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lorawan-device-patch/src/rng.rs

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+//! RNG based on the `wyrand` pseudorandom number generator.
+//!
+//! This crate uses the random number generator for exactly two things:
+//!
+//! * Generating DevNonces for join requests
+//! * Selecting random channels when transmitting uplinks.
+//!
+//! The good news is that both these operations don't require true
+//! cryptographic randomness. In fact, in both cases, we don't even care about
+//! predictability! A pseudorandom number generator initialized with a seed
+//! generated by a true random number generator is plenty enough:
+//!
+//! * DevNonces must only be unique with a low chance of collision.
+//!   The 1.0.4 LoRaWAN spec even explicitly requires the DevNonces to be
+//!   a sequence of incrementing integers, which is obviously predictable.
+//! * No one cares if the channel selected for the next uplink is predictable,
+//!   as long as the channel selection yields an uniform distribution.
+//!
+//! By providing a PRNG `RngCore` implementation, we enable the crate users the
+//! flexibility of choosing whether they want to provide their own RNG, or just
+//! a seed to instantiate this PRNG to generate the random numbers for them.
+
+use fastrand::Rng;
+use rand_core::RngCore;
+
+#[derive(Clone)]
+/// A pseudorandom number generator utilizing Wyrand algorithm via
+/// the `fastrand` crate.
+pub struct Prng(Rng);
+
+impl Prng {
+    pub(crate) fn new(seed: u64) -> Self {
+        Self(Rng::with_seed(seed))
+    }
+}
+
+impl RngCore for Prng {
+    fn next_u32(&mut self) -> u32 {
+        self.0.u32(..)
+    }
+
+    fn next_u64(&mut self) -> u64 {
+        self.0.u64(..)
+    }
+
+    fn fill_bytes(&mut self, dest: &mut [u8]) {
+        rand_core::impls::fill_bytes_via_next(self, dest)
+    }
+
+    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), rand_core::Error> {
+        self.fill_bytes(dest);
+        Ok(())
+    }
+}