fix: upcasting of signed integer when block decomposing

Some parts of the code did not use the correct way to
decompose a clear integer into blocks which could be encrypted
or used in scalar ops.

The sign extension was not always properly done, leading for example
in the encryption of a negative integer stored on a i8 to a
SignedRadixCiphertext with a num_blocks greater than i8 to be incorrect:

```
let ct = cks.encrypt_signed(-1i8, 16) // 2_2 parameters
let d: i32 = cks.decrypt_signed(&ct);
assert_eq!(d, i32::from(-1i8)); // Fails
```

To fix, a BlockDecomposer::with_block_count function is added and used
This function will properly do the sign extension when needed

tfhe/src/integer/block_decomposition.rs

@@ -115,6 +115,21 @@ where
         Self::new_(value, bits_per_block, None, Some(padding_bit))
     }
 
+    /// Creates a block decomposer that will return `block_count` blocks
+    ///
+    /// * If T is signed, extra block will be sign extended
+    pub fn with_block_count(value: T, bits_per_block: u32, block_count: u32) -> Self {
+        let mut decomposer = Self::new(value, bits_per_block);
+        // If the new number of bits is less than the actual number of bits, it means
+        // data will be truncated
+        //
+        // If the new number of bits is greater than the actual number of bits, it means
+        // the right shift used internally will correctly sign extend for us
+        let num_bits_valid = block_count * bits_per_block;
+        decomposer.num_bits_valid = num_bits_valid;
+        decomposer
+    }
+
     pub fn new(value: T, bits_per_block: u32) -> Self {
         Self::new_(value, bits_per_block, None, None)
     }
@@ -238,7 +253,8 @@ where
     T: Recomposable,
 {
     pub fn value(&self) -> T {
-        if self.bit_pos >= T::BITS as u32 {
+        let is_signed = (T::ONE << (T::BITS as u32 - 1)) < T::ZERO;
+        if self.bit_pos >= (T::BITS as u32 - u32::from(is_signed)) {
             self.data
         } else {
             let valid_mask = (T::ONE << self.bit_pos) - T::ONE;
@@ -351,6 +367,49 @@ mod tests {
         assert_eq!(expected_blocks, blocks);
     }
 
+    #[test]
+    fn test_bit_block_decomposer_with_block_count() {
+        let bits_per_block = 3;
+        let expected_blocks = [0, 0, 6, 7, 7, 7, 7, 7, 7];
+        let value = i8::MIN;
+        for block_count in 1..expected_blocks.len() as u32 {
+            let blocks = BlockDecomposer::with_block_count(value, bits_per_block, block_count)
+                .iter_as::<u64>()
+                .collect::<Vec<_>>();
+            assert_eq!(expected_blocks[..block_count as usize], blocks);
+        }
+
+        let bits_per_block = 3;
+        let expected_blocks = [7, 7, 1, 0, 0, 0, 0, 0, 0];
+        let value = i8::MAX;
+        for block_count in 1..expected_blocks.len() as u32 {
+            let blocks = BlockDecomposer::with_block_count(value, bits_per_block, block_count)
+                .iter_as::<u64>()
+                .collect::<Vec<_>>();
+            assert_eq!(expected_blocks[..block_count as usize], blocks);
+        }
+
+        let bits_per_block = 2;
+        let expected_blocks = [0, 0, 0, 2, 3, 3, 3, 3, 3];
+        let value = i8::MIN;
+        for block_count in 1..expected_blocks.len() as u32 {
+            let blocks = BlockDecomposer::with_block_count(value, bits_per_block, block_count)
+                .iter_as::<u64>()
+                .collect::<Vec<_>>();
+            assert_eq!(expected_blocks[..block_count as usize], blocks);
+        }
+
+        let bits_per_block = 2;
+        let expected_blocks = [3, 3, 3, 1, 0, 0, 0, 0, 0, 0];
+        let value = i8::MAX;
+        for block_count in 1..expected_blocks.len() as u32 {
+            let blocks = BlockDecomposer::with_block_count(value, bits_per_block, block_count)
+                .iter_as::<u64>()
+                .collect::<Vec<_>>();
+            assert_eq!(expected_blocks[..block_count as usize], blocks);
+        }
+    }
+
     #[test]
     fn test_bit_block_decomposer_recomposer_carry_handling_in_between() {
         let value = u16::MAX as u32;

