diff --git a/cs_derive/src/lib.rs b/cs_derive/src/lib.rs
index a644692..45996c0 100644
--- a/cs_derive/src/lib.rs
+++ b/cs_derive/src/lib.rs
@@ -6,6 +6,7 @@ mod allocatable;
 mod selectable;
 pub(crate) mod utils;
 mod var_length_encodable;
+mod witness_var_length_encodable;
 mod witness_hook;
 
 #[proc_macro_derive(CSSelectable, attributes(CSSelectableBound))]
@@ -31,3 +32,9 @@ pub fn derive_witness_hook(input: TokenStream) -> TokenStream {
 pub fn derive_var_length_encodable(input: TokenStream) -> TokenStream {
     self::var_length_encodable::derive_var_length_encodable(input)
 }
+
+#[proc_macro_derive(WitVarLengthEncodable, attributes(WitnessVarLengthEncodableBound))]
+#[proc_macro_error::proc_macro_error]
+pub fn derive_witness_length_encodable(input: TokenStream) -> TokenStream {
+    self::witness_var_length_encodable::derive_witness_var_length_encodable(input)
+}
diff --git a/cs_derive/src/var_length_encodable/mod.rs b/cs_derive/src/var_length_encodable/mod.rs
index e59598d..b481c38 100644
--- a/cs_derive/src/var_length_encodable/mod.rs
+++ b/cs_derive/src/var_length_encodable/mod.rs
@@ -24,8 +24,6 @@ pub(crate) fn derive_var_length_encodable(
 
     let mut length_impls = TokenStream::new();
     let mut field_impls = TokenStream::new();
-    let mut witness_to_buffer_impls = TokenStream::new();
-    let mut witness_length_impls = TokenStream::new();
 
     let extra_bound = if let Some(bound) = fetch_attr_from_list(BOUND_ATTR_NAME, &attrs) {
         let bound = syn::parse_str::<WhereClause>(&bound).expect("must parse bound as WhereClause");
@@ -43,7 +41,7 @@ pub(crate) fn derive_var_length_encodable(
                 for field in named_fields.named.iter() {
                     let field_ident = field.ident.clone().expect("should have a field elem ident");
                     match field.ty {
-                        Type::Array(ref array_ty) => {
+                        Type::Array(ref _array_ty) => {
                             let field_impl = quote! {
                                 total_len += CircuitVarLengthEncodable::<F>::encoding_length(&self.#field_ident);
                             };
@@ -53,16 +51,8 @@ pub(crate) fn derive_var_length_encodable(
                                 CircuitVarLengthEncodable::<F>::encode_to_buffer(&self.#field_ident, cs, dst);
                             };
                             field_impls.extend(field_impl);
-                            let wit_to_buf_impl = quote! {
-                                <#array_ty as CircuitVarLengthEncodable<F>>::encode_witness_to_buffer(&witness.#field_ident, dst);
-                            };
-                            witness_to_buffer_impls.extend(wit_to_buf_impl);
-                            let wit_length_impl = quote! {
-                                total_len += <#array_ty as CircuitVarLengthEncodable<F>>::witness_encoding_length(&witness.#field_ident);
-                            };
-                            witness_length_impls.extend(wit_length_impl);
                         }
-                        Type::Path(ref path_ty) => {
+                        Type::Path(ref _path_ty) => {
                             let field_impl = quote! {
                                 total_len += CircuitVarLengthEncodable::<F>::encoding_length(&self.#field_ident);
                             };
@@ -71,14 +61,6 @@ pub(crate) fn derive_var_length_encodable(
                                 CircuitVarLengthEncodable::<F>::encode_to_buffer(&self.#field_ident, cs, dst);
                             };
                             field_impls.extend(field_impl);
-                            let wit_to_buf_impl = quote! {
-                                <#path_ty as CircuitVarLengthEncodable<F>>::encode_witness_to_buffer(&witness.#field_ident, dst);
-                            };
-                            witness_to_buffer_impls.extend(wit_to_buf_impl);
-                            let wit_length_impl = quote! {
-                                total_len += <#path_ty as CircuitVarLengthEncodable<F>>::witness_encoding_length(&witness.#field_ident);
-                            };
-                            witness_length_impls.extend(wit_length_impl);
                         }
                         _ => abort_call_site!("only array and path types are allowed"),
                     };
@@ -124,15 +106,6 @@ pub(crate) fn derive_var_length_encodable(
             fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, cs: &mut CS, dst: &mut Vec<Variable>) {
                 #field_impls
             }
-            fn witness_encoding_length(witness: &Self::Witness) -> usize {
-                let mut total_len = 0;
-                #witness_length_impls
-
-                total_len
-            }
-            fn encode_witness_to_buffer(witness: &Self::Witness, dst: &mut Vec<F>) {
-                #witness_to_buffer_impls
-            }
         }
     };
 
