Replace math rand with crypto rand

lib/service/util.go

@@ -0,0 +1,19 @@
+package service
+
+import (
+	"crypto/rand"
+	"math/big"
+)
+
+func randBytesFromStr(length int, from string) ([]byte, error) {
+	b := make([]byte, length)
+	fromLenBigInt := big.NewInt(int64(len(from)))
+	for i := range b {
+		r, err := rand.Int(rand.Reader, fromLenBigInt)
+		if err != nil {
+			return nil, err
+		}
+		b[i] = from[r.Int64()]
+	}
+	return b, nil
+}