BoxedUint: add cond_map and cond_and_then
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Unfortunately `BoxedUint` can't impl `subtle::ConditionallySelectable` due to a supertrait bound on `Copy`. See dalek-cryptography/subtle#94 This bound is required by `CtOption::map` and `CtOption::and_then` which are important for writing constant-time code. As a workaround which makes it still possible to leverate `CtOption`, this adds special `BoxedUint`-specialized combinators that are able to work around this issue by generating a placeholder (zero) value to pass to the provided callbacks in the event `CtOption` is none. This requires branching on the output of `CtOption` (which is unavoidable without an upstream fix in `subtle` itself), but still ensures that the provided callback function is called with a `BoxedUint` of a matching number of limbs regardless of whether the `CtOption` is some or none, which is the best we can do for now (and really quite close to what `subtle` is doing under the hood anyway).
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Unfortunately
BoxedUintcan't implsubtle::ConditionallySelectabledue to a supertrait bound onCopy. See dalek-cryptography/subtle#94This bound is required by
CtOption::mapandCtOption::and_thenwhich are important for writing constant-time code.As a workaround which makes it still possible to leverate
CtOption, this adds specialBoxedUint-specialized combinators that are able to work around this issue by generating a placeholder (zero) value to pass to the provided callbacks in the eventCtOptionis none.This requires branching on the output of
CtOption(which is unavoidable without an upstream fix insubtleitself), but still ensures that the provided callback function is called with aBoxedUintof a matching number of limbs regardless of whether theCtOptionis some or none, which is the best we can do for now (and really quite close to whatsubtleis doing under the hood anyway).