SDK drift: `SECP256K1_HALF_N` signature-canonicalization constant is a public export in TypeScript but private (`_`-prefixed) in Python

[realfishsam]

Drift

Both SDKs compute the same secp256k1 group-order-over-2 constant, used for the canonical low-s signature check in signature verification. In TypeScript it is a public, top-level export const. In Python the equivalent is underscore-prefixed and therefore private-by-convention, with no public counterpart exposed anywhere in the module.

This is the mirror image of already-tracked issue #1477 (EIP-712 schema constants like ORDER_PARAMS_FIELDS/SCHEMAS are private in TS but public in Python) — here the direction is reversed, and the constant in question is a different one (a cryptographic constant for the post-sign verification layer, not a schema-shape constant).

TypeScript SDK

sdks/typescript/pmxt/hosted-typed-data.ts:243-244

export const SECP256K1_HALF_N =
    0x7fffffffffffffffffffffffffffffff5d576e7357a4501ddfe92f46681b20a0n;

Used at hosted-typed-data.ts:627 (if (sValue > SECP256K1_HALF_N)).

Python SDK

sdks/python/pmxt/_hosted_typeddata.py:43-47

_SECP256K1_N = int(
    "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141",
    16,
)
_SECP256K1_HALF_N = _SECP256K1_N // 2

Used at _hosted_typeddata.py:299 (if s_value > _SECP256K1_HALF_N). No public SECP256K1_HALF_N or SECP256K1_N binding exists in the module.

Note: the values are numerically identical (verified) — this is a visibility/export gap, not a correctness bug. Python's own test suite (sdks/python/tests/test_hosted_typeddata.py:48) has to hand-roll a local copy (SECP256K1_HALF_N = SECP256K1_N // 2) rather than importing the module's version, confirming there is no reusable public binding on the Python side.

Expected

Python should expose SECP256K1_HALF_N (and ideally SECP256K1_N) as public module members, matching TypeScript's export, so downstream consumers doing their own EIP-712/ECDSA tooling have a reusable canonical constant on both SDKs.

Impact

Low severity — doesn't affect wire compatibility or signature validity. It's a genuine public-API asymmetry: TypeScript consumers can import and reuse SECP256K1_HALF_N; Python consumers cannot without reaching into a private, underscore-prefixed name.

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Found by automated SDK cross-language drift audit