diff --git a/lib/des/des.c b/lib/des/des.c
index c7311e263..d060b664c 100644
--- a/lib/des/des.c
+++ b/lib/des/des.c
@@ -45,13 +45,15 @@
  * [including the GNU Public Licence.]
  */
 
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
 #include <stdio.h>
 #include <stdlib.h>
-#ifndef MSDOS
+#include <string.h>
+#ifdef HAVE_UNISTD_H
 #include <unistd.h>
-#else
-#include <io.h>
-#define RAND
 #endif
 
 #include <time.h>
@@ -60,22 +62,16 @@
 #ifdef VMS
 #include <types.h>
 #include <stat.h>
-#else
-#ifndef _IRIX
+#endif
+#ifdef HAVE_SYS_TYPES_H
 #include <sys/types.h>
 #endif
+#ifdef HAVE_SYS_STAT_H
 #include <sys/stat.h>
 #endif
-#if defined(NOCONST)
-#define const
-#endif
 #include "des.h"
 
-#if defined(__STDC__) || defined(VMS) || defined(M_XENIX) || defined(MSDOS)
-#include <string.h>
-#endif
-
-#ifdef RAND
+#ifndef HAVE_RANDOM
 #define random rand
 #define srandom(s) srand(s)
 #endif
diff --git a/lib/des/destest.c b/lib/des/destest.c
index d6aede74c..6526c02af 100644
--- a/lib/des/destest.c
+++ b/lib/des/destest.c
@@ -45,14 +45,16 @@
  * [including the GNU Public Licence.]
  */
 
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
 #include <stdio.h>
 #include <stdlib.h>
-#ifndef MSDOS
-#include <unistd.h>
-#else
-#include <io.h>
-#endif
 #include <string.h>
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
 #include "des.h"
 
 /* tisk tisk - the test keys don't all have odd parity :-( */
diff --git a/lib/des/speed.c b/lib/des/speed.c
index c659cf5b1..1e5774c80 100644
--- a/lib/des/speed.c
+++ b/lib/des/speed.c
@@ -48,6 +48,10 @@
 /* 11-Sep-92 Andrew Daviel   Support for Silicon Graphics IRIX added */
 /* 06-Apr-92 Luke Brennan    Support for VMS and add extra signal calls */
 
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
 #ifndef MSDOS
 #define TIMES
 #endif