Add secret_from_seed helper

README.md

@@ -54,6 +54,15 @@ alpha = b"input data"
 ad = b"additional data"
 ```
 
+Deterministic key generation from a seed (matching ark-vrf):
+
+```python
+from dot_ring import Bandersnatch, secret_from_seed
+
+seed = (0).to_bytes(32, "little")
+public_key, secret_scalar = secret_from_seed(seed, Bandersnatch)
+```
+
 ### IETF VRF
 
 ```python

dot_ring/__init__.py

@@ -48,6 +48,7 @@
 from dot_ring.curve.specs.p384 import P384_NU, P384_RO
 from dot_ring.curve.specs.p521 import P521_NU, P521_RO
 from dot_ring.curve.specs.secp256k1 import Secp256k1_NU, Secp256k1_RO
+from dot_ring.keygen import secret_from_seed
 from dot_ring.vrf.ietf.ietf import IETF_VRF
 from dot_ring.vrf.pedersen.pedersen import PedersenVRF
 from dot_ring.vrf.ring.ring_vrf import RingVRF
@@ -114,4 +115,5 @@
     "BLS12_381_G2",
     "BLS12_381_G2_RO",
     "BLS12_381_G2_NU",
+    "secret_from_seed",
 ]

dot_ring/keygen.py

@@ -0,0 +1,59 @@
+from __future__ import annotations
+
+from typing import Literal, cast
+
+from dot_ring.curve.curve import CurveVariant
+from dot_ring.curve.specs.bandersnatch import Bandersnatch
+from dot_ring.ring_proof.helpers import Helpers
+from dot_ring.vrf.ietf.ietf import IETF_VRF
+
+
+def _hash_seed(curve: CurveVariant, seed: bytes, counter: int) -> bytes:
+    hasher = curve.curve.H_A()
+    hasher.update(seed)
+    if counter:
+        hasher.update(bytes([counter]))
+    if curve.curve._uses_xof():
+        length = curve.curve._default_xof_len()
+        return cast(bytes, hasher.digest(length))
+    return cast(bytes, hasher.digest())
+
+
+def secret_from_seed(seed: bytes, curve: CurveVariant = Bandersnatch) -> tuple[bytes, bytes]:
+    """
+    Deterministically derive a secret scalar and public key from a seed.
+
+    Mirrors ark-vrf's Secret::from_seed:
+    - Hash seed with curve's hash function
+    - Interpret hash output as little-endian integer, reduce modulo curve order
+    - If zero, append a counter byte and rehash
+
+    Returns:
+        (public_key_bytes, secret_scalar_bytes)
+    """
+    if not isinstance(seed, (bytes, bytearray)):
+        raise TypeError("seed must be bytes")
+    if not isinstance(curve, CurveVariant):
+        raise TypeError("curve must be a CurveVariant")
+
+    seed_bytes = bytes(seed)
+    order = curve.curve.ORDER
+    scalar_len = (order.bit_length() + 7) // 8
+
+    counter = 0
+    while True:
+        digest = _hash_seed(curve, seed_bytes, counter)
+        sk_int = int.from_bytes(digest, "little") % order
+        if sk_int != 0:
+            break
+        counter = (counter + 1) & 0xFF
+        if counter == 0:
+            raise RuntimeError("failed to derive non-zero secret scalar")
+
+    sk_bytes = Helpers.int_to_str(
+        sk_int,
+        cast(Literal["little", "big"], curve.curve.ENDIAN),
+        scalar_len,
+    )
+    pk_bytes = IETF_VRF[curve].get_public_key(sk_bytes)
+    return pk_bytes, sk_bytes

tests/test_keygen.py

@@ -0,0 +1,38 @@
+import pytest
+
+from dot_ring import Bandersnatch, secret_from_seed
+from dot_ring.vrf.ietf.ietf import IETF_VRF
+
+
+@pytest.mark.parametrize(
+    "seed, expected_pk, expected_sk",
+    [
+        (
+            0,
+            "5e465beb01dbafe160ce8216047f2155dd0569f058afd52dcea601025a8d161d",
+            "51c1537c18eea5c5969cb2ae45c1224cc245de5c5b8e6e25f48fb99f2786ee05",
+        ),
+        (
+            100,
+            "caf7eb70d84e27511179c83ac352f8d3e9b9661371520c54c9ad56781f374a32",
+            "ad20931d3f8cee57206bc1c3e5dad50677afb9fb712217c6a980867d3a56451c",
+        ),
+    ],
+)
+def test_secret_from_seed_vectors(seed: int, expected_pk: str, expected_sk: str) -> None:
+    pk, sk = secret_from_seed(seed.to_bytes(32, "little"), Bandersnatch)
+    assert pk.hex() == expected_pk
+    assert sk.hex() == expected_sk
+
+
+def test_secret_from_seed_public_key_roundtrip() -> None:
+    seed = (2**32 - 1).to_bytes(32, "little")
+    pk, sk = secret_from_seed(seed, Bandersnatch)
+    assert pk == IETF_VRF[Bandersnatch].get_public_key(sk)
+
+
+def test_secret_from_seed_type_errors() -> None:
+    with pytest.raises(TypeError):
+        secret_from_seed("not-bytes")  # type: ignore[arg-type]
+    with pytest.raises(TypeError):
+        secret_from_seed(b"\x00" * 32, "not-a-curve")  # type: ignore[arg-type]