diff --git a/cs_derive/src/witness_var_length_encodable/mod.rs b/cs_derive/src/witness_var_length_encodable/mod.rs
new file mode 100644
index 0000000..195cc6a
--- /dev/null
+++ b/cs_derive/src/witness_var_length_encodable/mod.rs
@@ -0,0 +1,98 @@
+use proc_macro2::{Span, TokenStream};
+use proc_macro_error::abort_call_site;
+use quote::quote;
+use syn::{
+    parse_macro_input, token::Comma, DeriveInput, GenericParam, Type,
+    WhereClause,
+};
+
+use crate::utils::*;
+
+const BOUND_ATTR_NAME: &'static str = "WitnessVarLengthEncodableBound";
+
+pub(crate) fn derive_witness_var_length_encodable(
+    input: proc_macro::TokenStream,
+) -> proc_macro::TokenStream {
+    let derived_input = parse_macro_input!(input as DeriveInput);
+    let DeriveInput {
+        ident,
+        generics,
+        data,
+        attrs,
+        ..
+    } = derived_input.clone();
+
+    let mut witness_to_buffer_impls = TokenStream::new();
+    let mut witness_length_impls = TokenStream::new();
+
+    let extra_bound = if let Some(bound) = fetch_attr_from_list(BOUND_ATTR_NAME, &attrs) {
+        let bound = syn::parse_str::<WhereClause>(&bound).expect("must parse bound as WhereClause");
+
+        Some(bound)
+    } else {
+        None
+    };
+
+    let bound = merge_where_clauses(generics.where_clause.clone(), extra_bound);
+
+    match data {
+        syn::Data::Struct(ref struct_data) => match struct_data.fields {
+            syn::Fields::Named(ref named_fields) => {
+                for field in named_fields.named.iter() {
+                    let field_ident = field.ident.clone().expect("should have a field elem ident");
+                    match field.ty {
+                        Type::Array(ref array_ty) => {
+                            let wit_to_buf_impl = quote! {
+                                <#array_ty as WitnessVarLengthEncodable<F>>::encode_witness_to_buffer(&witness.#field_ident, dst);
+                            };
+                            witness_to_buffer_impls.extend(wit_to_buf_impl);
+                            let wit_length_impl = quote! {
+                                total_len += <#array_ty as WitnessVarLengthEncodable<F>>::witness_encoding_length(&witness.#field_ident);
+                            };
+                            witness_length_impls.extend(wit_length_impl);
+                        }
+                        Type::Path(ref path_ty) => {
+                            let wit_to_buf_impl = quote! {
+                                <#path_ty as WitnessVarLengthEncodable<F>>::encode_witness_to_buffer(&witness.#field_ident, dst);
+                            };
+                            witness_to_buffer_impls.extend(wit_to_buf_impl);
+                            let wit_length_impl = quote! {
+                                total_len += <#path_ty as WitnessVarLengthEncodable<F>>::witness_encoding_length(&witness.#field_ident);
+                            };
+                            witness_length_impls.extend(wit_length_impl);
+                        }
+                        _ => abort_call_site!("only array and path types are allowed"),
+                    };
+                }
+            }
+            _ => abort_call_site!("only named fields are allowed!"),
+        },
+        _ => abort_call_site!("only struct types are allowed!"),
+    }
+
+    let comma = Comma(Span::call_site());
+
+    let field_generic_param = syn::parse_str::<GenericParam>(&"F: SmallField").unwrap();
+    let has_engine_param = has_proper_small_field_parameter(&generics.params, &field_generic_param);
+    if has_engine_param == false {
+        panic!("Expected to have `F: SmallField` somewhere in bounds");
+    }
+
+    let type_params_of_allocated_struct = get_type_params_from_generics(&generics, &comma);
+
+    let expanded = quote! {
+        impl #generics WitnessVarLengthEncodable<F> for #ident<#type_params_of_allocated_struct> #bound {
+            fn witness_encoding_length(witness: &Self::Witness) -> usize {
+                let mut total_len = 0;
+                #witness_length_impls
+
+                total_len
+            }
+            fn encode_witness_to_buffer(witness: &Self::Witness, dst: &mut Vec<F>) {
+                #witness_to_buffer_impls
+            }
+        }
+    };
+
+    proc_macro::TokenStream::from(expanded)
+}
diff --git a/src/gadgets/boolean/mod.rs b/src/gadgets/boolean/mod.rs
index ed95ddb..855c779 100644
--- a/src/gadgets/boolean/mod.rs
+++ b/src/gadgets/boolean/mod.rs
@@ -702,7 +702,7 @@ impl<F: SmallField> Boolean<F> {
     }
 }
 
