diff --git a/src/detail/probe-encoder.cpp b/src/detail/probe-encoder.cpp
index 2a5fa08..f040d2e 100644
--- a/src/detail/probe-encoder.cpp
+++ b/src/detail/probe-encoder.cpp
@@ -24,25 +24,25 @@
 namespace ndncert {
 
 Block
-probeEncoder::encodeApplicationParameters(std::vector<std::tuple<std::string, std::string>>&& parameters)
+probeEncoder::encodeApplicationParameters(std::multimap<std::string, std::string>&& parameters)
 {
   Block content(ndn::tlv::ApplicationParameters);
-  for (size_t i = 0; i < parameters.size(); ++i) {
-    content.push_back(makeStringBlock(tlv::ParameterKey, std::get<0>(parameters[i])));
-    content.push_back(makeStringBlock(tlv::ParameterValue, std::get<1>(parameters[i])));
+  for (const auto& items : parameters) {
+    content.push_back(makeStringBlock(tlv::ParameterKey, items.first));
+    content.push_back(makeStringBlock(tlv::ParameterValue, items.second));
   }
   content.encode();
   return content;
 }
 
-std::vector<std::tuple<std::string, std::string>>
+std::multimap<std::string, std::string>
 probeEncoder::decodeApplicationParameters(const Block& block)
 {
-  std::vector<std::tuple<std::string, std::string>> result;
+  std::multimap<std::string, std::string> result;
   block.parse();
   for (size_t i = 0; i < block.elements().size() - 1; i++) {
     if (block.elements()[i].type() == tlv::ParameterKey && block.elements()[i + 1].type() == tlv::ParameterValue) {
-      result.emplace_back(readString(block.elements().at(i)), readString(block.elements().at(i + 1)));
+      result.emplace(readString(block.elements().at(i)), readString(block.elements().at(i + 1)));
       i ++;
     }
   }
diff --git a/src/detail/probe-encoder.hpp b/src/detail/probe-encoder.hpp
index 03d216e..3985f93 100644
--- a/src/detail/probe-encoder.hpp
+++ b/src/detail/probe-encoder.hpp
@@ -29,7 +29,7 @@
 
   // For Client use
   Block
-  encodeApplicationParameters(std::vector<std::tuple<std::string, std::string>>&& parameters);
+  encodeApplicationParameters(std::multimap<std::string, std::string>&& parameters);
 
   void
   decodeDataContent(const Block& block, std::vector<std::pair<Name, int>>& availableNames,
@@ -41,7 +41,7 @@
                     optional<size_t> maxSuffixLength = nullopt,
                     optional<std::vector<std::shared_ptr<security::Certificate>>> redirectionItems = nullopt);
 
-  std::vector<std::tuple<std::string, std::string>>
+  std::multimap<std::string, std::string>
   decodeApplicationParameters(const Block& block);
 } // namespace ProbeEncoder
 } // namespace ndncert
diff --git a/src/identity-challenge/challenge-credential.cpp b/src/identity-challenge/challenge-credential.cpp
index 7e72805..a9ebe1c 100644
--- a/src/identity-challenge/challenge-credential.cpp
+++ b/src/identity-challenge/challenge-credential.cpp
@@ -121,13 +121,13 @@
 }
 
 // For Client
-std::vector<std::tuple<std::string, std::string>>
+std::multimap<std::string, std::string>
 ChallengeCredential::getRequestedParameterList(Status status, const std::string& challengeStatus)
 {
-  std::vector<std::tuple<std::string, std::string>> result;
+  std::multimap<std::string, std::string> result;
   if (status == Status::BEFORE_CHALLENGE) {
-    result.push_back(std::make_tuple(PARAMETER_KEY_CREDENTIAL_CERT, "Please provide the certificate issued by a trusted CA."));
-    result.push_back(std::make_tuple(PARAMETER_KEY_PROOF_OF_PRIVATE_KEY, "Please sign a Data packet with request ID as the content."));
+    result.emplace(PARAMETER_KEY_CREDENTIAL_CERT, "Please provide the certificate issued by a trusted CA.");
+    result.emplace(PARAMETER_KEY_PROOF_OF_PRIVATE_KEY, "Please sign a Data packet with request ID as the content.");
     return result;
   }
   NDN_THROW(std::runtime_error("Unexpected status or challenge status."));
@@ -135,7 +135,7 @@
 
