined(ADCSRA) && defined(ADCSRB) && defined(ADSC) && defined(ADEN))
#endif
#ifndef RNG_CONF_USE_RADIO_CLOCK
#define RNG_CONF_USE_RADIO_CLOCK ((!RNG_CONF_USE_ADC) && RF230BB)
#endif
#define TEMPORAL_AGITATION() do { static uint8_t agitator; agitator*=97; agitator+=101; _delay_us(agitator>>1); } while (0);
// -------------------------------------------------------------------------
#if RNG_CONF_USE_ADC
/* The hope is that there is enough noise in the LSB when pointing the
** ADC at the internal band-gap input and using the internal 2.56v
** AREF.
**
** TODO: Run some randomness tests on the output of this RNG!
*/
#define BITS_TO_SHIFT 9
#define ADC_CHAN_ADC1 ((0<<MUX4)|(0<<MUX3)|(0<<MUX2)|(0<<MUX1)|(1<<MUX0))
#define ADC_CHAN_BAND_GAP ((1<<MUX4)|(1<<MUX3)|(1<<MUX2)|(1<<MUX1)|(0<<MUX0))
#define ADC_REF_AREF ((0<<REFS1)|(0<<REFS0))
#define ADC_REF_AVCC ((0<<REFS1)|(1<<REFS0))
#define ADC_REF_INT ((1<<REFS1)|(1<<REFS0))
#define ADC_TRIG_FREE_RUN ((0<<ADTS2)|(0<<ADTS1)|(0<<ADTS0))
#define ADC_PS_128 ((1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0))
#define ADC_PS_2 ((0<<ADPS2)|(0<<ADPS1)|(1<<ADPS0))
#ifndef CONTIKI_CONF_RNG_ADC_CHANNEL
#define CONTIKI_CONF_RNG_ADC_CHANNEL ADC_CHAN_BAND_GAP
#endif
#ifndef CONTIKI_CONF_RNG_ADC_REF
#define CONTIKI_CONF_RNG_ADC_REF ADC_REF_INT
#endif
static uint8_t
extract_random_bit_() {
uint8_t ret = 0;
// Store the state so that we can restore it when we are done.
uint8_t sreg = SREG;
uint8_t adcsra = ADCSRA;
uint8_t admux = ADMUX;
uint8_t adcsrb = ADCSRB;
#ifdef PRR
uint8_t prr = PRR;
#endif
// Disable interrupts
cli();
#ifdef PRR
// Enable ADC module
PRR &= ~(1 << PRADC);
#endif
// Wait for any ADC conversion which
// might currently be happening to finish.
while(ADCSRA & (1<<ADSC));
// Configure the ADC module
ADCSRA = (1<<ADEN)|ADC_PS_128;
ADMUX = (uint8_t)CONTIKI_CONF_RNG_ADC_REF|(uint8_t)CONTIKI_CONF_RNG_ADC_CHANNEL;
ADCSRB = ADC_TRIG_FREE_RUN;
// This loop is where we try to come up with our
// random bit. Unfortunately, the time it takes
// for this to happen is non-deterministic, but
// the result should be non-biased random bit.
do {
// Start conversion for first bit
ADCSRA |= (1<<ADSC);
// Wait for conversion to complete.
while(ADCSRA & (1<<ADSC));
ret = (ADC&1);
ret <<= 1;
// Start conversion for second bit
ADCSRA |= (1<<ADSC);
// Wait for conversion to complete.
while(ADCSRA & (1<<ADSC));
ret |= (ADC&1);
// Toggling the reference voltage
// seems to help introduce noise.
ADMUX^=(1<<REFS1);
// We only want to exit the loop if the first
// and second sampled bits are different.
// This is preliminary conditioning.
} while((ret==0)||(ret==3));
// Toss out the other bit, we only care about one of them.
ret &= 1;
ADCSRA=0;
// Restore the state
ADCSRB = adcsrb;
ADMUX = admux;
ADCSRA = adcsra;
#ifdef PRR
PRR = prr;
#endif
SREG = sreg;
return ret;
}
// -------------------------------------------------------------------------
#elif RNG_CONF_USE_RADIO_CLOCK
/* Here we are hoping to find some noise in the clock skew
** of two different oscilating crystals. On the RZUSBstick,
** there are two such crystals: An 8MHz crystal for the
** microcontroller, and a 16MHz crystal and for the radio.
** The MCLK pin of the RF230 chip is conveniently connected
** to pin 6 of port D. First we need to have the radio
** output the 16MHz signal (it defaults to 1MHz), and then
** we can try to find some noise by sampling pin 6 of port D.
**
** The suitability of this method as a real random number
** generator has yet to be determined. It is entirely possible
** that the perceived randomness of the output is due to
** the temporal agitator mechanism that I have employed.
** Use with caution!
**
** TODO: Run some randomness tests on the output of this RNG!
*/
#define BITS_TO_SHIFT 8
#include "radio/rf230bb/hal.h"
#include "radio/rf230bb/at86rf230_registermap.h"
#ifndef TRX_CTRL_0
#define TRX_CTRL_0 0x03
#endif
static uint8_t
extract_random_bit_() {
uint8_t ret;
uint8_t trx_ctrl_0 = hal_register_read(TRX_CTRL_0);
// Set radio clock output to 8MHz
hal_register_write(TRX_CTRL_0,0x8|5);
do {
TEMPORAL_AGITATION(); // WARNING: This step may hide lack of entropy!
ret = !!(PIND&(1<<6));
ret <<= 1;
ret |= !!(PIND&(1<<6));
} while((ret==0)||(ret==3));
// Toss out the other bit, we only care about one of them.
ret &= 1;
// Restore the clkm state
hal_register_write(TRX_CTRL_0,trx_ctrl_0);
return ret;
}
#endif
// -------------------------------------------------------------------------
static uint8_t
extract_random_bit() {
uint8_t ret;
// These next two lines attempt to sync ourselves to
// any pattern that might happen to be present in the
// raw random source stream. After this, we use the
// bias removal mechanism below to filter out the first
// sign of noise.
while(extract_random_bit_()==1);
while(extract_random_bit_()==0);
do {
ret = extract_random_bit_();
ret <<= 1;
ret |= extract_random_bit_();
} while((ret==0)||(ret==3));
// Toss out the other bit, we only care about one of them.
ret &= 1;
return ret;
}
uint8_t
rng_get_uint8() {
uint8_t ret = 0, i;
for(i=0;i<BITS_TO_SHIFT;i++) {
// Leftshift.
ret <<= 1;
// Add a random bit.
ret |= extract_random_bit();
}
return ret;
}