diff --git a/src/lean_spec/subspecs/xmss/__init__.py b/src/lean_spec/subspecs/xmss/__init__.py
new file mode 100644
index 00000000..9e381730
--- /dev/null
+++ b/src/lean_spec/subspecs/xmss/__init__.py
@@ -0,0 +1,22 @@
+"""
+This package provides a Python specification for the Generalized XMSS
+hash-based signature scheme.
+
+It exposes the core data structures and the main interface functions.
+"""
+
+from .constants import LIFETIME, MESSAGE_LENGTH
+from .interface import key_gen, sign, verify
+from .structures import HashTreeOpening, PublicKey, SecretKey, Signature
+
+__all__ = [
+    "key_gen",
+    "sign",
+    "verify",
+    "PublicKey",
+    "Signature",
+    "SecretKey",
+    "HashTreeOpening",
+    "LIFETIME",
+    "MESSAGE_LENGTH",
+]
diff --git a/src/lean_spec/subspecs/xmss/constants.py b/src/lean_spec/subspecs/xmss/constants.py
new file mode 100644
index 00000000..e2f14bb0
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+++ b/src/lean_spec/subspecs/xmss/constants.py
@@ -0,0 +1,101 @@
+"""
+Defines the cryptographic constants for the XMSS specification.
+
+This specification corresponds to the "hashing-optimized" Top Level Target Sum
+instantiation from the canonical Rust implementation.
+
+.. note::
+   This specification uses the **KoalaBear** prime field, which is consistent
+   with the formal analysis in the reference papers (e.g., Section 5 of the
+   "LeanSig" technical note: https://eprint.iacr.org/2025/1332).
+
+   The canonical Rust implementation currently uses the `BabyBear` field for
+   practical reasons but is expected to align with this
+   specification in the future.
+"""
+
+from ..koalabear import Fp
+
+# =================================================================
+# Core Scheme Configuration
+# =================================================================
+
+MESSAGE_LENGTH: int = 32
+"""The length in bytes for all messages to be signed."""
+
+LOG_LIFETIME: int = 32
+"""The base-2 logarithm of the scheme's maximum lifetime."""
+
+LIFETIME: int = 1 << LOG_LIFETIME
+"""
+The maximum number of epochs supported by this configuration.
+
+An individual key pair can be active for a smaller sub-range.
+"""
+
+
+# =================================================================
+# Target Sum WOTS Parameters
+# =================================================================
+
+DIMENSION: int = 64
+"""The total number of hash chains, `v`."""
+
+BASE: int = 8
+"""The alphabet size for the digits of the encoded message."""
+
+FINAL_LAYER: int = 77
+"""The number of top layers of the hypercube to map the hash output into."""
+
+TARGET_SUM: int = 375
+"""The required sum of all codeword chunks for a signature to be valid."""
+
+
+# =================================================================
+# Hash and Encoding Length Parameters (in field elements)
+# =================================================================
+
+PARAMETER_LEN: int = 5
+"""
+The length of the public parameter `P`.
+
+It is used to specialize the hash function.
+"""
+
+TWEAK_LEN_FE: int = 2
+"""The length of a domain-separating tweak."""
+
+MSG_LEN_FE: int = 9
+"""The length of a message after being encoded into field elements."""
+
+RAND_LEN_FE: int = 7
+"""The length of the randomness `rho` used during message encoding."""
+
+HASH_LEN_FE: int = 8
+"""The output length of the main tweakable hash function."""
+
+CAPACITY: int = 9
+"""The capacity of the Poseidon2 sponge, defining its security level."""
+
+POS_OUTPUT_LEN_PER_INV_FE: int = 15
+"""Output length per invocation for the message hash."""
+
+POS_INVOCATIONS: int = 1
+"""Number of invocations for the message hash."""
+
+POS_OUTPUT_LEN_FE: int = POS_OUTPUT_LEN_PER_INV_FE * POS_INVOCATIONS
+"""Total output length for the message hash."""
