diff --git a/src/cache.h b/src/cache.h
index 82aa6e2..410d447 100644
--- a/src/cache.h
+++ b/src/cache.h
@@ -163,7 +163,7 @@ class StatCache
 
         // Delete stat cache
         bool DelStat(const char* key, bool lock_already_held = false);
-        bool DelStat(std::string& key, bool lock_already_held = false)
+        bool DelStat(const std::string& key, bool lock_already_held = false)
         {
             return DelStat(key.c_str(), lock_already_held);
         }
diff --git a/src/curl.cpp b/src/curl.cpp
index a1eb733..51a3671 100644
--- a/src/curl.cpp
+++ b/src/curl.cpp
@@ -761,9 +761,9 @@ storage_class_t S3fsCurl::SetStorageClass(storage_class_t storage_class)
     return old;
 }
 
-bool S3fsCurl::PushbackSseKeys(std::string& onekey)
+bool S3fsCurl::PushbackSseKeys(const std::string& input)
 {
-    onekey = trim(onekey);
+    std::string onekey = trim(input);
     if(onekey.empty()){
         return false;
     }
@@ -2770,8 +2770,9 @@ bool S3fsCurl::LoadIAMRoleFromMetaData()
     return (0 == result);
 }
 
-bool S3fsCurl::AddSseRequestHead(sse_type_t ssetype, std::string& ssevalue, bool is_only_c, bool is_copy)
+bool S3fsCurl::AddSseRequestHead(sse_type_t ssetype, const std::string& input, bool is_only_c, bool is_copy)
 {
+    std::string ssevalue = input;
     switch(ssetype){
         case sse_type_t::SSE_DISABLE:
             return true;
@@ -3133,7 +3134,7 @@ int S3fsCurl::PutRequest(const char* tpath, headers_t& meta, int fd)
     return result;
 }
 
-int S3fsCurl::PreGetObjectRequest(const char* tpath, int fd, off_t start, off_t size, sse_type_t ssetype, std::string& ssevalue)
+int S3fsCurl::PreGetObjectRequest(const char* tpath, int fd, off_t start, off_t size, sse_type_t ssetype, const std::string& ssevalue)
 {
     S3FS_PRN_INFO3("[tpath=%s][start=%lld][size=%lld]", SAFESTRPTR(tpath), static_cast<long long>(start), static_cast<long long>(size));
 
diff --git a/src/curl.h b/src/curl.h
index 3d1c93e..9075f57 100644
--- a/src/curl.h
+++ b/src/curl.h
@@ -240,7 +240,7 @@ class S3fsCurl
         static bool SetIAMRoleFromMetaData(const char* response);
         static bool LoadEnvSseCKeys();
         static bool LoadEnvSseKmsid();
-        static bool PushbackSseKeys(std::string& onekey);
+        static bool PushbackSseKeys(const std::string& onekey);
         static bool AddUserAgent(CURL* hCurl);
 
         static int CurlDebugFunc(CURL* hcurl, curl_infotype type, char* data, size_t size, void* userptr);
@@ -352,18 +352,18 @@ class S3fsCurl
         bool DestroyCurlHandle(bool restore_pool = true, bool clear_internal_data = true);
 
         bool LoadIAMRoleFromMetaData();
-        bool AddSseRequestHead(sse_type_t ssetype, std::string& ssevalue, bool is_only_c, bool is_copy);
+        bool AddSseRequestHead(sse_type_t ssetype, const std::string& ssevalue, bool is_only_c, bool is_copy);
         bool GetResponseCode(long& responseCode, bool from_curl_handle = true);
         int RequestPerform(bool dontAddAuthHeaders=false);
         int DeleteRequest(const char* tpath);
         bool PreHeadRequest(const char* tpath, const char* bpath = NULL, const char* savedpath = NULL, int ssekey_pos = -1);
-        bool PreHeadRequest(std::string& tpath, std::string& bpath, std::string& savedpath, int ssekey_pos = -1) {
+        bool PreHeadRequest(const std::string& tpath, const std::string& bpath, const std::string& savedpath, int ssekey_pos = -1) {
           return PreHeadRequest(tpath.c_str(), bpath.c_str(), savedpath.c_str(), ssekey_pos);
         }
         int HeadRequest(const char* tpath, headers_t& meta);
         int PutHeadRequest(const char* tpath, headers_t& meta, bool is_copy);
         int PutRequest(const char* tpath, headers_t& meta, int fd);
-        int PreGetObjectRequest(const char* tpath, int fd, off_t start, off_t size, sse_type_t ssetype, std::string& ssevalue);
+        int PreGetObjectRequest(const char* tpath, int fd, off_t start, off_t size, sse_type_t ssetype, const std::string& ssevalue);
         int GetObjectRequest(const char* tpath, int fd, off_t start = -1, off_t size = -1);
         int CheckBucket();
         int ListBucketRequest(const char* tpath, const char* query);
diff --git a/src/curl_util.h b/src/curl_util.h
index 889733d..b9fcb73 100644
--- a/src/curl_util.h
+++ b/src/curl_util.h
@@ -27,8 +27,6 @@
 // Functions
 //----------------------------------------------
 std::string GetContentMD5(int fd);
-unsigned char* md5hexsum(int fd, off_t start, ssize_t size);
-std::string md5sum(int fd, off_t start, ssize_t size);
 struct curl_slist* curl_slist_sort_insert(struct curl_slist* list, const char* data);
 struct curl_slist* curl_slist_sort_insert(struct curl_slist* list, const char* key, const char* value);
 std::string get_sorted_header_keys(const struct curl_slist* list);
diff --git a/src/string_util.cpp b/src/string_util.cpp
index 6e847e6..e1d073b 100644
--- a/src/string_util.cpp
+++ b/src/string_util.cpp
@@ -245,7 +245,7 @@ bool takeout_str_dquart(std::string& str)
 //
 // ex. target="http://......?keyword=value&..."
 //
-bool get_keyword_value(std::string& target, const char* keyword, std::string& value)
+bool get_keyword_value(const std::string& target, const char* keyword, std::string& value)
 {
     if(!keyword){
         return false;
diff --git a/src/string_util.h b/src/string_util.h
index 427e8f7..84f6bc3 100644
--- a/src/string_util.h
+++ b/src/string_util.h
@@ -87,7 +87,7 @@ std::string urlEncode2(const std::string &s);
 std::string urlDecode(const std::string& s);
 
 bool takeout_str_dquart(std::string& str);
-bool get_keyword_value(std::string& target, const char* keyword, std::string& value);
+bool get_keyword_value(const std::string& target, const char* keyword, std::string& value);
 
 //
 // For binary string