Bug fixes

- fixed `encoding.rs/decode_coeff_i64` for the case `k < basek2`
- `glwe/external_product.rs/external_product_scratch_space` did not allocate enough space for [`a_dft`](829b8be610/core/src/glwe/external_product.rs (L83)) when `k_out < k_ggsw`. Fixed by replacing `in_size` with `ggsw_size`.
- `glwe/ops.rs/add` sets `basek` and `k`of `self` GLWECiphertext whic is invalid because the `size` which depends on `inner` data remains unchanged.
- (not a fix) expose `backend` from `core`.

core/src/glwe/external_product.rs

@@ -18,7 +18,7 @@ impl GLWECiphertext<Vec<u8>> {
         let in_size: usize = k_in.div_ceil(basek).div_ceil(digits);
         let out_size: usize = k_out.div_ceil(basek);
         let ggsw_size: usize = k_ggsw.div_ceil(basek);
-        let vmp: usize = module.bytes_of_vec_znx_dft(rank + 1, in_size)
+        let vmp: usize = module.bytes_of_vec_znx_dft(rank + 1, ggsw_size)
             + module.vmp_apply_tmp_bytes(
                 out_size,
                 in_size,

core/src/glwe/ops.rs

@@ -14,6 +14,7 @@ pub trait GLWEOps: GLWECiphertextToMut + Infos + SetMetaData {
             assert_eq!(b.n(), module.n());
             assert_eq!(self.n(), module.n());
             assert_eq!(a.basek(), b.basek());
+            assert_eq!(self.basek(), a.basek());
             assert!(self.rank() >= a.rank().max(b.rank()));
         }
 
@@ -46,8 +47,8 @@ pub trait GLWEOps: GLWECiphertextToMut + Infos + SetMetaData {
             });
         });
 
-        self.set_basek(a.basek());
-        self.set_k(a.k().max(b.k()));
+        // self.set_basek(a.basek());
+        // self.set_k(a.k().max(b.k()));
     }
 
     fn add_inplace<A>(&mut self, module: &Module<FFT64>, a: &A)

core/src/lib.rs

@@ -19,6 +19,7 @@ pub use glwe::{GLWECiphertext, GLWEOps, GLWEPacker, GLWEPlaintext, GLWEPublicKey
 pub(crate) use glwe::{GLWECiphertextToMut, GLWECiphertextToRef};
 pub use lwe::{LWECiphertext, LWESecret};
 
+pub use backend;
 pub use backend::Scratch;
 pub use backend::ScratchOwned;