diff --git a/crypto/hkdf/hkdf.c b/crypto/hkdf/hkdf.c
index be51bc3..bb7f5a4 100644
--- a/crypto/hkdf/hkdf.c
+++ b/crypto/hkdf/hkdf.c
@@ -15,6 +15,7 @@
 #include <openssl/hkdf.h>
 
 #include <assert.h>
+#include <string.h>
 
 #include <openssl/err.h>
 #include <openssl/hmac.h>
diff --git a/crypto/hkdf/hkdf_test.c b/crypto/hkdf/hkdf_test.c
index a098fba..7467fe0 100644
--- a/crypto/hkdf/hkdf_test.c
+++ b/crypto/hkdf/hkdf_test.c
@@ -13,6 +13,7 @@
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
 
 #include <stdio.h>
+#include <string.h>
 
 #include <openssl/bio.h>
 #include <openssl/crypto.h>