diff --git a/src/openssl.hxx b/src/openssl.hxx
index c8d14d7..09fe18f 100644
--- a/src/openssl.hxx
+++ b/src/openssl.hxx
@@ -17,6 +17,17 @@
 #include <openssl/err.h>
 #include <vector>
 
+#include <openssl/opensslv.h>
+#ifndef OPENSSL_VERSION_NUMBER
+#  error OpenSSL Version Number not Found
+#elif OPENSSL_VERSION_NUMBER < 0x01000000L
+#  define OPENSSL_0
+#  define V0_CONST const
+#else
+#  define OPENSSL_1
+#  define V0_CONST /* no const in openssl 1.0.0 */
+#endif
+
 #include <openssl/x509v3.h> // BASIC_CONSTRAINTS
 #include <openssl/bn.h>
 #include <cryptaux.hxx>
@@ -724,7 +735,7 @@ namespace openssl {
       //! Initialize from DER encoded cerificate.
       X509(const std::string& der): _x509(0) {
         OPENSSL_LOG("log");
-        unsigned char* c((unsigned char*)der.begin().operator->());
+        V0_CONST unsigned char* c((unsigned char*)der.begin().operator->());
         if (!(_x509=d2i_X509(0, &c, der.size())) ||
             (const char*)c!=der.begin().operator->()+der.size())
           throw x509_decoding_failed(der);
@@ -734,7 +745,7 @@ namespace openssl {
         unsigned char* d(0);
         int len(i2d_X509(o._x509, &d));
         if (!len) throw x509_copy_failed();
-        unsigned char* d2(d);
+        V0_CONST unsigned char* d2(d);
         _x509 = d2i_X509(0, &d2, len);
         OPENSSL_free(d);
         if (!_x509) throw x509_copy_failed();
@@ -755,7 +766,7 @@ namespace openssl {
         unsigned char* d(0);
         int len(i2d_X509(o._x509, &d));
         if (!len) throw x509_copy_failed();
-        unsigned char* d2(d);
+        V0_CONST unsigned char* d2(d);
         _x509 = d2i_X509(0, &d2, len);
         OPENSSL_free(d);
         if (!_x509) throw x509_copy_failed();