diff --git a/src/Event.cpp b/src/Event.cpp
index 28e597e3bd7ddec8d5d4430189767786053a1521..4daba6c755c8b56d8c4da44ac11b802ec6a939c8 100644
--- a/src/Event.cpp
+++ b/src/Event.cpp
@@ -51,12 +51,8 @@ Event Event::next() const {
     if(s == 0) return Event();
     size_t i = m_impl->m_container->size()+1;
     if(keys[0].size() <= i) return Event();
-    if(keys[0][i] != '%') return Event();
-    std::stringstream strEventNumber;
-    strEventNumber << std::hex << keys[0].substr(i+1);
-    EventNumber n;
-    strEventNumber >> n;
-
+    EventNumber n = Impl::parseEventNumberFromKeyString(&keys[0][i]);
+    if(n == InvalidEventNumber) return Event();
     return Event(m_impl->m_datastore, m_impl->m_level, m_impl->m_container, n);
 }
 
diff --git a/src/Run.cpp b/src/Run.cpp
index 9b73c1c98d4e7c81fb1c7d3b117024f6562ca845..9efdf55a0fdd8e26be878b4121733c22ebe2fbdf 100644
--- a/src/Run.cpp
+++ b/src/Run.cpp
@@ -53,12 +53,8 @@ Run Run::next() const {
     if(s == 0) return Run();
     size_t i = m_impl->m_container->size()+1;
     if(keys[0].size() <= i) return Run();
-    if(keys[0][i] != '%') return Run();
-    std::stringstream strRunNumber;
-    strRunNumber << std::hex << keys[0].substr(i+1);
-    RunNumber rn;
-    strRunNumber >> rn;
-
+    RunNumber rn = Impl::parseRunNumberFromKeyString(&keys[0][i]);
+    if(rn == InvalidRunNumber) return Run();
     return Run(m_impl->m_datastore, m_impl->m_level, m_impl->m_container, rn);
 }
 
diff --git a/src/SubRun.cpp b/src/SubRun.cpp
index a3fb4854412de61e500fde065f49d03313a45bb4..5cb378b019468048b3449dff531ab727c690450b 100644
--- a/src/SubRun.cpp
+++ b/src/SubRun.cpp
@@ -56,12 +56,8 @@ SubRun SubRun::next() const {
     if(s == 0) return SubRun();
     size_t i = m_impl->m_container->size()+1;
     if(keys[0].size() <= i) return SubRun();
-    if(keys[0][i] != '%') return SubRun();
-    std::stringstream strSubRunNumber;
-    strSubRunNumber << std::hex << keys[0].substr(i+1);
-    SubRunNumber rn;
-    strSubRunNumber >> rn;
-
+    SubRunNumber rn = Impl::parseSubRunNumberFromKeyString(&keys[0][i]);
+    if(rn == InvalidSubRunNumber) return SubRun();
     return SubRun(m_impl->m_datastore, m_impl->m_level, m_impl->m_container, rn);
 }
 
diff --git a/src/private/EventImpl.hpp b/src/private/EventImpl.hpp
index da837c040ee4bad96561fcf573881632dd1330b4..458d254a2d69bda242da3932efac37e6784b9578 100644
--- a/src/private/EventImpl.hpp
+++ b/src/private/EventImpl.hpp
@@ -6,6 +6,7 @@
 #ifndef __HEPNOS_PRIVATE_EVENT_IMPL_H
 #define __HEPNOS_PRIVATE_EVENT_IMPL_H
 
+#include <boost/predef/other/endian.h>
 #include <sstream>
 #include <iomanip>
 #include <memory>
@@ -29,9 +30,47 @@ class Event::Impl {
         , m_event_nr(n) {}
 
         static std::string makeKeyStringFromEventNumber(const EventNumber& n) {
+            std::string str(1+sizeof(n),'\0');
+            str[0] = '%';
+#ifndef HEPNOS_READABLE_NUMBERS
+#if BOOST_ENDIAN_BIG_BYTE
+            std::memcpy(&str[1], &n, sizeof(n));
+            return str;
+#else
+            unsigned i = sizeof(n);
+            auto n2 = n;
+            while(n2 != 0) {
+                str[i] = n2 & 0xff;
+                n2 = n2 >> 8;
+                i -= 1;
+            }
+            return str;
+#endif
+#else
             std::stringstream strstr;
             strstr << "%" << std::setfill('0') << std::setw(16) << std::hex << n;
             return strstr.str();
+#endif
+        }
+
+        static EventNumber parseEventNumberFromKeyString(const char* str) {
+            if(str[0] != '%') return InvalidEventNumber;
+            EventNumber n;
+#ifdef HEPNOS_READABLE_NUMBERS
+            std::stringstream strEventNumber;
+            strEventNumber << std::hex << std::string(str+1, 16);
+            strEventNumber >> n;
+#else
+#if BOOST_ENDIAN_BIG_BYTE
+            std::memcpy(&n, &str[1], sizeof(n));
+#else
+            n = 0;
+            for(unsigned i=0; i < sizeof(n); i++) {
+                n = 256*n + str[i+1];
+            }
+#endif
+#endif
+            return n;
         }
 