+
+
+# =================================================================
+# Domain Separator Prefixes for Tweaks
+# =================================================================
+
+TWEAK_PREFIX_CHAIN = Fp(value=0x00)
+"""The unique prefix for tweaks used in Winternitz-style hash chains."""
+
+TWEAK_PREFIX_TREE = Fp(value=0x01)
+"""The unique prefix for tweaks used when hashing Merkle tree nodes."""
+
+TWEAK_PREFIX_MESSAGE = Fp(value=0x02)
+"""The unique prefix for tweaks used in the initial message hashing step."""
diff --git a/src/lean_spec/subspecs/xmss/interface.py b/src/lean_spec/subspecs/xmss/interface.py
new file mode 100644
index 00000000..e8e7e492
--- /dev/null
+++ b/src/lean_spec/subspecs/xmss/interface.py
@@ -0,0 +1,109 @@
+"""
+Defines the core interface for the Generalized XMSS signature scheme.
+
+Specification for the high-level functions (`key_gen`, `sign`, `verify`)
+that constitute the public API of the signature scheme. For the purpose of this
+specification, these are defined as placeholders with detailed documentation.
+"""
+
+from __future__ import annotations
+
+from typing import Tuple
+
+from .structures import PublicKey, SecretKey, Signature
+
+
+def key_gen(
+    activation_epoch: int, num_active_epochs: int
+) -> Tuple[PublicKey, SecretKey]:
+    """
+    Generates a new cryptographic key pair. This is a **randomized** algorithm.
+
+    This function is a placeholder. In a real implementation, it would involve
+    generating a master secret, deriving all one-time keys, and constructing
+    the full Merkle tree.
+
+    Args:
+        activation_epoch: The starting epoch for which this key is active.
+        num_active_epochs: The number of consecutive epochs
+        the key is active for.
+
+    For the formal specification of this process, please refer to:
+    - "Hash-Based Multi-Signatures for Post-Quantum Ethereum": https://eprint.iacr.org/2025/055
+    - "Technical Note: LeanSig for Post-Quantum Ethereum": https://eprint.iacr.org/2025/1332
+    - The canonical Rust implementation: https://github.com/b-wagn/hash-sig
+    """
+    raise NotImplementedError(
+        "key_gen is not part of this specification. "
+        "See the Rust reference implementation."
+    )
+
+
+def sign(sk: SecretKey, epoch: int, message: bytes) -> Signature:
+    """
+    Produces a digital signature for a given message at a specific epoch. This
+    is a **randomized** algorithm.
+
+    This function is a placeholder. The signing process involves encoding the
+    message, generating a one-time signature, and providing a Merkle path.
+
+    **CRITICAL**: This function must never be called twice with the same secret
+    key and epoch for different messages, as this would compromise security.
+
+    For the formal specification of this process, please refer to:
+    - "Hash-Based Multi-Signatures for Post-Quantum Ethereum": https://eprint.iacr.org/2025/055
+    - "Technical Note: LeanSig for Post-Quantum Ethereum": https://eprint.iacr.org/2025/1332
+    - The canonical Rust implementation: https://github.com/b-wagn/hash-sig
+    """
+    raise NotImplementedError(
+        "sign is not part of this specification. "
+        "See the Rust reference implementation."
+    )
+
+
+def verify(pk: PublicKey, epoch: int, message: bytes, sig: Signature) -> bool:
+    r"""
+    Verifies a digital signature against a public key, message, and epoch. This
+    is a **deterministic** algorithm.
+
+    This function is a placeholder. The complete verification logic is detailed
+    below and will be implemented in a future update.
+
+    ### Verification Algorithm
+
+    1.  **Re-encode Message**: The verifier uses the randomness `rho` from the
+        signature to re-compute the codeword $x = (x_1, \dots, x_v)$ from the
+        message `m`.
+        This includes calculating the checksum or checking the target sum.
+
+    2.  **Reconstruct One-Time Public Key**: For each intermediate hash $y_i$
+        in the signature, the verifier completes the corresponding hash chain.
