xmss: add simpler validation

src/lean_spec/subspecs/xmss/_validation.py

@@ -0,0 +1,29 @@
+"""Internal validation utilities for the XMSS scheme."""
+
+from __future__ import annotations
+
+from typing import Any
+
+
+def enforce_strict_types(instance: Any, **field_types: type) -> None:
+    """
+    Validate that specified fields are exact types, not subclasses.
+
+    This is a helper function to be called from Pydantic model validators.
+
+    It enforces that field values are exactly the declared type, preventing
+    type confusion attacks where a malicious subclass could override behavior.
+
+    Args:
+        instance: The model instance being validated.
+        **field_types: Mapping of field names to their exact expected types.
+
+    Raises:
+        TypeError: If any field is a subclass rather than the exact type.
+    """
+    for field_name, expected_type in field_types.items():
+        value = getattr(instance, field_name)
+        if type(value) is not expected_type:
+            raise TypeError(
+                f"{field_name} must be exactly {expected_type.__name__}, not a subclass"
+            )

src/lean_spec/subspecs/xmss/interface.py

@@ -18,6 +18,7 @@
 )
 from lean_spec.types import StrictBaseModel, Uint64
 
+from ._validation import enforce_strict_types
 from .constants import (
     PROD_CONFIG,
     TEST_CONFIG,
@@ -60,18 +61,16 @@ class GeneralizedXmssScheme(StrictBaseModel):
     """Random data generator for key generation."""
 
     @model_validator(mode="after")
-    def enforce_strict_types(self) -> "GeneralizedXmssScheme":
+    def _validate_strict_types(self) -> "GeneralizedXmssScheme":
         """Reject subclasses to prevent type confusion attacks."""
-        if type(self.config) is not XmssConfig:
-            raise TypeError("config must be exactly XmssConfig, not a subclass")
-        if type(self.prf) is not Prf:
-            raise TypeError("prf must be exactly Prf, not a subclass")
-        if type(self.hasher) is not TweakHasher:
-            raise TypeError("hasher must be exactly TweakHasher, not a subclass")
-        if type(self.encoder) is not TargetSumEncoder:
-            raise TypeError("encoder must be exactly TargetSumEncoder, not a subclass")
-        if type(self.rand) is not Rand:
-            raise TypeError("rand must be exactly Rand, not a subclass")
+        enforce_strict_types(
+            self,
+            config=XmssConfig,
+            prf=Prf,
+            hasher=TweakHasher,
+            encoder=TargetSumEncoder,
+            rand=Rand,
+        )
         return self
 
     def key_gen(

src/lean_spec/subspecs/xmss/message_hash.py

@@ -39,6 +39,7 @@
 from lean_spec.types import StrictBaseModel, Uint64
 
 from ..koalabear import Fp, P
+from ._validation import enforce_strict_types
 from .constants import (
     PROD_CONFIG,
     TEST_CONFIG,
@@ -64,12 +65,9 @@ class MessageHasher(StrictBaseModel):
     """Poseidon hash engine."""
 
     @model_validator(mode="after")
-    def enforce_strict_types(self) -> "MessageHasher":
+    def _validate_strict_types(self) -> "MessageHasher":
         """Reject subclasses to prevent type confusion attacks."""
-        if type(self.config) is not XmssConfig:
-            raise TypeError("config must be exactly XmssConfig, not a subclass")
-        if type(self.poseidon) is not PoseidonXmss:
-            raise TypeError("poseidon must be exactly PoseidonXmss, not a subclass")
+        enforce_strict_types(self, config=XmssConfig, poseidon=PoseidonXmss)
         return self
 
     def encode_message(self, message: bytes) -> list[Fp]:

src/lean_spec/subspecs/xmss/poseidon.py

@@ -33,6 +33,7 @@
     Poseidon2Params,
     permute,
 )
+from ._validation import enforce_strict_types
 from .utils import int_to_base_p
 
 
@@ -46,12 +47,9 @@ class PoseidonXmss(StrictBaseModel):
     """Poseidon2 parameters for 24-width permutation."""
 
     @model_validator(mode="after")
-    def enforce_strict_types(self) -> "PoseidonXmss":
+    def _validate_strict_types(self) -> "PoseidonXmss":
         """Reject subclasses to prevent type confusion attacks."""
-        if type(self.params16) is not Poseidon2Params:
-            raise TypeError("params16 must be exactly Poseidon2Params, not a subclass")
-        if type(self.params24) is not Poseidon2Params:
-            raise TypeError("params24 must be exactly Poseidon2Params, not a subclass")
+        enforce_strict_types(self, params16=Poseidon2Params, params24=Poseidon2Params)
         return self
 
     def compress(self, input_vec: list[Fp], width: int, output_len: int) -> list[Fp]:

src/lean_spec/subspecs/xmss/prf.py

@@ -17,6 +17,7 @@
 from lean_spec.subspecs.koalabear import Fp
 from lean_spec.types import StrictBaseModel, Uint64
 
+from ._validation import enforce_strict_types
 from .constants import (
     PRF_KEY_LENGTH,
     PROD_CONFIG,
@@ -109,10 +110,9 @@ class Prf(StrictBaseModel):
     """Configuration parameters for the PRF."""
 