 Block
 ChallengeCredential::genChallengeRequestTLV(Status status, const std::string& challengeStatus,
-                                            std::vector<std::tuple<std::string, std::string>>&& params)
+                                            std::multimap<std::string, std::string>&& params)
 {
   Block request(tlv::EncryptedPayload);
   if (status == Status::BEFORE_CHALLENGE) {
@@ -170,7 +170,7 @@
 }
 
 void
-ChallengeCredential::fulfillParameters(std::vector<std::tuple<std::string, std::string>>& params,
+ChallengeCredential::fulfillParameters(std::multimap<std::string, std::string>& params,
                                        KeyChain& keyChain, const Name& issuedCertName, const RequestId& requestId)
 {
   auto& pib = keyChain.getPib();
diff --git a/src/identity-challenge/challenge-credential.hpp b/src/identity-challenge/challenge-credential.hpp
index 72db981..7b48483 100644
--- a/src/identity-challenge/challenge-credential.hpp
+++ b/src/identity-challenge/challenge-credential.hpp
@@ -56,15 +56,15 @@
   handleChallengeRequest(const Block& params, ca::RequestState& request) override;
 
   // For Client
-  std::vector<std::tuple<std::string, std::string>>
+  std::multimap<std::string, std::string>
   getRequestedParameterList(Status status, const std::string& challengeStatus) override;
 
   Block
   genChallengeRequestTLV(Status status, const std::string& challengeStatus,
-                         std::vector<std::tuple<std::string, std::string>>&& params) override;
+                         std::multimap<std::string, std::string>&& params) override;
 
   static void
-  fulfillParameters(std::vector<std::tuple<std::string, std::string>>& params,
+  fulfillParameters(std::multimap<std::string, std::string>& params,
                     KeyChain& keyChain, const Name& issuedCertName, const RequestId& requestId);
 
   // challenge parameters
diff --git a/src/identity-challenge/challenge-email.cpp b/src/identity-challenge/challenge-email.cpp
index 332ef96..8b08d82 100644
--- a/src/identity-challenge/challenge-email.cpp
+++ b/src/identity-challenge/challenge-email.cpp
@@ -103,18 +103,18 @@
 }
 
 // For Client
-std::vector<std::tuple<std::string, std::string>>
+std::multimap<std::string, std::string>
 ChallengeEmail::getRequestedParameterList(Status status, const std::string& challengeStatus)
 {
-  std::vector<std::tuple<std::string, std::string>> result;
+  std::multimap<std::string, std::string> result;
   if (status == Status::BEFORE_CHALLENGE && challengeStatus == "") {
-    result.push_back(std::make_tuple(PARAMETER_KEY_EMAIL, "Please input your email address"));
+    result.emplace(PARAMETER_KEY_EMAIL, "Please input your email address");
   }
   else if (status == Status::CHALLENGE && challengeStatus == NEED_CODE) {
-    result.push_back(std::make_tuple(PARAMETER_KEY_CODE, "Please input your verification code"));
+    result.emplace(PARAMETER_KEY_CODE, "Please input your verification code");
   }
   else if (status == Status::CHALLENGE && challengeStatus == WRONG_CODE) {
-    result.push_back(std::make_tuple(PARAMETER_KEY_CODE, "Incorrect code, please try again"));
+    result.emplace(PARAMETER_KEY_CODE, "Incorrect code, please try again");
   }
   else {
     NDN_THROW(std::runtime_error("Unexpected status or challenge status."));
@@ -124,24 +124,24 @@
 
