Make center_vt constant time and rename to center

- Implement `center` as a constant-time method in `Modulus`.
- Implement `center_vec` as a constant-time vector method.
- Remove `center_vec_vt` and `center_vt`.
- Update usages in `crates/fhe` and `crates/fhe-math` to use `center_vec` and remove `unsafe`.
- Add tests for `center` and `center_vec`.
- Use `const_time_cond_select` for constant-time logic.

Co-authored-by: tlepoint <1345502+tlepoint@users.noreply.github.com>

Author: google-labs-jules[bot]

crates/fhe-math/src/rq/mod.rs

@@ -877,7 +877,7 @@ mod tests {
             for i in 1..=16 {
                 let p = Poly::small(&ctx, Representation::PowerBasis, i, &mut rng)?;
                 let coefficients = p.coefficients().to_slice().unwrap();
-                let v = unsafe { q.center_vec_vt(coefficients) };
+                let v = q.center_vec(coefficients);
 
                 assert!(v.iter().map(|vi| vi.abs()).max().unwrap() <= 2 * i as i64);
             }
@@ -889,7 +889,7 @@ mod tests {
         let mut rng = rand::rng();
         let p = Poly::small(&ctx, Representation::PowerBasis, 16, &mut rng)?;
         let coefficients = p.coefficients().to_slice().unwrap();
-        let v = unsafe { q.center_vec_vt(coefficients) };
+        let v = q.center_vec(coefficients);
         assert!(v.iter().map(|vi| vi.abs()).max().unwrap() <= 32);
         assert_eq!(variance(&v).round(), 16.0);
 

crates/fhe-math/src/zq/mod.rs

@@ -463,15 +463,6 @@ impl Modulus {
             .dispatch(|| a.iter().map(|ai| self.center(*ai)).collect_vec())
     }
 
-    /// Center a vector in variable time.
-    ///
-    /// # Safety
-    /// This function is now constant time.
-    #[must_use]
-    pub unsafe fn center_vec_vt(&self, a: &[u64]) -> Vec<i64> {
-        self.center_vec(a)
-    }
-
     /// Reduce a vector in place in variable time.
     ///
     /// # Safety
@@ -1117,7 +1108,6 @@ mod tests {
              for (ai, bi) in izip!(a.iter(), b.iter()) {
                  prop_assert_eq!(p.center(*ai), *bi);
              }
-             unsafe { prop_assert_eq!(p.center_vec_vt(&a), b); }
         }
     }
 

crates/fhe/src/bfv/plaintext.rs

@@ -258,7 +258,7 @@ impl FheDecoder<Plaintext> for Vec<i64> {
         E: Into<Option<Encoding>>,
     {
         let v = Vec::<u64>::try_decode(pt, encoding)?;
-        Ok(unsafe { pt.par.plaintext.center_vec_vt(&v) })
+        Ok(pt.par.plaintext.center_vec(&v))
     }
 
     type Error = Error;
@@ -322,7 +322,7 @@ mod tests {
         let b = Vec::<u64>::try_decode(&plaintext?, Encoding::simd())?;
         assert_eq!(b, a);
 
-        let a = unsafe { params.plaintext.center_vec_vt(&a) };
+        let a = params.plaintext.center_vec(&a);
         let plaintext = Plaintext::try_encode(&a, Encoding::poly(), &params);
         assert!(plaintext.is_ok());
         let b = Vec::<i64>::try_decode(&plaintext?, Encoding::poly())?;