diff --git a/caching/emitter.h b/caching/emitter.h
index a2a84dc..c70b56e 100644
--- a/caching/emitter.h
+++ b/caching/emitter.h
@@ -1,11 +1,11 @@
 #ifndef CACHE_EMITTER_H
 #define CACHE_EMITTER_H
 
-#include <boost/shared_ptr.hpp>
-#include <string>
-
 #include "persistent-data/block.h"
 
+#include <boost/shared_ptr.hpp>
+#include <vector>
+
 //----------------------------------------------------------------
 
 namespace caching {
@@ -35,7 +35,7 @@ namespace caching {
 		virtual void end_hints() = 0;
 
 		virtual void hint(pd::block_address cblock,
-				  std::string const &data) = 0;
+				  std::vector<unsigned char> const &data) = 0;
 	};
 }
 
diff --git a/caching/restore_emitter.cc b/caching/restore_emitter.cc
index c705a25..8269ad7 100644
--- a/caching/restore_emitter.cc
+++ b/caching/restore_emitter.cc
@@ -88,13 +88,16 @@ namespace {
 		}
 
 		virtual void begin_hints() {
+			// noop
 		}
 
 		virtual void end_hints() {
+			// noop
 		}
 
 		virtual void hint(pd::block_address cblock,
-				  std::string const &data) {
+				  vector<unsigned char> const &data) {
+
 		}
 
 	private:
diff --git a/caching/xml_format.cc b/caching/xml_format.cc
index 9d0f439..f3b9610 100644
--- a/caching/xml_format.cc
+++ b/caching/xml_format.cc
@@ -12,12 +12,12 @@ using namespace std;
 
 namespace {
 	// base64 encoding?
-	string encode(string const &data) {
-		return data;
+	string encode(vector<unsigned char> const &data) {
+		return "";
 	}
 
-	string decode(string const &data) {
-		return data;
+	vector<unsigned char> decode(string const &data) {
+		return vector<unsigned char>();
 	}
 
 	//--------------------------------
@@ -84,7 +84,7 @@ namespace {
 		}
 
 		virtual void hint(block_address cblock,
-				  std::string const &data) {
+				  vector<unsigned char> const &data) {
 			out_ << "<hint"
 			     << " cache_block=\"" << cblock << "\""
 			     << " data=\"" << encode(data) << "\""