-use crate::gadgets::traits::encodable::CircuitVarLengthEncodable;
+use crate::gadgets::traits::encodable::{CircuitVarLengthEncodable, WitnessVarLengthEncodable};
 
 impl<F: SmallField> CircuitVarLengthEncodable<F> for Boolean<F> {
     #[inline(always)]
@@ -712,6 +712,9 @@ impl<F: SmallField> CircuitVarLengthEncodable<F> for Boolean<F> {
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, _cs: &mut CS, dst: &mut Vec<Variable>) {
         dst.push(self.variable);
     }
+}
+
+impl<F: SmallField> WitnessVarLengthEncodable<F> for Boolean<F> {
     fn witness_encoding_length(_witness: &Self::Witness) -> usize {
         1
     }
diff --git a/src/gadgets/num/mod.rs b/src/gadgets/num/mod.rs
index ce323ed..7967061 100644
--- a/src/gadgets/num/mod.rs
+++ b/src/gadgets/num/mod.rs
@@ -1214,7 +1214,7 @@ pub fn dot_product_using_dot_product_gate<F: SmallField, CS: ConstraintSystem<F>
     result
 }
 
-use crate::gadgets::traits::encodable::CircuitVarLengthEncodable;
+use crate::gadgets::traits::encodable::{CircuitVarLengthEncodable, WitnessVarLengthEncodable};
 
 impl<F: SmallField> CircuitVarLengthEncodable<F> for Num<F> {
     #[inline(always)]
@@ -1224,6 +1224,9 @@ impl<F: SmallField> CircuitVarLengthEncodable<F> for Num<F> {
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, _cs: &mut CS, dst: &mut Vec<Variable>) {
         dst.push(self.get_variable());
     }
+}
+
+impl<F: SmallField> WitnessVarLengthEncodable<F> for Num<F> {
     #[inline(always)]
     fn witness_encoding_length(_witness: &Self::Witness) -> usize {
         1
diff --git a/src/gadgets/queue/mod.rs b/src/gadgets/queue/mod.rs
index 20f3221..30234bf 100644
--- a/src/gadgets/queue/mod.rs
+++ b/src/gadgets/queue/mod.rs
@@ -582,14 +582,14 @@ pub fn simulate_new_tail<
 use crate::gadgets::traits::encodable::CircuitVarLengthEncodable;
 use crate::serde_utils::BigArraySerde;
 
-#[derive(Derivative, CSAllocatable, CSSelectable, CSVarLengthEncodable, WitnessHookable)]
+#[derive(Derivative, CSAllocatable, CSSelectable, CSVarLengthEncodable, WitVarLengthEncodable, WitnessHookable)]
 #[derivative(Clone, Copy, Debug)]
 pub struct QueueState<F: SmallField, const N: usize> {
     pub head: [Num<F>; N],
     pub tail: QueueTailState<F, N>,
 }
 