 Block
 ChallengeEmail::genChallengeRequestTLV(Status status, const std::string& challengeStatus,
-                                       std::vector<std::tuple<std::string, std::string>>&& params)
+                                       std::multimap<std::string, std::string>&& params)
 {
   Block request(tlv::EncryptedPayload);
   if (status == Status::BEFORE_CHALLENGE) {
-    if (params.size() != 1 || std::get<0>(params[0]) != PARAMETER_KEY_EMAIL) {
+    if (params.size() != 1 || params.find(PARAMETER_KEY_EMAIL) == params.end()) {
       NDN_THROW(std::runtime_error("Wrong parameter provided."));
     }
     request.push_back(makeStringBlock(tlv::SelectedChallenge, CHALLENGE_TYPE));
     request.push_back(makeStringBlock(tlv::ParameterKey, PARAMETER_KEY_EMAIL));
-    request.push_back(makeStringBlock(tlv::ParameterValue, std::get<1>(params[0])));
+    request.push_back(makeStringBlock(tlv::ParameterValue, params.find(PARAMETER_KEY_EMAIL)->second));
   }
   else if (status == Status::CHALLENGE && (challengeStatus == NEED_CODE || challengeStatus == WRONG_CODE)) {
-    if (params.size() != 1 || std::get<0>(params[0]) != PARAMETER_KEY_CODE) {
+    if (params.size() != 1 || params.find(PARAMETER_KEY_CODE) == params.end()) {
       NDN_THROW(std::runtime_error("Wrong parameter provided."));
     }
     request.push_back(makeStringBlock(tlv::SelectedChallenge, CHALLENGE_TYPE));
     request.push_back(makeStringBlock(tlv::ParameterKey, PARAMETER_KEY_CODE));
-    request.push_back(makeStringBlock(tlv::ParameterValue, std::get<1>(params[0])));
+    request.push_back(makeStringBlock(tlv::ParameterValue, params.find(PARAMETER_KEY_CODE)->second));
   }
   else {
     NDN_THROW(std::runtime_error("Unexpected status or challenge status."));
diff --git a/src/identity-challenge/challenge-email.hpp b/src/identity-challenge/challenge-email.hpp
index 9dbcedd..c005b7e 100644
--- a/src/identity-challenge/challenge-email.hpp
+++ b/src/identity-challenge/challenge-email.hpp
@@ -60,12 +60,12 @@
   handleChallengeRequest(const Block& params, ca::RequestState& request) override;
 
   // For Client
-  std::vector<std::tuple<std::string, std::string>>
+  std::multimap<std::string, std::string>
   getRequestedParameterList(Status status, const std::string& challengeStatus) override;
 
   Block
   genChallengeRequestTLV(Status status, const std::string& challengeStatus,
-                         std::vector<std::tuple<std::string, std::string>>&& params) override;
+                         std::multimap<std::string, std::string>&& params) override;
 
   // challenge status
   static const std::string NEED_CODE;
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
diff --git a/src/identity-challenge/challenge-pin.cpp b/src/identity-challenge/challenge-pin.cpp
index 34b987d..1ae6753 100644
--- a/src/identity-challenge/challenge-pin.cpp
+++ b/src/identity-challenge/challenge-pin.cpp
@@ -84,18 +84,18 @@
 }
 
 // For Client
-std::vector<std::tuple<std::string, std::string>>
+std::multimap<std::string, std::string>
 ChallengePin::getRequestedParameterList(Status status, const std::string& challengeStatus)
 {
-  std::vector<std::tuple<std::string, std::string>> result;
+  std::multimap<std::string, std::string> result;
   if (status == Status::BEFORE_CHALLENGE) {
     // do nothing
   }
   else if (status == Status::CHALLENGE && challengeStatus == NEED_CODE) {
-    result.push_back(std::make_tuple(PARAMETER_KEY_CODE, "Please input your PIN code"));
+    result.emplace(PARAMETER_KEY_CODE, "Please input your PIN code");
   }
   else if (status == Status::CHALLENGE && challengeStatus == WRONG_CODE) {
-    result.push_back(std::make_tuple(PARAMETER_KEY_CODE, "Incorrect PIN code, please try again"));
+    result.emplace(PARAMETER_KEY_CODE, "Incorrect PIN code, please try again");
   }
   else {
     NDN_THROW(std::runtime_error("Unexpected status or challenge status."));
@@ -105,19 +105,19 @@
 
