Fixes in fpylll.algorithms.babai #274
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malb
reviewed
Jun 6, 2024
| # zero vector at the end | ||
| A.swap_rows(0, B.nrows) | ||
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| for j in range(B.ncols): | ||
| A[-1, j] = t[j] | ||
| A[-1, -1] = ceil(A[-2].norm()) | ||
| # precise norm | ||
| A[-1, -1] = isqrt(sum(x**2 for x in A[-2])) + 1 |
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This would be wrong if the sqrt is an integer, no?
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Technically yes, but it doesn't matter if it's too big only too small
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It should at least be documented lest future readers stumble over this
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True, does this look better?
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Thanks! |
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This PR contains one fix and one improvement.
fpylll.algorithms.babai, when the norm of the largest vector is calculated,ceil(A[-2].norm())is used which casts the value to a double after which it is rounded. This is prone to large imprecision issues and when sufficiently large integers are used it may lead to issues with infinity. We instead opt for calculating the norm in Python and usingmath.isqrtwhich is precise.babaiwill now also be passed ontoLLL.reduction, which could be useful in some cases.