-#[derive(Derivative, CSAllocatable, CSSelectable, CSVarLengthEncodable, WitnessHookable)]
+#[derive(Derivative, CSAllocatable, CSSelectable, CSVarLengthEncodable, WitVarLengthEncodable, WitnessHookable)]
 #[derivative(Clone, Copy, Debug)]
 pub struct QueueTailState<F: SmallField, const N: usize> {
     pub tail: [Num<F>; N],
diff --git a/src/gadgets/queue/queue_optimizer/sponge_optimizer.rs b/src/gadgets/queue/queue_optimizer/sponge_optimizer.rs
index 2ccb759..0efb1c3 100644
--- a/src/gadgets/queue/queue_optimizer/sponge_optimizer.rs
+++ b/src/gadgets/queue/queue_optimizer/sponge_optimizer.rs
@@ -1,6 +1,6 @@
 use super::*;
 
-#[derive(Derivative, CSAllocatable, CSSelectable, CSVarLengthEncodable, WitnessHookable)]
+#[derive(Derivative, CSAllocatable, CSSelectable, CSVarLengthEncodable, WitVarLengthEncodable, WitnessHookable)]
 #[derivative(Clone, Debug, Copy)]
 pub struct SpongeRoundRequest<F: SmallField, const SWIDTH: usize> {
     pub initial_state: [Num<F>; SWIDTH],
diff --git a/src/gadgets/recursion/allocated_vk.rs b/src/gadgets/recursion/allocated_vk.rs
index d8b8f97..fdf75c9 100644
--- a/src/gadgets/recursion/allocated_vk.rs
+++ b/src/gadgets/recursion/allocated_vk.rs
@@ -101,7 +101,7 @@ where
     }
 }
 
-use crate::gadgets::traits::encodable::CircuitVarLengthEncodable;
+use crate::gadgets::traits::encodable::{CircuitVarLengthEncodable, WitnessVarLengthEncodable};
 
 impl<F: SmallField, H: RecursiveTreeHasher<F, Num<F>>> CircuitVarLengthEncodable<F>
     for AllocatedVerificationKey<F, H>
@@ -123,7 +123,11 @@ impl<F: SmallField, H: RecursiveTreeHasher<F, Num<F>>> CircuitVarLengthEncodable
             el.encode_to_buffer(cs, dst);
         }
     }
+}
 
+impl<F: SmallField, H: RecursiveTreeHasher<F, Num<F>>> WitnessVarLengthEncodable<F>
+    for AllocatedVerificationKey<F, H>
+{
     fn witness_encoding_length(witness: &Self::Witness) -> usize {
         let cap_size = witness.setup_merkle_tree_cap.len();
         assert!(cap_size > 0);
@@ -138,3 +142,4 @@ impl<F: SmallField, H: RecursiveTreeHasher<F, Num<F>>> CircuitVarLengthEncodable
         }
     }
 }
+
diff --git a/src/gadgets/recursion/recursive_tree_hasher.rs b/src/gadgets/recursion/recursive_tree_hasher.rs
index 4a04fa2..60737a6 100644
--- a/src/gadgets/recursion/recursive_tree_hasher.rs
+++ b/src/gadgets/recursion/recursive_tree_hasher.rs
@@ -25,7 +25,8 @@ pub trait CircuitTreeHasher<F: SmallField, B: Sized + CSAllocatable<F>>:
         + Eq
         + std::fmt::Debug
         + CSAllocatable<F>
-        + CircuitVarLengthEncodable<F>;
+        + CircuitVarLengthEncodable<F>
+        + WitnessVarLengthEncodable<F>;
 
     fn new<CS: ConstraintSystem<F>>(cs: &mut CS) -> Self;
     fn placeholder_output<CS: ConstraintSystem<F>>(cs: &mut CS) -> Self::CircuitOutput;
@@ -87,6 +88,8 @@ pub trait RecursiveTreeHasher<F: SmallField, B: Sized + CSAllocatable<F>>:
 
 use crate::gadgets::round_function::CircuitSimpleAlgebraicSponge;
 
+use super::traits::encodable::WitnessVarLengthEncodable;
+
 impl<
         F: SmallField,
         const AW: usize,
diff --git a/src/gadgets/traits/encodable.rs b/src/gadgets/traits/encodable.rs
index 36b13af..818d3df 100644
--- a/src/gadgets/traits/encodable.rs
+++ b/src/gadgets/traits/encodable.rs
@@ -7,6 +7,11 @@ pub trait CircuitEncodable<F: SmallField, const N: usize>:
     'static + Send + Sync + CSAllocatable<F>
 {
     fn encode<CS: ConstraintSystem<F>>(&self, cs: &mut CS) -> [Variable; N];
+}
+
+pub trait WitnessEncodable<F: SmallField, const N: usize>:
+    'static + Send + Sync + CSAllocatable<F>
+{
     fn encode_witness(witness: &Self::Witness, dst: &mut Vec<F>);
 }
 
