Merge pull request #27 from Chainscore/feat/zk-row-blinding

Add ZK-row random blinding to ring proof witness columns

Author: prasad-kumkar

dot_ring/ring_proof/columns/columns.py

@@ -2,10 +2,11 @@
 
 import json
 import os
+import secrets
 from dataclasses import dataclass
 from typing import cast
 
-from dot_ring.ring_proof.constants import DEFAULT_SIZE, MAX_RING_SIZE, OMEGAS, S_PRIME, SeedPoint
+from dot_ring.ring_proof.constants import DEFAULT_SIZE, MAX_RING_SIZE, OMEGAS, S_PRIME, ZK_ROWS, SeedPoint
 from dot_ring.ring_proof.curve.bandersnatch import TwistedEdwardCurve as TE
 from dot_ring.ring_proof.helpers import Helpers as H
 from dot_ring.ring_proof.params import RingProofParams
@@ -27,12 +28,30 @@ class Column:
     commitment: G1Point | None = None
     size: int = DEFAULT_SIZE
 
-    def interpolate(self, domain_omega: int = OMEGAS[DEFAULT_SIZE], prime: int = S_PRIME) -> None:
-        """Fill `self.coeffs` from `self.evals` using FFT interpolation."""
+    def interpolate(
+        self,
+        domain_omega: int = OMEGAS[DEFAULT_SIZE],
+        prime: int = S_PRIME,
+        hidden: bool = False,
+        test_vectors: bool = False,
+    ) -> None:
+        """Fill `self.coeffs` from `self.evals` using FFT interpolation.
+
+        When ``hidden=True`` and ``test_vectors=False``, the last
+        ``ZK_ROWS`` positions are filled with cryptographically random
+        field elements (random blinding) to preserve zero-knowledge.
+        """
         if self.coeffs is None:
-            if len(self.evals) > self.size:
-                raise ValueError(f"{self.name} evals length {len(self.evals)} exceeds column size {self.size}")
-            self.evals += [0] * (self.size - len(self.evals))
+            if hidden and not test_vectors:
+                capacity = self.size - ZK_ROWS
+                if len(self.evals) > capacity:
+                    raise ValueError(f"{self.name} evals length {len(self.evals)} exceeds capacity {capacity} (size={self.size}, ZK_ROWS={ZK_ROWS})")
+                self.evals += [0] * (capacity - len(self.evals))
+                self.evals += [secrets.randbelow(prime) for _ in range(ZK_ROWS)]
+            else:
+                if len(self.evals) > self.size:
+                    raise ValueError(f"{self.name} evals length {len(self.evals)} exceeds column size {self.size}")
+                self.evals += [0] * (self.size - len(self.evals))
             self.coeffs = poly_interpolate_fft(self.evals, domain_omega, prime)
 
     def commit(self) -> None:
@@ -53,6 +72,7 @@ class WitnessColumnBuilder:
     prime: int = S_PRIME
     max_ring_size: int = MAX_RING_SIZE
     padding_rows: int = 4
+    test_vectors: bool = False
 
     @classmethod
     def from_params(
@@ -73,6 +93,7 @@ def from_params(
             prime=params.prime,
             max_ring_size=params.max_ring_size,
             padding_rows=params.padding_rows,
+            test_vectors=params.test_vectors,
         )
 
     def _bits_vector(self) -> list[int]:
@@ -120,7 +141,7 @@ def build(self) -> tuple[Column, Column, Column, Column]:
             Column("accip", acc_ip, size=self.size),
         ]
         for col in columns:
-            col.interpolate(self.omega, self.prime)
+            col.interpolate(self.omega, self.prime, hidden=True, test_vectors=self.test_vectors)
             col.commit()
         return (columns[0], columns[1], columns[2], columns[3])
 

dot_ring/ring_proof/constants.py

@@ -62,6 +62,8 @@
 
 MAX_RING_SIZE: int = 255  # Upper bound enforced by the constraint system
 
+ZK_ROWS: int = 3  # Number of random blinding rows for zero-knowledge (matches Rust ZK_ROWS)
+
 
