Potential fix for code scanning alert no. 35: Multiplication result converted to larger type

[michaelbeale-IL]

Potential fix for https://github.com/IntelLabs/HEC-openfhe-development-test/security/code-scanning/35

To fix the problem, the multiplication should be performed using the larger integer type so that if the result would exceed the range of a 32-bit unsigned integer, the correct (unwrapped) value will be produced. The best way to achieve this is to cast one of the operands (bStep or j) to size_t or std::vector<>::size_type before doing the multiplication, so that the multiplication will be promoted to the larger type. Update all instances of bStep * j that index into vectors or are used as arguments requiring larger types to use the cast.

For the code in question, change line 1608:

Ciphertext<DCRTPoly> inner = EvalMultExt(cc->KeySwitchExt(ct, true), A[bStep * j]);

to:

Ciphertext<DCRTPoly> inner = EvalMultExt(cc->KeySwitchExt(ct, true), A[static_cast<size_t>(bStep) * j]);

You may similarly want to update the subsequent uses (e.g., A[bStep * j + i], FindAutomorphismIndex2nComplex(bStep * j, M)) for consistency and safety, but only the highlighted multiplication needs fixing per the error report.

No new methods, imports, or external dependencies are required.

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