 Block
 ChallengePin::genChallengeRequestTLV(Status status, const std::string& challengeStatus,
-                                     std::vector<std::tuple<std::string, std::string>>&& params)
+                                     std::multimap<std::string, std::string>&& params)
 {
   Block request(tlv::EncryptedPayload);
   if (status == Status::BEFORE_CHALLENGE) {
     request.push_back(makeStringBlock(tlv::SelectedChallenge, CHALLENGE_TYPE));
   }
   else if (status == Status::CHALLENGE && (challengeStatus == NEED_CODE || challengeStatus == WRONG_CODE)) {
-    if (params.size() != 1 || std::get<0>(params[0]) != PARAMETER_KEY_CODE) {
+    if (params.size() != 1 || params.find(PARAMETER_KEY_CODE) == params.end()) {
       NDN_THROW(std::runtime_error("Wrong parameter provided."));
     }
     request.push_back(makeStringBlock(tlv::SelectedChallenge, CHALLENGE_TYPE));
     request.push_back(makeStringBlock(tlv::ParameterKey, PARAMETER_KEY_CODE));
-    request.push_back(makeStringBlock(tlv::ParameterValue, std::get<1>(params[0])));
+    request.push_back(makeStringBlock(tlv::ParameterValue, params.find(PARAMETER_KEY_CODE)->second));
   }
   else {
     NDN_THROW(std::runtime_error("Unexpected status or challenge status."));
diff --git a/src/identity-challenge/challenge-pin.hpp b/src/identity-challenge/challenge-pin.hpp
index 10b23d4..757870d 100644
--- a/src/identity-challenge/challenge-pin.hpp
+++ b/src/identity-challenge/challenge-pin.hpp
@@ -55,12 +55,12 @@
   handleChallengeRequest(const Block& params, ca::RequestState& request) override;
 
   // For Client
-  std::vector<std::tuple<std::string, std::string>>
+  std::multimap<std::string, std::string>
   getRequestedParameterList(Status status, const std::string& challengeStatus) override;
 
   Block
   genChallengeRequestTLV(Status status, const std::string& challengeStatus,
-                         std::vector<std::tuple<std::string, std::string>>&& params) override;
+                         std::multimap<std::string, std::string>&& params) override;
 
   // challenge status
   static const std::string NEED_CODE;
diff --git a/src/name-assignment/assignment-func.cpp b/src/name-assignment/assignment-func.cpp
index 22eb35e..d65eefd 100644
--- a/src/name-assignment/assignment-func.cpp
+++ b/src/name-assignment/assignment-func.cpp
@@ -24,8 +24,7 @@
 namespace ndn {
 namespace ndncert {
 
-NameAssignmentFunc::NameAssignmentFunc(const std::string& factoryType, const std::string& format)
-  : FACTORY_TYPE(factoryType)
+NameAssignmentFunc::NameAssignmentFunc(const std::string& format)
 {
   size_t index = 0, startIndex = 0;
   while ((index = format.find("/", startIndex)) != std::string::npos) {
diff --git a/src/name-assignment/assignment-func.hpp b/src/name-assignment/assignment-func.hpp
index e122ed2..9ed8e22 100644
--- a/src/name-assignment/assignment-func.hpp
+++ b/src/name-assignment/assignment-func.hpp
@@ -22,6 +22,7 @@
 #define NDNCERT_ASSIGNMENT_FUNC_HPP
 
 #include "detail/ca-request-state.hpp"
+#include <map>
 
 namespace ndn {
 namespace ndncert {
@@ -29,7 +30,7 @@
 class NameAssignmentFunc : noncopyable
 {
 protected:
-  explicit NameAssignmentFunc(const std::string& factoryType, const std::string& format = "");
+  explicit NameAssignmentFunc(const std::string& format = "");
 
 public:
   virtual ~NameAssignmentFunc() = default;
@@ -45,9 +46,7 @@
    * @return a vector containing the possible namespaces derived from the parameters.
    */
   virtual std::vector<PartialName>
-  assignName(const std::vector<std::tuple<std::string, std::string>>& params) = 0;
-
-  const std::string FACTORY_TYPE;
+  assignName(const std::multimap<std::string, std::string> &params) = 0;
 
 public:
   template <class AssignmentType>
@@ -62,7 +61,7 @@
   static unique_ptr<NameAssignmentFunc>
   createNameAssignmentFunc(const std::string& challengeType, const std::string& format = "");
 