tfhe/src/integer/encryption.rs

@@ -92,9 +92,7 @@ where
 // We need to concretize the iterator type to be able to pass callbacks consuming the iterator,
 // having an opaque return impl Iterator does not allow to take callbacks at this moment, not sure
 // the Fn(impl Trait) syntax can be made to work nicely with the rest of the language
-pub(crate) type ClearRadixBlockIterator<T> = std::iter::Take<
-    std::iter::Chain<std::iter::Map<BlockDecomposer<T>, fn(T) -> u64>, std::iter::Repeat<u64>>,
->;
+pub(crate) type ClearRadixBlockIterator<T> = std::iter::Map<BlockDecomposer<T>, fn(T) -> u64>;
 
 pub(crate) fn create_clear_radix_block_iterator<T>(
     message: T,
@@ -105,12 +103,7 @@ where
     T: DecomposableInto<u64>,
 {
     let bits_in_block = message_modulus.0.ilog2();
-    let decomposer = BlockDecomposer::new(message, bits_in_block);
-
-    decomposer
-        .iter_as::<u64>()
-        .chain(std::iter::repeat(0u64))
-        .take(num_blocks)
+    BlockDecomposer::with_block_count(message, bits_in_block, num_blocks as u32).iter_as::<u64>()
 }
 
 pub(crate) fn encrypt_crt<BlockKey, Block, CrtCiphertextType, F>(

tfhe/src/integer/gpu/server_key/radix/mod.rs

@@ -187,10 +187,12 @@ impl CudaServerKey {
             PBSOrder::BootstrapKeyswitch => self.key_switching_key.output_key_lwe_size(),
         };
 
-        let decomposer = BlockDecomposer::new(scalar, self.message_modulus.0.ilog2())
-            .iter_as::<u64>()
-            .chain(std::iter::repeat(0))
-            .take(num_blocks);
+        let decomposer = BlockDecomposer::with_block_count(
+            scalar,
+            self.message_modulus.0.ilog2(),
+            num_blocks as u32,
+        )
+        .iter_as::<u64>();
         let mut cpu_lwe_list = LweCiphertextList::new(
             0,
             lwe_size,

tfhe/src/integer/server_key/radix/tests.rs

@@ -48,6 +48,7 @@ create_parameterized_test_classical_params!(integer_encrypt_decrypt_128_bits);
 create_parameterized_test_classical_params!(integer_encrypt_decrypt_128_bits_specific_values);
 create_parameterized_test_classical_params!(integer_encrypt_decrypt_256_bits_specific_values);
 create_parameterized_test_classical_params!(integer_encrypt_decrypt_256_bits);
+create_parameterized_test_classical_params!(integer_encrypt_auto_cast);
 create_parameterized_test_classical_params!(integer_unchecked_add);
 create_parameterized_test_classical_params!(integer_smart_add);
 create_parameterized_test!(
@@ -157,7 +158,7 @@ fn integer_encrypt_decrypt_128_bits(param: ClassicPBSParameters) {
     let (cks, _) = KEY_CACHE.get_from_params(param, IntegerKeyKind::Radix);
 
     let mut rng = rand::thread_rng();
-    let num_block = (128f64 / (param.message_modulus.0 as f64).log(2.0)).ceil() as usize;
+    let num_block = 128u32.div_ceil(param.message_modulus.0.ilog2()) as usize;
     for _ in 0..10 {
         let clear = rng.gen::<u128>();
 
@@ -172,7 +173,7 @@ fn integer_encrypt_decrypt_128_bits(param: ClassicPBSParameters) {
 fn integer_encrypt_decrypt_128_bits_specific_values(param: ClassicPBSParameters) {
     let (cks, sks) = KEY_CACHE.get_from_params(param, IntegerKeyKind::Radix);
 
-    let num_block = (128f64 / (param.message_modulus.0 as f64).log(2.0)).ceil() as usize;
+    let num_block = 128u32.div_ceil(param.message_modulus.0.ilog2()) as usize;
     {
         let a = u64::MAX as u128;
         let ct = cks.encrypt_radix(a, num_block);
@@ -220,7 +221,7 @@ fn integer_encrypt_decrypt_128_bits_specific_values(param: ClassicPBSParameters)
 fn integer_encrypt_decrypt_256_bits_specific_values(param: ClassicPBSParameters) {
     let (cks, _) = KEY_CACHE.get_from_params(param, IntegerKeyKind::Radix);
 
