diff --git a/src/identity-challenge/challenge-module.hpp b/src/identity-challenge/challenge-module.hpp
index 1e2ca81..02e5027 100644
--- a/src/identity-challenge/challenge-module.hpp
+++ b/src/identity-challenge/challenge-module.hpp
@@ -54,12 +54,12 @@
   handleChallengeRequest(const Block& params, ca::RequestState& request) = 0;
 
   // For Client
-  virtual std::vector<std::tuple<std::string, std::string>>
+  virtual std::multimap<std::string, std::string>
   getRequestedParameterList(Status status, const std::string& challengeStatus) = 0;
 
   virtual Block
   genChallengeRequestTLV(Status status, const std::string& challengeStatus,
-                         std::vector<std::tuple<std::string, std::string>>&& params) = 0;
+                         std::multimap<std::string, std::string>&& params) = 0;
 
   // helpers
   static std::string