diff --git a/include/cloudkey.hpp b/include/cloudkey.hpp
index ac4a15d..af2325d 100644
--- a/include/cloudkey.hpp
+++ b/include/cloudkey.hpp
@@ -193,7 +193,7 @@ void ikskgen(KeySwitchingKey<P>& ksk, const Key<typename P::domainP>& domainkey,
     for (int l = 0; l < P::domainP::k; l++)
         for (int i = 0; i < P::domainP::n; i++)
             for (int j = 0; j < P::t; j++)
-                for (uint32_t k = 0; k < (1 << P::basebit) - 1; k++)
+                for (uint32_t k = 0; k < 1U << (P::basebit-1); k++)
                     ksk[l * P::domainP::n + i][j][k] =
                         tlweSymEncrypt<typename P::targetP>(
                             domainkey[l * P::domainP::n + i] * (k + 1) *
diff --git a/include/keyswitch.hpp b/include/keyswitch.hpp
index 383acd5..f0977cb 100644
--- a/include/keyswitch.hpp
+++ b/include/keyswitch.hpp
@@ -8,22 +8,31 @@
 
 namespace TFHEpp {
 
+template <class P>
+constexpr typename P::domainP::T iksoffsetgen()
+{
+    typename P::domainP::T offset = 0;
+    for (int i = 1; i <= P::t; i++)
+        offset += (1ULL<<P::basebit) / 2 *
+                  (1ULL << (std::numeric_limits<typename P::domainP::T>::digits -
+                            i * P::basebit));
+    return offset;
+}
+
 template <class P>
 void IdentityKeySwitch(TLWE<typename P::targetP> &res,
                        const TLWE<typename P::domainP> &tlwe,
                        const KeySwitchingKey<P> &ksk)
 {
-    constexpr uint32_t mask = (1U << P::basebit) - 1;
     res = {};
     constexpr uint domain_digit =
         std::numeric_limits<typename P::domainP::T>::digits;
     constexpr uint target_digit =
         std::numeric_limits<typename P::targetP::T>::digits;
-    constexpr typename P::domainP::T prec_offset =
+    constexpr typename P::domainP::T roundoffset =
         (P::basebit * P::t) < domain_digit
             ? 1ULL << (domain_digit - (1 + P::basebit * P::t))
             : 0;
-
     if constexpr (domain_digit == target_digit)
         res[P::targetP::k * P::targetP::n] =
             tlwe[P::domainP::k * P::domainP::n];
@@ -37,16 +46,25 @@ void IdentityKeySwitch(TLWE<typename P::targetP> &res,
             static_cast<typename P::targetP::T>(
                 tlwe[P::domainP::k * P::domainP::n])
             << (target_digit - domain_digit);
+
+//Koga's Optimization
+    constexpr typename P::domainP::T offset = iksoffsetgen<P>();
+    constexpr typename P::domainP::T mask = (1ULL << P::basebit) - 1; 
+    constexpr typename P::domainP::T halfbase = 1ULL << (P::basebit - 1);
+
     for (int i = 0; i < P::domainP::k * P::domainP::n; i++) {
-        const typename P::domainP::T aibar = tlwe[i] + prec_offset;
+        const typename P::domainP::T aibar = tlwe[i] + offset + roundoffset;
         for (int j = 0; j < P::t; j++) {
-            const uint32_t aij =
-                (aibar >> (std::numeric_limits<typename P::domainP::T>::digits -
+            const int32_t aij =
+                ((aibar >> (std::numeric_limits<typename P::domainP::T>::digits -
                            (j + 1) * P::basebit)) &
-                mask;
-            if (aij != 0)
-                for (int k = 0; k <= P::targetP::k * P::targetP::n; k++)
-                    res[k] -= ksk[i][j][aij - 1][k];
+                mask)-halfbase;
+            if(aij > 0)
+            for (int k = 0; k <= P::targetP::k * P::targetP::n; k++)
+                res[k] -= ksk[i][j][aij - 1][k];
+            else if(aij < 0)
+            for (int k = 0; k <= P::targetP::k * P::targetP::n; k++)
+                res[k] += ksk[i][j][std::abs(aij) - 1][k];
         }
     }
 }
diff --git a/include/params.hpp b/include/params.hpp
index 2eca7c8..98e03db 100644
--- a/include/params.hpp
+++ b/include/params.hpp
@@ -147,7 +147,7 @@ using BootstrappingKeyRAINTT =
 
 template <class P>
 using KeySwitchingKey = std::array<
-    std::array<std::array<TLWE<typename P::targetP>, (1 << P::basebit) - 1>,
+    std::array<std::array<TLWE<typename P::targetP>, (1 << (P::basebit-1))>,
                P::t>,
     P::domainP::k * P::domainP::n>;
 template <class P>