@@ -20,7 +25,11 @@ pub trait CircuitVarLengthEncodable<F: SmallField>:
 {
     fn encoding_length(&self) -> usize;
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, cs: &mut CS, dst: &mut Vec<Variable>);
+}
 
+pub trait WitnessVarLengthEncodable<F: SmallField>:
+    'static + Send + Sync + CSAllocatable<F>
+{
     fn witness_encoding_length(witness: &Self::Witness) -> usize;
     fn encode_witness_to_buffer(witness: &Self::Witness, dst: &mut Vec<F>);
 }
@@ -52,7 +61,11 @@ impl<F: SmallField, const N: usize, T: CircuitVarLengthEncodable<F>> CircuitVarL
             el.encode_to_buffer(cs, dst);
         }
     }
+}
 
+impl<F: SmallField, const N: usize, T: WitnessVarLengthEncodable<F>> WitnessVarLengthEncodable<F>
+    for [T; N]
+{
     fn witness_encoding_length(witness: &Self::Witness) -> usize {
         debug_assert!(N > 0);
 
@@ -74,6 +87,9 @@ impl<F: SmallField> CircuitVarLengthEncodable<F> for () {
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, _cs: &mut CS, _dst: &mut Vec<Variable>) {
         // do nothing
     }
+}
+
+impl<F: SmallField> WitnessVarLengthEncodable<F> for () {
     #[inline(always)]
     fn witness_encoding_length(_witness: &Self::Witness) -> usize {
         0
diff --git a/src/gadgets/u16/mod.rs b/src/gadgets/u16/mod.rs
index 083240f..2041552 100644
--- a/src/gadgets/u16/mod.rs
+++ b/src/gadgets/u16/mod.rs
@@ -559,7 +559,7 @@ impl<F: SmallField> UInt16<F> {
     }
 }
 
-use crate::gadgets::traits::encodable::CircuitVarLengthEncodable;
+use crate::gadgets::traits::encodable::{CircuitVarLengthEncodable, WitnessVarLengthEncodable};
 
 impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt16<F> {
     #[inline(always)]
@@ -569,6 +569,9 @@ impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt16<F> {
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, _cs: &mut CS, dst: &mut Vec<Variable>) {
         dst.push(self.variable);
     }
+}
+
+impl<F: SmallField> WitnessVarLengthEncodable<F> for UInt16<F> {
     #[inline(always)]
     fn witness_encoding_length(_witness: &Self::Witness) -> usize {
         1
diff --git a/src/gadgets/u160/mod.rs b/src/gadgets/u160/mod.rs
index c90cae0..da6965c 100644
--- a/src/gadgets/u160/mod.rs
+++ b/src/gadgets/u160/mod.rs
@@ -245,7 +245,7 @@ use crate::gadgets::traits::selectable::MultiSelectable;
 // so we degrade to default impl via normal select
 impl<F: SmallField> MultiSelectable<F> for UInt160<F> {}
 
-use crate::gadgets::traits::encodable::CircuitVarLengthEncodable;
+use crate::gadgets::traits::encodable::{CircuitVarLengthEncodable, WitnessVarLengthEncodable};
 
 impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt160<F> {
     #[inline(always)]
@@ -255,6 +255,9 @@ impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt160<F> {
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, cs: &mut CS, dst: &mut Vec<Variable>) {
         CircuitVarLengthEncodable::<F>::encode_to_buffer(&self.inner, cs, dst);
     }
+}
+
+impl<F: SmallField> WitnessVarLengthEncodable<F> for UInt160<F> {
     #[inline(always)]
     fn witness_encoding_length(_witness: &Self::Witness) -> usize {
         5
diff --git a/src/gadgets/u256/mod.rs b/src/gadgets/u256/mod.rs
index 420e816..9006642 100644
--- a/src/gadgets/u256/mod.rs
+++ b/src/gadgets/u256/mod.rs
@@ -403,7 +403,7 @@ use crate::gadgets::traits::selectable::MultiSelectable;
 // so we degrade to default impl via normal select
 impl<F: SmallField> MultiSelectable<F> for UInt256<F> {}
 
