Modular inversion improvements #263
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@fjarri needs a rebase |
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#267 should go in first, then I'll rebase on top of that. |
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Rebased. |
tarcieri
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Sep 2, 2023
fjarri
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Sep 2, 2023
| /// Computes the multiplicative inverse of `self` mod `modulus`. | ||
| /// Returns `(inverse, CtChoice::TRUE)` if an inverse exists, | ||
| /// otherwise `(undefined, CtChoice::FALSE)`. | ||
| pub fn inv_mod(&self, modulus: &Self) -> (Self, CtChoice) { |
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Just realized, it could have been made const.
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rem()which rendered it uselessinv_mod2k()constant-time both inselfandkinv_mod2k()toinv_mod2k_vartime()inv_mod2k_vartime()clearer and fix the attributioninv_mod()that supports both even and odd moduli (works via RNS andinv_mod2k()/inv_odd_mod(); another approach is binary GCD (see Constant-time GCD #227 for more info) which may or may not be faster, but is definitely more complicated)Note that this PR does not introduce any backwards-incompatible changes (except for the performance regression for the users of
inv_mod2k()).Possible additions:
inv_mod2k_bounded(), for the cases where a bound onkis known.inv_mod_vartime()(that callsinv_mod2k_vartime()internally, which would lead to a significant performance gain)(questions involving breaking changes are moved to #268)