Fix GCM forbidden_attack breaking on Sage 10.7+

[idrassi]

New Sage versions removed integer_representation() on NTL-backed GF(2^128) elements. This breaks forbidden_attack._from_gf2e.
Update the conversion to use Sage's from_integer()/to_integer() API and preserve the original bit order via a manual 128-bit reversal (reverse=True is not reliably supported).

Changes

• Replace integer_representation usage with from_integer/to_integer.
• Manually reverse 128-bit order to match original mapping.
• Retain legacy conversion path for older Sage where needed.

Tests

• sage -python -m pytest test/test_gcm.py
• sage -python -m pytest

    ============================= test session starts ==============================
    platform linux -- Python 3.12.12, pytest-9.0.2, pluggy-1.6.0
    ...
    -- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
    ================== 86 passed, 2 warnings in 193.74s (0:03:13) ==================

Fixes issue #31

[jvdsn]

(reverse=True is not reliably supported).

how so?

[idrassi]

@jvdsn
I have an issue using reverse=TRUE in from_integer call. See error trace below.

I'm running SageMath 10.7 under Conda on Linux Ubuntu 25.04 (I don't use sage from Ubuntu repository, I used official installation procedure).

So, to overcome this issue, I had to implement manual reversal. Somehow, although reverse is documented, NTL implementation for GF(2^m) doesn't seem to support it which is kinda strange.

Did I miss something?

    def _to_gf2e(n):
        n = int(n) & MASK128
        if _HAS_NEW_API:
>           return gf2e.from_integer(n, reverse=True)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E           TypeError: FiniteField_ntl_gf2e.from_integer() got an unexpected keyword argument 'reverse'

attacks/gcm/forbidden_attack.py:16: TypeError

[idrassi]

Simpler fix in d42a3df