fix(shortint): fix compression encoding change not being taken into a…

…ccount

- this maps better to what was optimized and will dramatically diminish the
pfail as we now have 2 more bits for the LUT redundancy

tfhe/src/shortint/engine/mod.rs

@@ -85,6 +85,32 @@ pub(crate) fn fill_accumulator<F, C>(
     carry_modulus: CarryModulus,
     f: F,
 ) -> u64
+where
+    C: ContainerMut<Element = u64>,
+    F: Fn(u64) -> u64,
+{
+    fill_accumulator_with_encoding(
+        accumulator,
+        polynomial_size,
+        glwe_size,
+        message_modulus,
+        carry_modulus,
+        message_modulus,
+        carry_modulus,
+        f,
+    )
+}
+
+pub(crate) fn fill_accumulator_with_encoding<F, C>(
+    accumulator: &mut GlweCiphertext<C>,
+    polynomial_size: PolynomialSize,
+    glwe_size: GlweSize,
+    input_message_modulus: MessageModulus,
+    input_carry_modulus: CarryModulus,
+    output_message_modulus: MessageModulus,
+    output_carry_modulus: CarryModulus,
+    f: F,
+) -> u64
 where
     C: ContainerMut<Element = u64>,
     F: Fn(u64) -> u64,
@@ -97,25 +123,25 @@ where
     accumulator_view.get_mut_mask().as_mut().fill(0);
 
     // Modulus of the msg contained in the msg bits and operations buffer
-    let modulus_sup = (message_modulus.0 * carry_modulus.0) as usize;
+    let input_modulus_sup = (input_message_modulus.0 * input_carry_modulus.0) as usize;
 
     // N/(p/2) = size of each block
-    let box_size = polynomial_size.0 / modulus_sup;
+    let box_size = polynomial_size.0 / input_modulus_sup;
 
     // Value of the shift we multiply our messages by
-    let delta = (1_u64 << 63) / (message_modulus.0 * carry_modulus.0);
+    let output_delta = (1_u64 << 63) / (output_message_modulus.0 * output_carry_modulus.0);
 
     let mut body = accumulator_view.get_mut_body();
     let accumulator_u64 = body.as_mut();
 
     // Tracking the max value of the function to define the degree later
     let mut max_value = 0;
 
-    for i in 0..modulus_sup {
+    for i in 0..input_modulus_sup {
         let index = i * box_size;
         let f_eval = f(i as u64);
         max_value = max_value.max(f_eval);
-        accumulator_u64[index..index + box_size].fill(f_eval * delta);
+        accumulator_u64[index..index + box_size].fill(f_eval * output_delta);
     }
 
     let half_box_size = box_size / 2;

tfhe/src/shortint/list_compression/compression.rs

@@ -7,9 +7,9 @@ use crate::core_crypto::prelude::{
 };
 use crate::shortint::ciphertext::CompressedCiphertextList;
 use crate::shortint::engine::ShortintEngine;
-use crate::shortint::parameters::NoiseLevel;
+use crate::shortint::parameters::{CarryModulus, MessageModulus, NoiseLevel};
 use crate::shortint::server_key::{
-    apply_programmable_bootstrap, generate_lookup_table, unchecked_scalar_mul_assign,
+    apply_programmable_bootstrap, generate_lookup_table_with_encoding, unchecked_scalar_mul_assign,
 };
 use crate::shortint::{Ciphertext, CiphertextModulus, MaxNoiseLevel};
 use rayon::iter::ParallelIterator;
@@ -131,13 +131,28 @@ impl DecompressionKey {
             return None;
         }
 