-    let num_block = (256f64 / (param.message_modulus.0 as f64).log(2.0)).ceil() as usize;
+    let num_block = 256u32.div_ceil(param.message_modulus.0.ilog2()) as usize;
     {
         let a = (u64::MAX as u128) << 64;
         let b = 0;
@@ -245,7 +246,7 @@ fn integer_encrypt_decrypt_256_bits(param: ClassicPBSParameters) {
     let (cks, _) = KEY_CACHE.get_from_params(param, IntegerKeyKind::Radix);
 
     let mut rng = rand::thread_rng();
-    let num_block = (256f64 / (param.message_modulus.0 as f64).log(2.0)).ceil() as usize;
+    let num_block = 256u32.div_ceil(param.message_modulus.0.ilog2()) as usize;
 
     for _ in 0..10 {
         let clear0 = rng.gen::<u128>();
@@ -261,11 +262,52 @@ fn integer_encrypt_decrypt_256_bits(param: ClassicPBSParameters) {
     }
 }
 
+fn integer_encrypt_auto_cast(param: ClassicPBSParameters) {
+    // The goal is to test that encrypting a value stored in a type
+    // for which the bit count does not match the target block count of the encrypted
+    // radix properly applies upcasting/downcasting
+
+    let (cks, _) = KEY_CACHE.get_from_params(param, IntegerKeyKind::Radix);
+    let mut rng = rand::thread_rng();
+
+    let num_blocks = 32u32.div_ceil(param.message_modulus.0.ilog2()) as usize;
+
+    // Positive signed value
+    let value = rng.gen_range(0..=i32::MAX);
+    let ct = cks.encrypt_signed_radix(value, num_blocks * 2);
+    let d: i64 = cks.decrypt_signed_radix(&ct);
+    assert_eq!(i64::from(value), d);
+
+    let ct = cks.encrypt_signed_radix(value, num_blocks.div_ceil(2));
+    let d: i16 = cks.decrypt_signed_radix(&ct);
+    assert_eq!(value as i16, d);
+
+    // Negative signed value
+    let value = rng.gen_range(i8::MIN..0);
+    let ct = cks.encrypt_signed_radix(value, num_blocks * 2);
+    let d: i64 = cks.decrypt_signed_radix(&ct);
+    assert_eq!(i64::from(value), d);
+
+    let ct = cks.encrypt_signed_radix(value, num_blocks.div_ceil(2));
+    let d: i16 = cks.decrypt_signed_radix(&ct);
+    assert_eq!(value as i16, d);
+
+    // Unsigned value
+    let value = rng.gen::<u32>();
+    let ct = cks.encrypt_radix(value, num_blocks * 2);
+    let d: u64 = cks.decrypt_radix(&ct);
+    assert_eq!(u64::from(value), d);
+
+    let ct = cks.encrypt_radix(value, num_blocks.div_ceil(2));
+    let d: u16 = cks.decrypt_radix(&ct);
+    assert_eq!(value as u16, d);
+}
+
 fn integer_smart_add_128_bits(param: ClassicPBSParameters) {
     let (cks, sks) = KEY_CACHE.get_from_params(param, IntegerKeyKind::Radix);
 
     let mut rng = rand::thread_rng();
-    let num_block = (128f64 / (param.message_modulus.0 as f64).log(2.0)).ceil() as usize;
+    let num_block = 128u32.div_ceil(param.message_modulus.0.ilog2()) as usize;
 
     for _ in 0..100 {
         let clear_0 = rng.gen::<u128>();

tfhe/src/integer/server_key/radix_parallel/scalar_add.rs

@@ -276,15 +276,13 @@ impl ServerKey {
             self.full_propagate_parallelized(ct);
         }
 
-        let scalar_blocks = BlockDecomposer::new(scalar, self.message_modulus().0.ilog2())
-            .iter_as::<u8>()
-            .chain(std::iter::repeat(if scalar < Scalar::ZERO {
-                (self.message_modulus().0 - 1) as u8
-            } else {
-                0
-            }))
-            .take(ct.blocks().len())
-            .collect();
+        let scalar_blocks = BlockDecomposer::with_block_count(
+            scalar,
+            self.message_modulus().0.ilog2(),
+            ct.blocks().len() as u32,
+        )
+        .iter_as::<u8>()
+        .collect();
 
         const COMPUTE_OVERFLOW: bool = false;
         const INPUT_CARRY: bool = false;

tfhe/src/integer/server_key/radix_parallel/scalar_comparison.rs

@@ -757,12 +757,10 @@ impl ServerKey {
             .map(|chunk_of_two| self.pack_block_chunk(chunk_of_two))
             .collect::<Vec<_>>();
 
