nrf: make GetRNG available to all chips

All nrf chips have a cryptographically secure RNG on board. Therefore, I've made the code more portable so that it works on all nrf chips. I did remove a number of exported functions. I am of the opinion that these should only be made available once we have an agreed upon API for multiple chips. People who want to have greater control over the RNG should use the device/nrf package directly instead. I have also changed the behavior to always enable digital error correction. Enabling it seems like a more conservative (and secure) default to me. Again, people who would like to have it disabled can use the device/nrf package directly.

Author: aykevl

src/crypto/rand/rand_baremetal.go

@@ -1,5 +1,5 @@
-//go:build nrf52840 || stm32 || (sam && atsamd51) || (sam && atsame5x)
-// +build nrf52840 stm32 sam,atsamd51 sam,atsame5x
+//go:build nrf || stm32 || (sam && atsamd51) || (sam && atsame5x)
+// +build nrf stm32 sam,atsamd51 sam,atsame5x
 
 package rand
 

src/machine/machine_nrf.go

@@ -350,3 +350,30 @@ func (i2c *I2C) readByte() (byte, error) {
 i2c.Bus.EVENTS_RXDREADY.Set(0)
 return byte(i2c.Bus.RXD.Get()), nil
 }
+
+var rngStarted = false
+
+// GetRNG returns 32 bits of non-deterministic random data based on internal thermal noise.
+// According to Nordic's documentation, the random output is suitable for cryptographic purposes.
+func GetRNG() (ret uint32, err error) {
+// There's no apparent way to check the status of the RNG peripheral's task, so simply start it
+// to avoid deadlocking while waiting for output.
+if !rngStarted {
+nrf.RNG.TASKS_START.Set(1)
+nrf.RNG.SetCONFIG_DERCEN(nrf.RNG_CONFIG_DERCEN_Enabled)
+rngStarted = true
+}
+
+// The RNG returns one byte at a time, so stack up four bytes into a single uint32 for return.
+for i := 0; i < 4; i++ {
+// Wait for data to be ready.
+for nrf.RNG.EVENTS_VALRDY.Get() == 0 {
+}
+// Append random byte to output.
+ret = (ret << 8) ^ nrf.RNG.GetVALUE()
+// Unset the EVENTS_VALRDY register to avoid reading the same random output twice.
+nrf.RNG.EVENTS_VALRDY.Set(0)
+}
+
+return ret, nil
+}