test(shortint): remove oprf test flakiness

tfhe/src/shortint/oprf.rs

@@ -1,7 +1,8 @@
 use super::Ciphertext;
+use crate::core_crypto::fft_impl::common::modulus_switch;
 use crate::core_crypto::prelude::{
-    keyswitch_lwe_ciphertext, lwe_ciphertext_plaintext_add_assign, LweCiphertext, LweSize,
-    Plaintext,
+    keyswitch_lwe_ciphertext, lwe_ciphertext_plaintext_add_assign, CiphertextModulusLog,
+    LweCiphertext, LweSize, Plaintext,
 };
 use crate::shortint::ciphertext::Degree;
 use crate::shortint::engine::ShortintEngine;
@@ -41,6 +42,21 @@ impl ServerKey {
         ct
     }
 
+    pub(crate) fn create_random_from_seed_modulus_switched(
+        &self,
+        seed: Seed,
+        lwe_size: LweSize,
+        log_modulus: CiphertextModulusLog,
+    ) -> LweCiphertext<Vec<u64>> {
+        let mut ct = self.create_random_from_seed(seed, lwe_size);
+
+        for i in ct.as_mut() {
+            *i = modulus_switch(*i, log_modulus) << (64 - log_modulus.0);
+        }
+
+        ct
+    }
+
     /// Uniformly generates a random encrypted value in `[0, 2^random_bits_count[`
     /// `2^random_bits_count` must be smaller than the message modulus
     /// The encryted value is oblivious to the server
@@ -107,7 +123,13 @@ impl ServerKey {
 
         let in_lwe_size = self.bootstrapping_key.input_lwe_dimension().to_lwe_size();
 
-        let seeded = self.create_random_from_seed(seed, in_lwe_size);
+        let seeded = self.create_random_from_seed_modulus_switched(
+            seed,
+            in_lwe_size,
+            self.bootstrapping_key
+                .polynomial_size()
+                .to_blind_rotation_input_modulus_log(),
+        );
 
         let p = 1 << random_bits_count;
 
@@ -160,13 +182,8 @@ impl ServerKey {
 
 #[cfg(test)]
 pub(crate) mod test {
-    use crate::core_crypto::commons::generators::DeterministicSeeder;
-    use crate::core_crypto::prelude::{
-        decrypt_lwe_ciphertext, DefaultRandomGenerator, GlweSecretKey, LweSecretKey,
-    };
-    use crate::shortint::engine::ShortintEngine;
+    use crate::core_crypto::prelude::decrypt_lwe_ciphertext;
     use crate::shortint::{ClientKey, ServerKey};
-    use itertools::Itertools;
     use rayon::prelude::*;
     use statrs::distribution::ContinuousCDF;
     use std::collections::HashMap;
@@ -177,35 +194,14 @@ pub(crate) mod test {
     }
 
     #[test]
-    // This test is seeded which prevents flakiness
-    // The noise added by the KS and the MS before the PRF LUT evaluation can make this test fail
-    // if the seeded input is close to a boundary between 2 encoded values
-    // Using another KS key can, with a non-neglibgible probability,
-    // change the output of the PRF after decoding
     fn oprf_compare_plain_ci_run_filter() {
-        let parameters = crate::shortint::parameters::PARAM_MESSAGE_2_CARRY_2_KS_PBS;
-
-        let glwe_sk = (0..parameters.glwe_dimension.0 * parameters.polynomial_size.0)
-            .map(|i| if i % 2 == 0 { 0 } else { 1 })
-            .collect_vec();
-
-        let lwe_sk = (0..parameters.lwe_dimension.0)
-            .map(|i| if i % 2 == 0 { 0 } else { 1 })
-            .collect_vec();
-
-        let ck = ClientKey {
-            glwe_secret_key: GlweSecretKey::from_container(glwe_sk, parameters.polynomial_size),
-            lwe_secret_key: LweSecretKey::from_container(lwe_sk),
-            parameters: parameters.into(),
-        };
-
-        let mut deterministic_seeder = DeterministicSeeder::<DefaultRandomGenerator>::new(Seed(0));
-
-        let mut engine = ShortintEngine::new_from_seeder(&mut deterministic_seeder);
-
-        let sk = engine.new_server_key(&ck);
+        use crate::shortint::gen_keys;
+        use crate::shortint::parameters::PARAM_MESSAGE_2_CARRY_2_KS_PBS;
+        let (ck, sk) = gen_keys(PARAM_MESSAGE_2_CARRY_2_KS_PBS);
 
-        oprf_compare_plain_from_seed(Seed(0), &ck, &sk);
+        for seed in 0..1000 {
+            oprf_compare_plain_from_seed(Seed(seed), &ck, &sk);
+        }
     }
 
     fn oprf_compare_plain_from_seed(seed: Seed, ck: &ClientKey, sk: &ServerKey) {
@@ -227,7 +223,13 @@ pub(crate) mod test {
 
         let lwe_size = sk.bootstrapping_key.input_lwe_dimension().to_lwe_size();
 
-        let ct = sk.create_random_from_seed(seed, lwe_size);
+        let ct = sk.create_random_from_seed_modulus_switched(
+            seed,
+            lwe_size,
+            sk.bootstrapping_key
+                .polynomial_size()
+                .to_blind_rotation_input_modulus_log(),
+        );
 
         let sk = ck.small_lwe_secret_key();