diff --git a/src/crypto-support/crypto-helper.hpp b/src/crypto-support/crypto-helper.hpp
index ad8aa22..1ff0149 100644
--- a/src/crypto-support/crypto-helper.hpp
+++ b/src/crypto-support/crypto-helper.hpp
@@ -30,6 +30,7 @@
 
 static const int INFO_LEN = 10;
 static const uint8_t INFO[] = {0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9};
+static const int AES_128_KEY_LEN = 16;
 
 struct ECDH_CTX {
   int EC_NID;