 __all__ = [
     "S_PRIME",
@@ -78,4 +80,5 @@
     "D_512",
     "D_2048",
     "MAX_RING_SIZE",
+    "ZK_ROWS",
 ]

dot_ring/ring_proof/params.py

@@ -102,6 +102,7 @@ class RingProofParams:
     prime: int = S_PRIME
     base_root: int = OMEGA_2048
     base_root_size: int = 2048
+    test_vectors: bool = False
     cv: ClassVar[CurveVariant] = Bandersnatch
 
     def __post_init__(self) -> None:
@@ -166,6 +167,7 @@ def from_ring_size(
         prime: int = S_PRIME,
         base_root: int = OMEGA_2048,
         base_root_size: int = 2048,
+        test_vectors: bool = False,
     ) -> RingProofParams:
         """
         Automatically construct RingProofParams based on ring size.
@@ -210,4 +212,5 @@ def from_ring_size(
             prime=prime,
             base_root=base_root,
             base_root_size=base_root_size,
+            test_vectors=test_vectors,
         )

tests/test_bandersnatch_ark.py

@@ -121,7 +121,7 @@ def test_ring_proof():
         ad = bytes.fromhex(item["ad"])
         keys = RingVRF[Bandersnatch].parse_keys(bytes.fromhex(item["ring_pks"]))
         start = time()
-        params = RingProofParams()
+        params = RingProofParams(test_vectors=True)
         ring = Ring(keys, params)
         ring_root = RingRoot.from_ring(ring, params)
         ring_time = time()

tests/test_ring_vrf/test_ring_vrf.py

@@ -24,7 +24,7 @@ def test_ring_proof():
         keys = RingVRF[Bandersnatch].parse_keys(bytes.fromhex(item["ring_pks"]))
 
         start_time = time.time()
-        params = RingProofParams()
+        params = RingProofParams(test_vectors=True)
         ring = Ring(keys, params)
         ring_root = RingRoot.from_ring(ring, params)
         ring_time = time.time()

tests/test_vectors.py

@@ -271,8 +271,8 @@ def verify_ring_vector(vector: dict[str, Any], curve) -> None:
         for i in range(0, len(ring_pks_bytes), point_len):
             ring_pks.append(ring_pks_bytes[i : i + point_len])
 
-        # Construct ring and ring root
-        params = RingProofParams()
+        # Construct ring and ring root (test_vectors=True for deterministic proofs)
+        params = RingProofParams(test_vectors=True)
         ring = Ring(ring_pks, params)
         ring_root = RingRoot.from_ring(ring, params)
 
@@ -417,8 +417,8 @@ def test_wrong_ring_root(self):
         alpha = b"test_input"
         ad = b"test_ad"
 
-        # Construct rings and ring roots
-        params = RingProofParams()
+        # Construct rings and ring roots (test_vectors=True for deterministic proofs)
+        params = RingProofParams(test_vectors=True)
         ring_obj1 = Ring(ring1, params)
         ring_root1 = RingRoot.from_ring(ring_obj1, params)
         ring_obj2 = Ring(ring2, params)
@@ -470,3 +470,35 @@ def test_pedersen_deterministic(self):
         assert proof1.ok.point_to_string() == proof2.ok.point_to_string()
         assert proof1.s == proof2.s
         assert proof1.sb == proof2.sb
+
+    def test_ring_nondeterministic(self):
+        """Ring VRF proofs with default params (test_vectors=False) should be non-deterministic.
+
+        Two proofs from the same inputs should differ due to random ZK-row blinding,
+        but both must still verify correctly.
+        """
+        sk = bytes.fromhex("0101010101010101010101010101010101010101010101010101010101010101")
+        pk = RingVRF[Bandersnatch].get_public_key(sk)
+
+        ring_keys = [pk]
+        for i in range(7):
+            other_sk = (i + 2).to_bytes(32, "little")
+            ring_keys.append(RingVRF[Bandersnatch].get_public_key(other_sk))
+
+        alpha = b"deterministic_test"
+        ad = b"test_ad"
+
+        # Default params: test_vectors=False → random ZK-row blinding
+        params = RingProofParams(test_vectors=False)
+        ring = Ring(ring_keys, params)
+        ring_root = RingRoot.from_ring(ring, params)
+
+        proof1 = RingVRF[Bandersnatch].prove(alpha, ad, sk, pk, ring, ring_root)
+        proof2 = RingVRF[Bandersnatch].prove(alpha, ad, sk, pk, ring, ring_root)
+
+        # Proof bytes should differ due to random blinding
+        assert proof1.to_bytes() != proof2.to_bytes(), "Ring proofs should be non-deterministic with random ZK-row blinding"
+
+        # Both proofs must still verify
+        assert proof1.verify(alpha, ad, ring, ring_root), "First proof verification failed"
+        assert proof2.verify(alpha, ad, ring, ring_root), "Second proof verification failed"