diff --git a/compilers/concrete-compiler/compiler/lib/Runtime/wrappers.cpp b/compilers/concrete-compiler/compiler/lib/Runtime/wrappers.cpp
index 4f3dd8729..514045d02 100644
--- a/compilers/concrete-compiler/compiler/lib/Runtime/wrappers.cpp
+++ b/compilers/concrete-compiler/compiler/lib/Runtime/wrappers.cpp
@@ -606,13 +606,78 @@ void memref_keyswitch_lwe_u64(uint64_t *out_allocated, uint64_t *out_aligned,
                               uint32_t output_dimension, uint32_t ksk_index,
                               mlir::concretelang::RuntimeContext *context) {
   assert(out_stride == 1 && ct0_stride == 1);
-  // Get keyswitch key
-  const uint64_t *keyswitch_key = context->keyswitch_key_buffer(ksk_index);
-  // Get stack parameter
-  concrete_cpu_keyswitch_lwe_ciphertext_u64(
-      out_aligned + out_offset, ct0_aligned + ct0_offset, keyswitch_key,
-      decomposition_level_count, decomposition_base_log, input_dimension,
-      output_dimension);
+  
+  // The input ciphertext is a vector of u64 of size input_dimension + 1. It has the following 
+  // memory repr:
+  // [a1, a2, ..., an, b]          with ai the mask elements, and b the body.
+  uint64_t* input_ct = ct0_aligned + ct0_offset;
+  uint64_t input_size = input_dimension + 1;
+
+  // The output ciphertext is a vector of u64 of size output_dimension + 1. It has the same kind of 
+  // as the input ct.
+  uint64_t* output_ct = out_aligned + out_offset;
+  uint64_t output_size = output_dimension + 1;
+
+  // We retrieve the ksk from the context. It is a three dimensional contiguous tensor with the 
+  // following repr:
+  // [I1, I2, ..., Iv]             with v the lwe dimension of the input key.
+  // 
+  // Each Ii contains the encryptions of the decomposition of one bit of the input key. It has the 
+  // following repr:
+  // [D1, D2, ..., Dl]             with l the number of decomposition levels used for the ksk.
+  // 
+  // Each Di is the encryption under the output key, of one level of the decomposition of the key 
+  // bit. It has the same repr as athe :
+  // [D1, D2, ..., Dl]             with l the number of decomposition levels used for the ksk.
+  const uint64_t *ksk = context->keyswitch_key_buffer(ksk_index);
+
+  // We begin by zeroing out the output ct
+  for (size_t i = 0; i < output_size; i++){
+    output_ct[i] = 0;
+  }
+
+  // We copy the body of the input ct to the body of the output ct
+  output_ct[output_size-1] = input_ct[input_size-1];
+
+  // We loop through the mask elements of the input ct and the corresponding key bits in the ksk.
+  for (size_t i = 0; i < input_dimension; i ++){
+    // We retrieve the block for the ith input key bit.
+    const uint64_t *ksk_block = ksk + i * decomposition_level_count * output_size;
+    // We retrieve the mask element
+    uint64_t mask_elm = input_ct[i];
+
+    // We compute the closest representable with the decomposition
+    uint64_t non_rep_bit_count = 64 - decomposition_level_count * decomposition_base_log;
+    uint64_t shift = non_rep_bit_count - 1;
+    uint64_t closest = mask_elm >> shift;
+    closest += (uint64_t) 1;
+    closest &= (uint64_t) - 2;
+    closest <<= shift;
+
+    // We initialize the decomposition
+    uint64_t dec_state = closest >> non_rep_bit_count;
+    uint64_t dec_mod_b_mask = ((uint64_t) 1 << decomposition_base_log) - (uint64_t) 1;
+
+    // We loop through the levels of the decomposition
+    for (size_t j = 0; j < decomposition_level_count; j++){
+
+      // We retrieve the encryption of the jth decomposition of ith bit of the input key.
+      const uint64_t *ksk_ct = ksk_block + j * output_size;
+
+      // We get the decomposed iterate
+      uint64_t decomposed = dec_state & dec_mod_b_mask;
+      dec_state >>= decomposition_base_log;
+      uint64_t carry = ((decomposed - (uint64_t) 1) | dec_state) & decomposed;
+      carry >>= decomposition_base_log - 1;
+      dec_state += carry;
+      decomposed -= carry << decomposition_base_log;
+
+      // We accumulate in the output ct
+      for (size_t k = 0; k < output_size; k++){
+        output_ct[k] -= ksk_ct[k] * decomposed;
+      }
+    }
+  }
 }
 
 void memref_batched_add_lwe_ciphertexts_u64(