chore: do not differentiate variable vs fixed length for Poseidon2 (#…

…11740)

Remove the terminator for poseidon2 that was used to distinguish between
fixed and variable size hashes.

barretenberg/cpp/src/barretenberg/crypto/poseidon2/poseidon2.cpp

@@ -7,7 +7,7 @@ namespace bb::crypto {
 template <typename Params>
 typename Poseidon2<Params>::FF Poseidon2<Params>::hash(const std::vector<typename Poseidon2<Params>::FF>& input)
 {
-    return Sponge::hash_fixed_length(input);
+    return Sponge::hash_internal(input);
 }
 
 /**

barretenberg/cpp/src/barretenberg/crypto/poseidon2/sponge/sponge.hpp

@@ -125,12 +125,10 @@ template <typename FF, size_t rate, size_t capacity, size_t t, typename Permutat
      * @brief Use the sponge to hash an input string
      *
      * @tparam out_len
-     * @tparam is_variable_length. Distinguishes between hashes where the preimage length is constant/not constant
      * @param input
      * @return std::array<FF, out_len>
      */
-    template <size_t out_len, bool is_variable_length>
-    static std::array<FF, out_len> hash_internal(std::span<const FF> input)
+    template <size_t out_len> static std::array<FF, out_len> hash_internal(std::span<const FF> input)
     {
         size_t in_len = input.size();
         const uint256_t iv = (static_cast<uint256_t>(in_len) << 64) + out_len - 1;
@@ -140,30 +138,13 @@ template <typename FF, size_t rate, size_t capacity, size_t t, typename Permutat
             sponge.absorb(input[i]);
         }
 
-        // In the case where the hash preimage is variable-length, we append `1` to the end of the input, to distinguish
-        // from fixed-length hashes. (the combination of this additional field element + the hash IV ensures
-        // fixed-length and variable-length hashes do not collide)
-        if constexpr (is_variable_length) {
-            sponge.absorb(1);
-        }
-
         std::array<FF, out_len> output;
         for (size_t i = 0; i < out_len; ++i) {
             output[i] = sponge.squeeze();
         }
         return output;
     }
 
-    template <size_t out_len> static std::array<FF, out_len> hash_fixed_length(std::span<const FF> input)
-    {
-        return hash_internal<out_len, false>(input);
-    }
-    static FF hash_fixed_length(std::span<const FF> input) { return hash_fixed_length<1>(input)[0]; }
-
-    template <size_t out_len> static std::array<FF, out_len> hash_variable_length(std::span<FF> input)
-    {
-        return hash_internal<out_len, true>(input);
-    }
-    static FF hash_variable_length(std::span<FF> input) { return hash_variable_length<1>(input)[0]; }
+    static FF hash_internal(std::span<const FF> input) { return hash_internal<1>(input)[0]; }
 };
 } // namespace bb::crypto

barretenberg/cpp/src/barretenberg/stdlib/hash/poseidon2/poseidon2.cpp

@@ -14,7 +14,7 @@ template <typename C> field_t<C> poseidon2<C>::hash(C& builder, const std::vecto
      * This should just call the sponge variable length hash function
      *
      */
-    return Sponge::hash_fixed_length(builder, inputs);
+    return Sponge::hash_internal(builder, inputs);
 }
 
 /**

barretenberg/cpp/src/barretenberg/stdlib/hash/poseidon2/sponge/sponge.hpp

@@ -134,7 +134,7 @@ template <size_t rate, size_t capacity, size_t t, typename Permutation, typename
      * @param input
      * @return std::array<field_t, out_len>
      */
-    template <size_t out_len, bool is_variable_length>
+    template <size_t out_len>
     static std::array<field_t, out_len> hash_internal(Builder& builder, std::span<const field_t> input)
     {
         size_t in_len = input.size();
@@ -145,38 +145,16 @@ template <size_t rate, size_t capacity, size_t t, typename Permutation, typename
             sponge.absorb(input[i]);
         }
 
-        // In the case where the hash preimage is variable-length, we append `1` to the end of the input, to distinguish
-        // from fixed-length hashes. (the combination of this additional field element + the hash IV ensures
-        // fixed-length and variable-length hashes do not collide)
-        if constexpr (is_variable_length) {
-            sponge.absorb(1);
-        }
-
         std::array<field_t, out_len> output;
         for (size_t i = 0; i < out_len; ++i) {
             output[i] = sponge.squeeze();
         }
         return output;
     }
 
-    template <size_t out_len>
-    static std::array<field_t, out_len> hash_fixed_length(Builder& builder, std::span<const field_t> input)
-    {
-        return hash_internal<out_len, false>(builder, input);
-    }
-    static field_t hash_fixed_length(Builder& builder, std::span<const field_t> input)
-    {
-        return hash_fixed_length<1>(builder, input)[0];
-    }
-
-    template <size_t out_len>
-    static std::array<field_t, out_len> hash_variable_length(Builder& builder, std::span<field_t> input)
-    {
-        return hash_internal<out_len, true>(builder, input);
-    }
-    static field_t hash_variable_length(Builder& builder, std::span<field_t> input)
+    static field_t hash_internal(Builder& builder, std::span<const field_t> input)
     {
-        return hash_variable_length<1>(builder, input)[0];
+        return hash_internal<1>(builder, input)[0];
     }
 };
 } // namespace bb::stdlib