+        Since $y_i$ was computed with $x_i$ steps, the verifier applies the
+        hash function an additional $w - 1 - x_i$ times to arrive at the
+        one-time public key component $pk_{ep,i}$.
+
+    3.  **Compute Merkle Leaf**: The verifier hashes the reconstructed one-time
+        public key components to compute the expected Merkle leaf for `epoch`.
+
+    4.  **Verify Merkle Path**: The verifier uses the `path` from the signature
+        to compute a candidate Merkle root starting from the computed leaf.
+        Verification succeeds if and only if this candidate root matches the
+        `root` in the `PublicKey`.
+
+    Args:
+        pk: The public key to verify against.
+        epoch: The epoch the signature corresponds to.
+        message: The message that was supposedly signed.
+        sig: The signature object to be verified.
+
+    Returns:
+        `True` if the signature is valid, `False` otherwise.
+
+    For the formal specification of this process, please refer to:
+    - "Hash-Based Multi-Signatures for Post-Quantum Ethereum": https://eprint.iacr.org/2025/055
+    - "Technical Note: LeanSig for Post-Quantum Ethereum": https://eprint.iacr.org/2025/1332
+    - The canonical Rust implementation: https://github.com/b-wagn/hash-sig
+    """
+    raise NotImplementedError(
+        "verify will be implemented in a future update to the specification."
+    )
diff --git a/src/lean_spec/subspecs/xmss/structures.py b/src/lean_spec/subspecs/xmss/structures.py
new file mode 100644
index 00000000..306e9948
--- /dev/null
+++ b/src/lean_spec/subspecs/xmss/structures.py
@@ -0,0 +1,75 @@
+"""Defines the data structures for the Generalized XMSS signature scheme."""
+
+from typing import Annotated, List
+
+from pydantic import BaseModel, ConfigDict, Field
+
+from ..koalabear import Fp
+from .constants import HASH_LEN_FE, PARAMETER_LEN, RAND_LEN_FE
+
+HashDigest = Annotated[
+    List[Fp], Field(min_length=HASH_LEN_FE, max_length=HASH_LEN_FE)
+]
+"""
+A type alias representing a hash digest.
+"""
+
+Parameter = Annotated[
+    List[Fp], Field(min_length=PARAMETER_LEN, max_length=PARAMETER_LEN)
+]
+"""
+A type alias representing the public parameter `P`.
+"""
+
+Randomness = Annotated[
+    List[Fp], Field(min_length=RAND_LEN_FE, max_length=RAND_LEN_FE)
+]
+"""
+A type alias representing the randomness `rho`.
+"""
+
+
+class HashTreeOpening(BaseModel):
+    """
+    A Merkle authentication path.
+
+    It contains a list of sibling nodes required to reconstruct the path
+    from a leaf node up to the Merkle root.
+    """
+
+    model_config = ConfigDict(frozen=True, arbitrary_types_allowed=True)
+    siblings: List[HashDigest] = Field(
+        ..., description="List of sibling hashes, from bottom to top."
+    )
+
+
+class PublicKey(BaseModel):
+    """The public key for the Generalized XMSS scheme."""
+
+    model_config = ConfigDict(frozen=True, arbitrary_types_allowed=True)
+    root: List[Fp] = Field(..., max_length=HASH_LEN_FE, min_length=HASH_LEN_FE)
+    parameter: Parameter = Field(
+        ..., max_length=PARAMETER_LEN, min_length=PARAMETER_LEN
+    )
+
+
+class Signature(BaseModel):
+    """A signature in the Generalized XMSS scheme."""
+
+    model_config = ConfigDict(frozen=True, arbitrary_types_allowed=True)
+    path: HashTreeOpening
+    rho: Randomness = Field(
+        ..., max_length=RAND_LEN_FE, min_length=RAND_LEN_FE
+    )
+    hashes: List[HashDigest]
+
+
+class SecretKey(BaseModel):
+    """
+    Placeholder for the secret key.
+
+    Note: The full secret key structure is not specified here as it is not
+    needed for verification.
+    """
+
+    pass