-use crate::gadgets::traits::encodable::CircuitVarLengthEncodable;
+use crate::gadgets::traits::encodable::{CircuitVarLengthEncodable, WitnessVarLengthEncodable};
 
 impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt256<F> {
     #[inline(always)]
@@ -413,6 +413,9 @@ impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt256<F> {
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, cs: &mut CS, dst: &mut Vec<Variable>) {
         CircuitVarLengthEncodable::<F>::encode_to_buffer(&self.inner, cs, dst);
     }
+}
+
+impl<F: SmallField> WitnessVarLengthEncodable<F> for UInt256<F> {
     #[inline(always)]
     fn witness_encoding_length(_witness: &Self::Witness) -> usize {
         8
diff --git a/src/gadgets/u32/mod.rs b/src/gadgets/u32/mod.rs
index d9807ba..18b2980 100644
--- a/src/gadgets/u32/mod.rs
+++ b/src/gadgets/u32/mod.rs
@@ -876,7 +876,7 @@ impl<F: SmallField> UInt32<F> {
     }
 }
 
-use crate::gadgets::traits::encodable::CircuitVarLengthEncodable;
+use crate::gadgets::traits::encodable::{CircuitVarLengthEncodable, WitnessVarLengthEncodable};
 
 impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt32<F> {
     #[inline(always)]
@@ -886,6 +886,9 @@ impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt32<F> {
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, _cs: &mut CS, dst: &mut Vec<Variable>) {
         dst.push(self.variable);
     }
+}
+
+impl<F: SmallField> WitnessVarLengthEncodable<F> for UInt32<F> {
     #[inline(always)]
     fn witness_encoding_length(_witness: &Self::Witness) -> usize {
         1
diff --git a/src/gadgets/u512/mod.rs b/src/gadgets/u512/mod.rs
index 4d05311..50ec3df 100644
--- a/src/gadgets/u512/mod.rs
+++ b/src/gadgets/u512/mod.rs
@@ -376,7 +376,7 @@ use crate::gadgets::traits::selectable::MultiSelectable;
 // so we degrade to default impl via normal select
 impl<F: SmallField> MultiSelectable<F> for UInt512<F> {}
 
-use crate::gadgets::traits::encodable::CircuitVarLengthEncodable;
+use crate::gadgets::traits::encodable::{CircuitVarLengthEncodable, WitnessVarLengthEncodable};
 
 impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt512<F> {
     #[inline(always)]
@@ -386,6 +386,9 @@ impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt512<F> {
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, cs: &mut CS, dst: &mut Vec<Variable>) {
         CircuitVarLengthEncodable::<F>::encode_to_buffer(&self.inner, cs, dst);
     }
+}
+
+impl<F: SmallField> WitnessVarLengthEncodable<F> for UInt512<F> {
     #[inline(always)]
     fn witness_encoding_length(_witness: &Self::Witness) -> usize {
         16
diff --git a/src/gadgets/u8/mod.rs b/src/gadgets/u8/mod.rs
index 486df33..12f39db 100644
--- a/src/gadgets/u8/mod.rs
+++ b/src/gadgets/u8/mod.rs
@@ -1,3 +1,5 @@
+use traits::encodable::WitnessVarLengthEncodable;
+
 use super::tables::ch4::Ch4Table;
 use super::tables::trixor4::TriXor4Table;
 use super::tables::xor8::Xor8Table;
@@ -577,6 +579,9 @@ impl<F: SmallField> CircuitVarLengthEncodable<F> for UInt8<F> {
     fn encode_to_buffer<CS: ConstraintSystem<F>>(&self, _cs: &mut CS, dst: &mut Vec<Variable>) {
         dst.push(self.variable);
     }
+}
+
+impl<F: SmallField> WitnessVarLengthEncodable<F> for UInt8<F> {
     #[inline(always)]
     fn witness_encoding_length(_witness: &Self::Witness) -> usize {
         1