Description
LoRaClass::random() reads the register once and returns the byte.
byte LoRaClass::random() {
return readRegister(REG_RSSI_WIDEBAND);
}This is wrong for two reasons:
-
LoRa chip must be set in a specific mode before that, you can't get random number at any given time. LoRa must be stopped and set to continuous mode first, and so on.
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There should be 8 reads, keeping only the least significant bit (LSB), and build the output byte from that.
See Chapter 4 of Random Number Generation for Cryptography.
To generate an N bit random number, perform N read operation of the register RegRssiWideband (address 0x2c) and use the LSB of the fetched value. The value from RegRssiWideband is derived from a wideband (4MHz) signal strength at the receiver input and the LSB of this value constantly and randomly changes.
Example code from LoRandom.h
#define RegOpMode 0x01
#define RegModemConfig1 0x1D
#define RegModemConfig2 0x1E
#define RegRssiWideband 0x2C
void setupLoRandom() {
LoRa.writeRegister(RegOpMode, 0b10001101);
// LoRa mode enable Receive Continuous mode
LoRa.writeRegister(RegModemConfig1, 0b01110010);
// ‘0111’ for 125kHz modulation Bandwidth
// 4/5 error coding rate
// Packets have up-front header
LoRa.writeRegister(RegModemConfig2, 0b01110000);
// ‘0111’ (SF7) = 6kbit/s
}
uint8_t getLoRandomByte() {
uint8_t x = 0;
for (uint8_t j = 0; j < 8; j++) {
x += (LoRa.readRegister(RegRssiWideband) & 0b00000001);
x = x << 1;
delay(1);
}
return x;
}You need to put the SX1276/8 in Receive Continuous mode, SF7, C/R 4/5, BW 125KHz (call to setupLoRandom(), and then read 8 bytes to get one random byte (call to getLoRandomByte()). After which you need to put back your LoRa chip to whatever settings you were using.
In BastWAN_LoRa_RNG I build a buffer of 256 random bytes at once.