-NDNCERT_PUBLIC_WITH_TESTS_ELSE_PRIVATE:
+NDNCERT_PUBLIC_WITH_TESTS_ELSE_PROTECTED:
     std::vector<std::string> m_nameFormat;
     
 private:
diff --git a/src/name-assignment/assignment-hash.cpp b/src/name-assignment/assignment-hash.cpp
index 8f0e641..0dec488 100644
--- a/src/name-assignment/assignment-hash.cpp
+++ b/src/name-assignment/assignment-hash.cpp
@@ -27,20 +27,19 @@
 NDNCERT_REGISTER_FUNCFACTORY(AssignmentHash, "hash");
 
 AssignmentHash::AssignmentHash(const std::string& format)
-  : NameAssignmentFunc("hash", format)
+  : NameAssignmentFunc(format)
 {}
 
 std::vector<PartialName>
-AssignmentHash::assignName(const std::vector<std::tuple<std::string, std::string>>& params)
+AssignmentHash::assignName(const std::multimap<std::string, std::string> &params)
 {
   std::vector<PartialName> resultList;
   Name result;
   for (const auto& item : m_nameFormat) {
-    auto it = std::find_if(params.begin(), params.end(),
-                           [&](const std::tuple<std::string, std::string>& e) { return std::get<0>(e) == item; });
+    auto it = params.find(item);
     if (it != params.end()) {
       util::Sha256 digest;
-      digest << std::get<1>(*it);
+      digest << it->second;
       result.append(digest.toString());
     }
     else {
diff --git a/src/name-assignment/assignment-hash.hpp b/src/name-assignment/assignment-hash.hpp
index 77e5e91..6acb30c 100644
--- a/src/name-assignment/assignment-hash.hpp
+++ b/src/name-assignment/assignment-hash.hpp
@@ -35,7 +35,7 @@
   AssignmentHash(const std::string& format = "");
 
   std::vector<PartialName>
-  assignName(const std::vector<std::tuple<std::string, std::string>>& params) override;
+  assignName(const std::multimap<std::string, std::string> &params) override;
 
 };
 
diff --git a/src/name-assignment/assignment-param.cpp b/src/name-assignment/assignment-param.cpp
index 4ebe637..b99d685 100644
--- a/src/name-assignment/assignment-param.cpp
+++ b/src/name-assignment/assignment-param.cpp
@@ -26,19 +26,19 @@
 NDNCERT_REGISTER_FUNCFACTORY(AssignmentParam, "param");
 
 AssignmentParam::AssignmentParam(const std::string& format)
-  : NameAssignmentFunc("param", format)
+  : NameAssignmentFunc(format)
 {}
 
 std::vector<PartialName>
-AssignmentParam::assignName(const std::vector<std::tuple<std::string, std::string>>& params)
+AssignmentParam::assignName(const std::multimap<std::string, std::string> &params)
 {
   std::vector<PartialName> resultList;
   Name result;
   for (const auto& item : m_nameFormat) {
     auto it = std::find_if(params.begin(), params.end(),
                            [&](const std::tuple<std::string, std::string>& e) { return std::get<0>(e) == item; });
-    if (it != params.end() && !std::get<1>(*it).empty()) {
-      result.append(std::get<1>(*it));
+    if (it != params.end() && !it->second.empty()) {
+      result.append(it->second);
     }
     else {
       return resultList;
diff --git a/src/name-assignment/assignment-param.hpp b/src/name-assignment/assignment-param.hpp
index 8a31dc6..cdf2cc5 100644
--- a/src/name-assignment/assignment-param.hpp
+++ b/src/name-assignment/assignment-param.hpp
@@ -35,7 +35,7 @@
   AssignmentParam(const std::string& format = "");
 
   std::vector<PartialName>
-  assignName(const std::vector<std::tuple<std::string, std::string>>& params) override;
+  assignName(const std::multimap<std::string, std::string> &params) override;
 
