Add Number func to random pkg

crypto/random/entropy_test.go

@@ -6,7 +6,6 @@ import (
 )
 
 func TestFeeder(t *testing.T) {
-
 	// wait for start / first round to complete
 	time.Sleep(1 * time.Millisecond)
 

crypto/random/get.go

@@ -1,8 +1,10 @@
 package random
 
 import (
+	"encoding/binary"
 	"errors"
 	"io"
+	"math"
 	"time"
 
 	"github.com/Safing/portbase/config"
@@ -94,3 +96,21 @@ func Bytes(n int) ([]byte, error) {
 
 	return rng.PseudoRandomData(uint(n)), nil
 }
+
+// Number returns a random number from 0 to (incl.) max.
+func Number(max uint64) (uint64, error) {
+	secureLimit := math.MaxUint64 - (math.MaxUint64 % max)
+	max++
+
+	for {
+		randomBytes, err := Bytes(8)
+		if err != nil {
+			return 0, err
+		}
+
+		candidate := binary.LittleEndian.Uint64(randomBytes)
+		if candidate < secureLimit {
+			return candidate % max, nil
+		}
+	}
+}

crypto/random/get_test.go

@@ -0,0 +1,35 @@
+package random
+
+import (
+	"testing"
+)
+
+func TestNumberRandomness(t *testing.T) {
+	if testing.Short() {
+		t.Skip()
+	}
+
+	var subjects uint64 = 10
+	var testSize uint64 = 10000
+
+	results := make([]uint64, int(subjects))
+	for i := 0; i < int(subjects*testSize); i++ {
+		n, err := Number(subjects - 1)
+		if err != nil {
+			t.Fatal(err)
+			return
+		}
+		results[int(n)]++
+	}
+
+	// catch big mistakes in the number function, eg. massive % bias
+	lowerMargin := testSize - testSize/50
+	upperMargin := testSize + testSize/50
+	for subject, result := range results {
+		if result < lowerMargin || result > upperMargin {
+			t.Errorf("subject %d is outside of margins: %d", subject, result)
+		}
+	}
+
+	t.Fatal(results)
+}