Add initial support for crypto callbacks.

[roberthdevries]

This patch does not add support for types of callbacks. Adding support for callbacks for cipher functions is complicated by issues with cffi code generation for the wc_CryptoInfo structure with two layers of anonymous unions.
Uncommenting more parts of the cipher struct causes errors regarding conflicting struct sizes of other parts of the wc_CryptoInfo struct.
Cffi cdef and the compiler seem to disagree.

However as it is, callbacks work for random and hash functions.

[JeremiahM37]

Skoll Code Review

Scan type: review
Overall recommendation: REQUEST_CHANGES
Findings: 14 total — 9 posted, 5 skipped
7 finding(s) posted as inline comments (see file-level comments below)

Posted findings

• [High] Importing wolfcrypt fails when CRYPTO_CB is disabled at build time — wolfcrypt/__init__.py:49 and wolfcrypt/cryptocb.py:33-58
• [High] Double unregister of device on context-manager exit + GC — wolfcrypt/cryptocb.py:73-77, 117-118
• [Medium] Return value of wolfCrypt_Init not checked — wolfcrypt/__init__.py:52
• [Medium] Debug print() statements left in production callback dispatcher — wolfcrypt/cryptocb.py:80, 84, 85, 107, 111
• [Medium] Hash digest dict KeyError on unmapped hash types — wolfcrypt/cryptocb.py:84, 90
• [Medium] Digest output buffer not size-validated — wolfcrypt/cryptocb.py:90
• [Medium] RNG output length not validated — wolfcrypt/cryptocb.py:96-97
• [Medium] No tests for hash callbacks despite PR claim — tests/test_cryptocb.py
• [Low] Free-floating string literal used as 'comment' in build_ffi.py is misleading — scripts/build_ffi.py:1376-1412

Skipped findings

• [Medium] Anonymous-union cdef relies on '...' partial layout — may not match reality
• [Low] ctx==NULL fallback would never be reached — wrong return code
• [Low] Type annotations on overridable methods declare positional args with leading underscores
• [Low] rng_callback signature passes raw cffi pointer with no type annotation
• [Info] Stale exception import in init.py guard

Review generated by Skoll

[JeremiahM37]

🔴 [High] Double unregister of device on context-manager exit + GC

Both __exit__ and __del__ call self._unregister(). When used as a context manager (the test does exactly this: with RngCryptoCallback(10): ...), the device is unregistered on __exit__ and again later when the object is garbage-collected. If a second CryptoCallback was registered with the same device_id between exit and GC (a reasonable usage pattern, e.g., in successive tests), the GC-time unregister will rip out the live registration belonging to the new object, causing test pollution / NULL callback dispatch. There is also no flag to record "already unregistered."

def __exit__(self, exc_type, exc_value, traceback) -> None:
    self._unregister()

def __del__(self) -> None:
    self._unregister()

def _unregister(self) -> None:
    _lib.wc_CryptoCb_UnRegisterDevice(self.device_id)

Recommendation: Track registration state with a _registered flag, set it to False after unregistering, and follow the same try/except AttributeError pattern used by Random.__del__ in wolfcrypt/random.py to avoid errors during interpreter shutdown.

[roberthdevries]

The tracking of the registration is done in the wolfcrypt library itself. Unregistering a device twice is a noop.
__del__ is not certain to be called at any time soon (unless a gc.collect() call is done explicitly) so if more fine-grained control is required to control the lifespan of the callback, a context manager is the typical solution in Python.

So the unregister in __exit__ must not be removed.

[JeremiahM37]

wc_CryptoCb_UnRegisterDevice looks up by devId only — no ownership check (cryptocb.c:428). The double-unregister is only a no-op if the slot is still INVALID_DEVID at the second call. In the cross-instance case (cb1 exits → cb2 registers same devId → cb1 GC'd), cb1's del will find cb2's live slot and ClearDev it. A _registered flag in del avoids this without removing exit's unregister.

[JeremiahM37]

Module-level dicts read _lib.WC_ALGO_TYPE_* / _lib.WC_HASH_TYPE_* at import, but those are only in the cdef when CRYPTO_CB is enabled — so import wolfcrypt.cryptocb fails with AttributeError before reaching the if _lib.CRYPTO_CB_ENABLED: guard. Move the dicts inside the guard, and make the CryptoCallback import in wolfcrypt/__init__.py conditional too.