diff --git a/util.h b/util.h
index cf0334a..4d090a6 100644
--- a/util.h
+++ b/util.h
@@ -4,11 +4,14 @@
 #include <cryptopp/filters.h>
 #include <cryptopp/hex.h>
 #include <cryptopp/sha3.h>
+#include <cstddef>
 
 namespace fortuna {
 
 class Util final {
 public:
+static constexpr const std::size_t gen_block_size{32}; // 256 bits
+
 static auto do_sha(const std::string& str_to_hash) -> const std::string {
 	// do sha256
 	std::string digest;