     @model_validator(mode="after")
-    def enforce_strict_types(self) -> "Prf":
+    def _validate_strict_types(self) -> "Prf":
         """Reject subclasses to prevent type confusion attacks."""
-        if type(self.config) is not XmssConfig:
-            raise TypeError("config must be exactly XmssConfig, not a subclass")
+        enforce_strict_types(self, config=XmssConfig)
         return self
 
     def key_gen(self) -> PRFKey:

src/lean_spec/subspecs/xmss/rand.py

@@ -7,6 +7,7 @@
 from lean_spec.types import StrictBaseModel
 
 from ..koalabear import Fp, P
+from ._validation import enforce_strict_types
 from .constants import PROD_CONFIG, TEST_CONFIG, XmssConfig
 from .types import HashDigestVector, Parameter, Randomness
 
@@ -18,10 +19,9 @@ class Rand(StrictBaseModel):
     """Configuration parameters for the random generator."""
 
     @model_validator(mode="after")
-    def enforce_strict_types(self) -> "Rand":
+    def _validate_strict_types(self) -> "Rand":
         """Reject subclasses to prevent type confusion attacks."""
-        if type(self.config) is not XmssConfig:
-            raise TypeError("config must be exactly XmssConfig, not a subclass")
+        enforce_strict_types(self, config=XmssConfig)
         return self
 
     def field_elements(self, length: int) -> list[Fp]:

src/lean_spec/subspecs/xmss/target_sum.py

@@ -10,6 +10,7 @@
 
 from lean_spec.types import StrictBaseModel, Uint64
 
+from ._validation import enforce_strict_types
 from .constants import PROD_CONFIG, TEST_CONFIG, XmssConfig
 from .message_hash import (
     PROD_MESSAGE_HASHER,
@@ -34,12 +35,9 @@ class TargetSumEncoder(StrictBaseModel):
     """Message hasher for encoding."""
 
     @model_validator(mode="after")
-    def enforce_strict_types(self) -> "TargetSumEncoder":
+    def _validate_strict_types(self) -> "TargetSumEncoder":
         """Reject subclasses to prevent type confusion attacks."""
-        if type(self.config) is not XmssConfig:
-            raise TypeError("config must be exactly XmssConfig, not a subclass")
-        if type(self.message_hasher) is not MessageHasher:
-            raise TypeError("message_hasher must be exactly MessageHasher, not a subclass")
+        enforce_strict_types(self, config=XmssConfig, message_hasher=MessageHasher)
         return self
 
     def encode(

src/lean_spec/subspecs/xmss/tweak_hash.py

@@ -31,6 +31,7 @@
 from lean_spec.types import StrictBaseModel, Uint64
 
 from ..koalabear import Fp
+from ._validation import enforce_strict_types
 from .constants import (
     PROD_CONFIG,
     TEST_CONFIG,
@@ -86,12 +87,9 @@ class TweakHasher(StrictBaseModel):
     """Poseidon permutation instance for hashing."""
 
     @model_validator(mode="after")
-    def enforce_strict_types(self) -> "TweakHasher":
+    def _validate_strict_types(self) -> "TweakHasher":
         """Reject subclasses to prevent type confusion attacks."""
-        if type(self.config) is not XmssConfig:
-            raise TypeError("config must be exactly XmssConfig, not a subclass")
-        if type(self.poseidon) is not PoseidonXmss:
-            raise TypeError("poseidon must be exactly PoseidonXmss, not a subclass")
+        enforce_strict_types(self, config=XmssConfig, poseidon=PoseidonXmss)
         return self
 
     def _encode_tweak(self, tweak: TreeTweak | ChainTweak, length: int) -> list[Fp]:
@@ -123,17 +121,18 @@ def _encode_tweak(self, tweak: TreeTweak | ChainTweak, length: int) -> list[Fp]:
         # Pack the tweak's integer fields into a single large integer.
         #
         # A hardcoded prefix is included for domain separation between tweak types.
-        if isinstance(tweak, TreeTweak):
-            # Packing scheme: (level << 40) | (index << 8) | PREFIX
-            acc = (tweak.level << 40) | (int(tweak.index) << 8) | TWEAK_PREFIX_TREE.value
-        else:
-            # Packing scheme: (epoch << 24) | (chain_index << 16) | (step << 8) | PREFIX
-            acc = (
-                (int(tweak.epoch) << 24)
-                | (tweak.chain_index << 16)
-                | (tweak.step << 8)
-                | TWEAK_PREFIX_CHAIN.value
-            )
+        match tweak:
+            case TreeTweak(level=level, index=index):
+                # Packing scheme: (level << 40) | (index << 8) | PREFIX
+                acc = (level << 40) | (int(index) << 8) | TWEAK_PREFIX_TREE.value
+            case ChainTweak(epoch=epoch, chain_index=chain_index, step=step):
+                # Packing scheme: (epoch << 24) | (chain_index << 16) | (step << 8) | PREFIX
+                acc = (
+                    (int(epoch) << 24)
+                    | (chain_index << 16)
+                    | (step << 8)
+                    | TWEAK_PREFIX_CHAIN.value
+                )
 
         # Decompose the packed integer `acc` into a list of base-P field elements.
         return int_to_base_p(acc, length)