-        let carry_extract = generate_lookup_table(
+        let encryption_cleartext_modulus = packed.message_modulus.0 * packed.carry_modulus.0;
+        // We multiply by message_modulus during compression so the actual modulus for the
+        // compression is smaller
+        let compression_cleartext_modulus = encryption_cleartext_modulus / packed.message_modulus.0;
+        let effective_compression_message_modulus = MessageModulus(compression_cleartext_modulus);
+        let effective_compression_carry_modulus = CarryModulus(1);
+
+        let decompression_rescale = generate_lookup_table_with_encoding(
             self.out_glwe_size(),
             self.out_polynomial_size(),
             packed.ciphertext_modulus,
+            // Input moduli are the effective compression ones
+            effective_compression_message_modulus,
+            effective_compression_carry_modulus,
+            // Output moduli are directly the ones stored in the list
             packed.message_modulus,
             packed.carry_modulus,
-            |x| x / packed.message_modulus.0,
+            // Here we do not divide by message_modulus
+            // Example: in the 2_2 case we are mapping a 2 bits message onto a 4 bits space, we
+            // want to keep the original 2 bits value in the 4 bits space, so we apply the identity
+            // and the encoding will rescale it for us.
+            |x| x,
         );
 
         let polynomial_size = packed.modulus_switched_glwe_ciphertext_list[0].polynomial_size();
@@ -181,14 +196,14 @@ impl DecompressionKey {
                 &self.blind_rotate_key,
                 &intermediate_lwe,
                 &mut output_br,
-                &carry_extract.acc,
+                &decompression_rescale.acc,
                 buffers,
             );
         });
 
         Some(Ciphertext::new(
             output_br,
-            carry_extract.degree,
+            decompression_rescale.degree,
             NoiseLevel::NOMINAL,
             packed.message_modulus,
             packed.carry_modulus,

tfhe/src/shortint/server_key/mod.rs

@@ -43,16 +43,17 @@ use crate::core_crypto::prelude::ComputationBuffers;
 use crate::shortint::ciphertext::{Ciphertext, Degree, MaxDegree, MaxNoiseLevel, NoiseLevel};
 use crate::shortint::client_key::ClientKey;
 use crate::shortint::engine::{
-    fill_accumulator, fill_accumulator_no_encoding, fill_many_lut_accumulator, ShortintEngine,
+    fill_accumulator, fill_accumulator_no_encoding, fill_accumulator_with_encoding,
+    fill_many_lut_accumulator, ShortintEngine,
 };
 use crate::shortint::parameters::{
     CarryModulus, CiphertextConformanceParams, CiphertextModulus, MessageModulus,
 };
 use crate::shortint::{EncryptionKeyChoice, PBSOrder};
-use ::tfhe_versionable::Versionize;
 use aligned_vec::ABox;
 use serde::{Deserialize, Serialize};
 use std::fmt::{Debug, Display, Formatter};
+use tfhe_versionable::Versionize;
 
 #[cfg(feature = "pbs-stats")]
 pub mod pbs_stats {
@@ -1563,6 +1564,52 @@ where
     }
 }
 
+/// Caller needs to ensure that the operation applied is coherent from an encoding perspective.
+///
+/// For example:
+///
+/// Input encoding has 2 bits and output encoding has 4 bits, applying the identity lut would map
+/// the following:
+///
+/// 0|00|xx -> 0|00|00
+/// 0|01|xx -> 0|00|01
+/// 0|10|xx -> 0|00|10
+/// 0|11|xx -> 0|00|11
+///
+/// The reason is the identity function is computed in the input space but the scaling is done in
+/// the output space, as there are more bits in the output space, the delta is smaller hence the
+/// apparent "division" happening.
+pub(crate) fn generate_lookup_table_with_encoding<F>(
+    glwe_size: GlweSize,
+    polynomial_size: PolynomialSize,
+    ciphertext_modulus: CiphertextModulus,
+    input_message_modulus: MessageModulus,
+    input_carry_modulus: CarryModulus,
+    output_message_modulus: MessageModulus,
+    output_carry_modulus: CarryModulus,
+    f: F,
+) -> LookupTableOwned
+where
+    F: Fn(u64) -> u64,
+{
+    let mut acc = GlweCiphertext::new(0, glwe_size, polynomial_size, ciphertext_modulus);
+    let max_value = fill_accumulator_with_encoding(
+        &mut acc,
+        polynomial_size,
+        glwe_size,
+        input_message_modulus,
+        input_carry_modulus,
+        output_message_modulus,
+        output_carry_modulus,
+        f,
+    );
+
+    LookupTableOwned {
+        acc,
+        degree: Degree::new(max_value),
+    }
+}
+
 #[derive(Copy, Clone)]
 pub struct PBSConformanceParameters {
     pub in_lwe_dimension: LweDimension,