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feat: SumMerkleDamgardDynamicLength #1634

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5e3c670
feat: `SumMerkleDamgardDynamicLength` with tests
Tabaie Oct 27, 2025
ead8711
Merge branch 'master' into feat/var-length-hash
Tabaie Oct 27, 2025
1ae9c91
docs: incorporate copilot suggestions
Tabaie Oct 27, 2025
976f197
Merge branch 'feat/var-length-hash' of https://github.com/Consensys/g…
Tabaie Oct 27, 2025
c9dd1c3
style: run goimports
Tabaie Oct 27, 2025
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15 changes: 15 additions & 0 deletions std/hash/hash.go
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ package hash

import (
"github.com/consensys/gnark/frontend"
"github.com/consensys/gnark/std/lookup/logderivlookup"
"github.com/consensys/gnark/std/math/uints"
)

Expand Down Expand Up @@ -136,3 +137,17 @@ func (h *merkleDamgardHasher) Write(data ...frontend.Variable) {
func (h *merkleDamgardHasher) Sum() frontend.Variable {
return h.state
}

// SumMerkleDamgardDynamicLength returns the hash of the data slice, truncated at the given length.
func SumMerkleDamgardDynamicLength(api frontend.API, f Compressor, initialState frontend.Variable, length frontend.Variable, data []frontend.Variable) frontend.Variable {
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@ivokub ivokub Oct 27, 2025

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Doesn't this function signature allow for length extension attacks? I.e. lets say:

  • IV = 0
  • f_0 = compress(IV, msg_0)
  • f_1 = compress(f_0, msg_1)

Then, when we do either:
SumMerkleDamgardDynamicLength(api, Compressor, IV, 2, [msg_0, msg_1])
or
SumMerkleDamgardDynamicLength(api, Compressor, f_0, 1, [msg_1, msg_xxx])
we would get the same result? And considering all are user inputs then it could imo lead to possible collisions.

We also provide variable-length mode for binary hashes (see interface BinaryFixedLengthHasher). But there depending on the underlying hash function (sha256, ripemd, sha3) the length is appended to the input, avoid collision problems.

resT := logderivlookup.New(api)
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@ivokub ivokub Oct 27, 2025

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And imo this method diverges from the rest of the APIs we use for hashers - otherwise we have defined FieldHasher for algebraic hash, and BinaryHasher/BinaryFixedLengthHasher for binary hashes (the ..FixedLengthHasher providing the same functionality i.e. allowing to dynamically set the input length to be hash).

I recommend following similar patter for consistency -- create FieldFixedLengthHasher interface

type FieldFixedLengthHasher interface {
    FieldHasher

    FixedLengthSum(length frontend.Variable) frontend.Variable
}

and then make in MiMC and Poseidon2 packages return this instead of FieldHasher.

state := initialState

resT.Insert(state)
for _, v := range data {
state = f.Compress(state, v)
resT.Insert(state)
}

return resT.Lookup(length)[0]
}
43 changes: 35 additions & 8 deletions std/hash/poseidon2/poseidon2_test.go
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@@ -1,41 +1,68 @@
package poseidon2

import (
"fmt"
"testing"

"github.com/consensys/gnark-crypto/ecc"
"github.com/consensys/gnark-crypto/ecc/bls12-377/fr/poseidon2"
"github.com/consensys/gnark-crypto/ecc/bls12-377/fr"
gcPoseidon2 "github.com/consensys/gnark-crypto/ecc/bls12-377/fr/poseidon2"
"github.com/consensys/gnark/frontend"
"github.com/consensys/gnark/std/hash"
"github.com/consensys/gnark/std/permutation/poseidon2"
"github.com/consensys/gnark/test"
)

type Poseidon2Circuit struct {
Input []frontend.Variable
Expected frontend.Variable `gnark:",public"`
Expected []frontend.Variable `gnark:",public"` // Expected[i] = H(Input[:i+1])
}

func (c *Poseidon2Circuit) Define(api frontend.API) error {
if len(c.Input) != len(c.Expected) {
return fmt.Errorf("length mismatch")
}
hsh, err := NewMerkleDamgardHasher(api)
if err != nil {
return err
}
hsh.Write(c.Input...)
api.AssertIsEqual(hsh.Sum(), c.Expected)

compressor, err := poseidon2.NewPoseidon2(api)
if err != nil {
return err
}

for i := range c.Input {
hsh.Write(c.Input[i])
api.AssertIsEqual(c.Expected[i], hsh.Sum())
api.AssertIsEqual(c.Expected[i], hash.SumMerkleDamgardDynamicLength(api, compressor, 0, i+1, c.Input))
}

return nil
}

func TestPoseidon2Hash(t *testing.T) {
assert := test.NewAssert(t)

var buf [fr.Bytes]byte
const nbInputs = 5
// prepare expected output
h := poseidon2.NewMerkleDamgardHasher()
h := gcPoseidon2.NewMerkleDamgardHasher()
expected := make([]frontend.Variable, nbInputs)
circInput := make([]frontend.Variable, nbInputs)
for i := range nbInputs {
_, err := h.Write([]byte{byte(i)})
buf[fr.Bytes-1] = byte(i)
_, err := h.Write(buf[:])
assert.NoError(err)
circInput[i] = i
expected[i] = h.Sum(nil)
}
res := h.Sum(nil)
assert.CheckCircuit(&Poseidon2Circuit{Input: make([]frontend.Variable, nbInputs)}, test.WithValidAssignment(&Poseidon2Circuit{Input: circInput, Expected: res}), test.WithCurves(ecc.BLS12_377)) // we have parametrized currently only for BLS12-377
assert.CheckCircuit(
&Poseidon2Circuit{
Input: make([]frontend.Variable, nbInputs),
Expected: make([]frontend.Variable, nbInputs),
}, test.WithValidAssignment(&Poseidon2Circuit{
Input: circInput,
Expected: expected,
}), test.WithCurves(ecc.BLS12_377)) // we have parametrized currently only for BLS12-377
}
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