diff --git a/src/crypto/drbg.c b/src/crypto/drbg.c
index 68aa7857..58e8fa7b 100644
--- a/src/crypto/drbg.c
+++ b/src/crypto/drbg.c
@@ -36,6 +36,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
  */
 
 #include <stdint.h>
+#include <string.h>
 #include <errno.h>
 #include <assert.h>
 #include <ipxe/entropy.h>
diff --git a/src/crypto/hash_df.c b/src/crypto/hash_df.c
index 4c2d28a9..9e3da181 100644
--- a/src/crypto/hash_df.c
+++ b/src/crypto/hash_df.c
@@ -36,6 +36,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
  */
 
 #include <stdint.h>
+#include <string.h>
 #include <byteswap.h>
 #include <ipxe/crypto.h>
 #include <ipxe/hash_df.h>
diff --git a/src/crypto/hmac_drbg.c b/src/crypto/hmac_drbg.c
index 64613d01..efbd67dc 100644
--- a/src/crypto/hmac_drbg.c
+++ b/src/crypto/hmac_drbg.c
@@ -36,6 +36,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
  */
 
 #include <stdint.h>
+#include <string.h>
 #include <errno.h>
 #include <ipxe/crypto.h>
 #include <ipxe/hmac.h>
diff --git a/src/tests/hash_df_test.c b/src/tests/hash_df_test.c
index 563704f4..201b4a18 100644
--- a/src/tests/hash_df_test.c
+++ b/src/tests/hash_df_test.c
@@ -33,6 +33,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #undef NDEBUG
 
 #include <assert.h>
+#include <string.h>
 #include <ipxe/hash_df.h>
 #include <ipxe/test.h>
 
diff --git a/src/tests/hmac_drbg_test.c b/src/tests/hmac_drbg_test.c
index cf066620..ad5584f1 100644
--- a/src/tests/hmac_drbg_test.c
+++ b/src/tests/hmac_drbg_test.c
@@ -33,6 +33,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #undef NDEBUG
 
 #include <assert.h>
+#include <string.h>
 #include <ipxe/hmac_drbg.h>
 #include <ipxe/test.h>