-        let padding_value = (packed_modulus - 1) * u64::from(b < Scalar::ZERO);
-        let mut b_blocks = BlockDecomposer::new(b, packed_modulus.ilog2())
-            .iter_as::<u64>()
-            .chain(std::iter::repeat(padding_value))
-            .take(a.len())
-            .collect::<Vec<_>>();
+        let mut b_blocks =
+            BlockDecomposer::with_block_count(b, packed_modulus.ilog2(), a.len() as u32)
+                .iter_as::<u64>()
+                .collect::<Vec<_>>();
 
         if !num_block_is_even && b < Scalar::ZERO {
             let last_index = b_blocks.len() - 1;
@@ -1058,25 +1056,23 @@ impl ServerKey {
         Scalar: DecomposableInto<u64>,
     {
         let is_superior = self.unchecked_scalar_gt_parallelized(lhs, rhs);
-        let luts = BlockDecomposer::new(rhs, self.message_modulus().0.ilog2())
-            .iter_as::<u64>()
-            .chain(std::iter::repeat(if rhs >= Scalar::ZERO {
-                0u64
-            } else {
-                self.message_modulus().0 - 1
-            }))
-            .take(lhs.blocks().len())
-            .map(|scalar_block| {
-                self.key
-                    .generate_lookup_table_bivariate(|is_superior, block| {
-                        if is_superior == 1 {
-                            block
-                        } else {
-                            scalar_block
-                        }
-                    })
-            })
-            .collect::<Vec<_>>();
+        let luts = BlockDecomposer::with_block_count(
+            rhs,
+            self.message_modulus().0.ilog2(),
+            lhs.blocks().len() as u32,
+        )
+        .iter_as::<u64>()
+        .map(|scalar_block| {
+            self.key
+                .generate_lookup_table_bivariate(|is_superior, block| {
+                    if is_superior == 1 {
+                        block
+                    } else {
+                        scalar_block
+                    }
+                })
+        })
+        .collect::<Vec<_>>();
 
         let new_blocks = lhs
             .blocks()
@@ -1097,25 +1093,23 @@ impl ServerKey {
         Scalar: DecomposableInto<u64>,
     {
         let is_inferior = self.unchecked_scalar_lt_parallelized(lhs, rhs);
-        let luts = BlockDecomposer::new(rhs, self.message_modulus().0.ilog2())
-            .iter_as::<u64>()
-            .chain(std::iter::repeat(if rhs >= Scalar::ZERO {
-                0u64
-            } else {
-                self.message_modulus().0 - 1
-            }))
-            .take(lhs.blocks().len())
-            .map(|scalar_block| {
-                self.key
-                    .generate_lookup_table_bivariate(|is_inferior, block| {
-                        if is_inferior == 1 {
-                            block
-                        } else {
-                            scalar_block
-                        }
-                    })
-            })
-            .collect::<Vec<_>>();
+        let luts = BlockDecomposer::with_block_count(
+            rhs,
+            self.message_modulus().0.ilog2(),
+            lhs.blocks().len() as u32,
+        )
+        .iter_as::<u64>()
+        .map(|scalar_block| {
+            self.key
+                .generate_lookup_table_bivariate(|is_inferior, block| {
+                    if is_inferior == 1 {
+                        block
+                    } else {
+                        scalar_block
+                    }
+                })
+        })
+        .collect::<Vec<_>>();
 
         let new_blocks = lhs
             .blocks()

tfhe/src/integer/server_key/radix_parallel/scalar_sub.rs

@@ -641,6 +641,7 @@ impl ServerKey {
         const INPUT_CARRY: bool = true;
         let flipped_scalar = !scalar;
         let decomposed_flipped_scalar =
+            // We don't use BlockDecomposer::with_block_count as we are doing something special
             BlockDecomposer::new(flipped_scalar, self.message_modulus().0.ilog2())
                 .iter_as::<u8>()
                 .chain(std::iter::repeat(if scalar < Scalar::ZERO {