diff --git a/rhizome.h b/rhizome.h
index 43bae6de..cb1bc718 100644
--- a/rhizome.h
+++ b/rhizome.h
@@ -274,6 +274,7 @@ int rhizome_bk_xor(const char *author,
 		   unsigned char bid[crypto_sign_edwards25519sha512batch_PUBLICKEYBYTES],
 		   unsigned char bkin[crypto_sign_edwards25519sha512batch_SECRETKEYBYTES],
 		   unsigned char bkout[crypto_sign_edwards25519sha512batch_SECRETKEYBYTES]);
+unsigned char *rhizome_bundle_shared_secret(rhizome_manifest *m);
 int rhizome_extract_privatekey(rhizome_manifest *m,const char *authorHex);
 int rhizome_queue_ignore_manifest(rhizome_manifest *m,
 				  struct sockaddr_in *peerip,int timeout);
diff --git a/rhizome_crypto.c b/rhizome_crypto.c
index a322ce0b..7447e525 100644
--- a/rhizome_crypto.c
+++ b/rhizome_crypto.c
@@ -22,6 +22,14 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 #include <stdlib.h>
 #include <ctype.h>
 
+/* Work out the encrypt/decrypt key for the supplied manifest.
+   If the manifest is not encrypted, then return NULL.
+*/
+unsigned char *rhizome_bundle_shared_secret(rhizome_manifest *m)
+{
+  return NULL;
+}
+
 int rhizome_strn_is_manifest_id(const char *id)
 {
   int i;