         std::string makeKeyStringFromEventNumber() const {
diff --git a/src/private/RunImpl.hpp b/src/private/RunImpl.hpp
index 85acbbe795988d84b36fd35de7a4df8407816f69..87ee061d74ffc957854861e55ab7dc84edfbd2ea 100644
--- a/src/private/RunImpl.hpp
+++ b/src/private/RunImpl.hpp
@@ -29,10 +29,48 @@ class Run::Impl {
         , m_container(container)
         , m_run_nr(rn) {}
 
-        static std::string makeKeyStringFromRunNumber(const RunNumber& nr) {
+        static std::string makeKeyStringFromRunNumber(const RunNumber& n) {
+            std::string str(1+sizeof(n),'\0');
+            str[0] = '%';
+#ifndef HEPNOS_READABLE_NUMBERS
+#if BOOST_ENDIAN_BIG_BYTE
+            std::memcpy(&str[1], &n, sizeof(n));
+            return str;
+#else
+            unsigned i = sizeof(n);
+            auto n2 = n;
+            while(n2 != 0) {
+                str[i] = n2 & 0xff;
+                n2 = n2 >> 8;
+                i -= 1;
+            }
+            return str;
+#endif
+#else
             std::stringstream strstr;
-            strstr << "%" << std::setfill('0') << std::setw(16) << std::hex << nr;
+            strstr << "%" << std::setfill('0') << std::setw(16) << std::hex << n;
             return strstr.str();
+#endif
+        }
+
+        static RunNumber parseRunNumberFromKeyString(const char* str) {
+            if(str[0] != '%') return InvalidRunNumber;
+            RunNumber n;
+#ifdef HEPNOS_READABLE_NUMBERS
+            std::stringstream strRunNumber;
+            strRunNumber << std::hex << std::string(str+1, 16);
+            strRunNumber >> n;
+#else
+#if BOOST_ENDIAN_BIG_BYTE
+            std::memcpy(&n, &str[1], sizeof(n));
+#else
+            n = 0;
+            for(unsigned i=0; i < sizeof(n); i++) {
+                n = 256*n + str[i+1];
+            }
+#endif
+#endif
+            return n;
         }
 
         std::string makeKeyStringFromRunNumber() const {
diff --git a/src/private/SubRunImpl.hpp b/src/private/SubRunImpl.hpp
index 75011d986c6f57141441f60836c006ab90cd9f1d..2a6e91882f28bbb60569c1ff2c477dfaf9c1c7ee 100644
--- a/src/private/SubRunImpl.hpp
+++ b/src/private/SubRunImpl.hpp
@@ -30,9 +30,47 @@ class SubRun::Impl {
         , m_subrun_nr(rn) {}
 
         static std::string makeKeyStringFromSubRunNumber(const SubRunNumber& n) {
+            std::string str(1+sizeof(n),'\0');
+            str[0] = '%';
+#ifndef HEPNOS_READABLE_NUMBERS
+#if BOOST_ENDIAN_BIG_BYTE
+            std::memcpy(&str[1], &n, sizeof(n));
+            return str;
+#else
+            unsigned i = sizeof(n);
+            auto n2 = n;
+            while(n2 != 0) {
+                str[i] = n2 & 0xff;
+                n2 = n2 >> 8;
+                i -= 1;
+            }
+            return str;
+#endif
+#else
             std::stringstream strstr;
             strstr << "%" << std::setfill('0') << std::setw(16) << std::hex << n;
             return strstr.str();
+#endif
+        }
+
+        static SubRunNumber parseSubRunNumberFromKeyString(const char* str) {
+            if(str[0] != '%') return InvalidSubRunNumber;
+            SubRunNumber n;
+#ifdef HEPNOS_READABLE_NUMBERS
+            std::stringstream strSubRunNumber;
+            strSubRunNumber << std::hex << std::string(str+1, 16);
+            strSubRunNumber >> n;
+#else
+#if BOOST_ENDIAN_BIG_BYTE
+            std::memcpy(&n, &str[1], sizeof(n));
+#else
+            n = 0;
+            for(unsigned i=0; i < sizeof(n); i++) {
+                n = 256*n + str[i+1];
+            }
+#endif
+#endif
+            return n;
         }
 
         std::string makeKeyStringFromSubRunNumber() const {