feature: make RNG implementation shared for rp2040/rp2350

Signed-off-by: deadprogram <[email protected]>

1 files changed, 41 insertions, 0 deletions

diff --git a/src/machine/machine_rp2_rng.go b/src/machine/machine_rp2_rng.go
new file mode 100644
index 000000000..e619f0500
--- /dev/null
+++ b/src/machine/machine_rp2_rng.go
@@ -0,0 +1,41 @@
+//go:build rp2040 || rp2350
+
+// Implementation based on code located here:
+// https://github.com/raspberrypi/pico-sdk/blob/master/src/rp2_common/pico_lwip/random.c
+
+package machine
+
+import (
+	"device/rp"
+)
+
+const numberOfCycles = 32
+
+// GetRNG returns 32 bits of semi-random data based on ring oscillator.
+//
+// Unlike some other implementations of GetRNG, these random numbers are not
+// cryptographically secure and must not be used for cryptographic operations
+// (nonces, etc).
+func GetRNG() (uint32, error) {
+	var val uint32
+	for i := 0; i < 4; i++ {
+		val = (val << 8) | uint32(roscRandByte())
+	}
+	return val, nil
+}
+
+var randomByte uint8
+
+func roscRandByte() uint8 {
+	var poly uint8
+	for i := 0; i < numberOfCycles; i++ {
+		if randomByte&0x80 != 0 {
+			poly = 0x35
+		} else {
+			poly = 0
+		}
+		randomByte = ((randomByte << 1) | uint8(rp.ROSC.GetRANDOMBIT()) ^ poly)
+		// TODO: delay a little because the random bit is a little slow
+	}
+	return randomByte
+}