 };
 
diff --git a/src/name-assignment/assignment-random.cpp b/src/name-assignment/assignment-random.cpp
index 59f6477..eaed861 100644
--- a/src/name-assignment/assignment-random.cpp
+++ b/src/name-assignment/assignment-random.cpp
@@ -27,11 +27,11 @@
 NDNCERT_REGISTER_FUNCFACTORY(AssignmentRandom, "random");
 
 AssignmentRandom::AssignmentRandom(const std::string& format)
-  : NameAssignmentFunc("random", format)
+  : NameAssignmentFunc(format)
 {}
 
 std::vector<PartialName>
-AssignmentRandom::assignName(const std::vector<std::tuple<std::string, std::string>>& params)
+AssignmentRandom::assignName(const std::multimap<std::string, std::string> &params)
 {
   std::vector<PartialName> resultList;
   resultList.emplace_back(to_string(random::generateSecureWord64()));
diff --git a/src/name-assignment/assignment-random.hpp b/src/name-assignment/assignment-random.hpp
index cf6509f..978a0b7 100644
--- a/src/name-assignment/assignment-random.hpp
+++ b/src/name-assignment/assignment-random.hpp
@@ -35,7 +35,7 @@
   AssignmentRandom(const std::string& format = "");
 
   std::vector<PartialName>
-  assignName(const std::vector<std::tuple<std::string, std::string>>& params) override;
+  assignName(const std::multimap<std::string, std::string> &params) override;
 };
 
 } // namespace ndncert
diff --git a/src/requester.cpp b/src/requester.cpp
index 7ace36c..5372db3 100644
--- a/src/requester.cpp
+++ b/src/requester.cpp
@@ -88,7 +88,7 @@
 }
 
 shared_ptr<Interest>
-Requester::genProbeInterest(const CaProfile& ca, std::vector<std::tuple<std::string, std::string>>&& probeInfo)
+Requester::genProbeInterest(const CaProfile& ca, std::multimap<std::string, std::string>&& probeInfo)
 {
   Name interestName = ca.m_caPrefix;
   interestName.append("CA").append("PROBE");
@@ -212,7 +212,7 @@
   return challenges;
 }
 
-std::vector<std::tuple<std::string, std::string>>
+std::multimap<std::string, std::string>
 Requester::selectOrContinueChallenge(RequestState& state, const std::string& challengeSelected)
 {
   auto challenge = ChallengeModule::createChallengeModule(challengeSelected);
@@ -225,7 +225,7 @@
 
 shared_ptr<Interest>
 Requester::genChallengeInterest(RequestState& state,
-                                std::vector<std::tuple<std::string, std::string>>&& parameters)
+                                std::multimap<std::string, std::string>&& parameters)
 {
   if (state.m_challengeType == "") {
     NDN_THROW(std::runtime_error("The challenge has not been selected."));
diff --git a/src/requester.hpp b/src/requester.hpp
index 825efd9..92cf00b 100644
--- a/src/requester.hpp
+++ b/src/requester.hpp
@@ -84,7 +84,7 @@
    * @return A shared pointer of to the encoded interest, ready to be sent.
    */
   static shared_ptr<Interest>
-  genProbeInterest(const CaProfile& ca, std::vector<std::tuple<std::string, std::string>>&& probeInfo);
+  genProbeInterest(const CaProfile& ca, std::multimap<std::string, std::string>&& probeInfo);
 
   /**
    * @brief Decodes the replied data for PROBE process from the CA.
@@ -147,7 +147,7 @@
    * @return The requirement list for the current stage of the challenge, in name, prompt mapping.
    * @throw std::runtime_error if the challenge is not supported.
    */
-  static std::vector<std::tuple<std::string, std::string>>
+  static std::multimap<std::string, std::string>
   selectOrContinueChallenge(RequestState& state, const std::string& challengeSelected);
 
   /**
@@ -160,7 +160,7 @@
    */
   static shared_ptr<Interest>
   genChallengeInterest(RequestState& state,
-                       std::vector<std::tuple<std::string, std::string>>&& parameters);
+                       std::multimap<std::string, std::string>&& parameters);
 
   /**
    * @brief Decodes the responded data from the CHALLENGE interest.