fix: fix the atomic pattern used to cast in trivium and a test in sho…

…rtint

- parameters are optimized for a clean ciphertext, the ciphertext being
keyswitched was noisy

apps/trivium/src/trans_ciphering/mod.rs

@@ -48,6 +48,8 @@ fn transcipher_from_1_1_stream(
 ) -> FheUint64 {
     assert_eq!(stream.len(), 64);
 
+    let id_lut = internal_server_key.generate_lookup_table(|x| x);
+
     let pairs = (0..32)
         .into_par_iter()
         .map(|i| {
@@ -57,10 +59,11 @@ fn transcipher_from_1_1_stream(
             let b0 = &stream[8 * byte_idx + 2 * pair_idx];
             let b1 = &stream[8 * byte_idx + 2 * pair_idx + 1];
 
-            casting_key.cast(
-                &internal_server_key
-                    .unchecked_add(b0, &internal_server_key.unchecked_scalar_mul(b1, 2)),
-            )
+            let mut combined = internal_server_key
+                .unchecked_add(b0, &internal_server_key.unchecked_scalar_mul(b1, 2));
+            internal_server_key.apply_lookup_table_assign(&mut combined, &id_lut);
+
+            casting_key.cast(&combined)
         })
         .collect::<Vec<_>>();
 

tfhe/src/shortint/key_switching_key/test.rs

@@ -129,7 +129,12 @@ fn gen_multi_keys_test_add_with_overflow_ci_run_filter() {
     let c3 = sk1.unchecked_scalar_mul(&c1, 2);
     let c4 = sk1.unchecked_add(&c3, &c2);
 
-    let output_of_cast = ksk.cast(&c4);
+    // The optimized atomic pattern requires a ciphertext with NoiseLevel::NOMINAL, i.e. a
+    // ciphertext fresh out of a bootstrap
+    let id_lut = sk1.generate_lookup_table(|x| x);
+    let c5 = sk1.apply_lookup_table(&c4, &id_lut);
+
+    let output_of_cast = ksk.cast(&c5);
     let clear = ck2.decrypt(&output_of_cast);
     assert_eq!(clear, 3);
     let ct_carry = sk2.carry_extract(&output_of_cast);