diff --git a/libarchive/archive_crypto.c b/libarchive/archive_crypto.c
index 8e792e52c..2caf57169 100644
--- a/libarchive/archive_crypto.c
+++ b/libarchive/archive_crypto.c
@@ -30,6 +30,12 @@
 #include "archive.h"
 #include "archive_crypto_private.h"
 
+/* In particular, force the configure probe to break if it tries
+ * to test a combination of OpenSSL and libmd. */
+#if defined(ARCHIVE_CRYPTO_OPENSSL) && defined(ARCHIVE_CRYPTO_LIBMD)
+#error Cannot use both OpenSSL and libmd.
+#endif
+
 /*
  * Message digest functions for Windows platform.
  */
diff --git a/libarchive/archive_crypto_private.h b/libarchive/archive_crypto_private.h
index 24754a017..f8b1fb3c3 100644
--- a/libarchive/archive_crypto_private.h
+++ b/libarchive/archive_crypto_private.h
@@ -79,6 +79,14 @@
 #endif
 
 /* libmd crypto headers */
+#if defined(ARCHIVE_CRYPTO_MD5_LIBMD) ||\
+  defined(ARCHIVE_CRYPTO_RMD160_LIBMD) ||\
+  defined(ARCHIVE_CRYPTO_SHA1_LIBMD) ||\
+  defined(ARCHIVE_CRYPTO_SHA256_LIBMD) ||\
+  defined(ARCHIVE_CRYPTO_SHA512_LIBMD)
+#define	ARCHIVE_CRYPTO_LIBMD 1
+#endif
+
 #if defined(ARCHIVE_CRYPTO_MD5_LIBMD)
 #include <md5.h>
 #endif
@@ -125,6 +133,7 @@
   defined(ARCHIVE_CRYPTO_SHA256_OPENSSL) ||\
   defined(ARCHIVE_CRYPTO_SHA384_OPENSSL) ||\
   defined(ARCHIVE_CRYPTO_SHA512_OPENSSL)
+#define	ARCHIVE_CRYPTO_OPENSSL 1
 #include <openssl/evp.h>
 #endif