diff --git a/storm/Big.cpp b/storm/Big.cpp
index 5200602..ce8aade 100644
--- a/storm/Big.cpp
+++ b/storm/Big.cpp
@@ -32,6 +32,10 @@ int32_t SBigCompare(BigData* a, BigData* b) {
     return Compare(a->Primary(), b->Primary());
 }
 
+void SBigCopy(BigData* a, BigData* b) {
+    a->m_primary = b->m_primary;
+}
+
 void SBigDel(BigData* num) {
     delete num;
 }
diff --git a/storm/Big.hpp b/storm/Big.hpp
index b820545..bf73ab1 100644
--- a/storm/Big.hpp
+++ b/storm/Big.hpp
@@ -12,6 +12,8 @@ void SBigBitLen(BigData* num, uint32_t* len);
 
 int32_t SBigCompare(BigData* a, BigData* b);
 
+void SBigCopy(BigData* a, BigData* b);
+
 void SBigDel(BigData* num);
 
 void SBigFromBinary(BigData* num, const void* data, uint32_t bytes);
diff --git a/test/Big.cpp b/test/Big.cpp
index 8b66f57..48ab0c6 100644
--- a/test/Big.cpp
+++ b/test/Big.cpp
@@ -213,6 +213,24 @@ TEST_CASE("SBigCompare", "[big]") {
     }
 }
 
+TEST_CASE("SBigCopy", "[big]") {
+    BigDataTest a;
+    BigDataTest b;
+
+    SECTION("copies data") {
+        uint8_t num[] = { 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9 };
+        SBigFromBinary(a, num, sizeof(num));
+        SBigFromUnsigned(b, 42);
+
+        CHECK(a->Primary().Count() == 4);
+
+        SBigCopy(a, b);
+
+        CHECK(a->Primary().Count() == 1);
+        CHECK(a->Primary()[0] == 42);
+    }
+}
+
 TEST_CASE("SBigFromBinary", "[big]") {
     SECTION("creates bigdata from 0") {
         BigData* num;