Commits on Nov 29, 2023

1. BoxedUint: add cond_map and cond_and_then …

   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).

   

   tarcieri committed Nov 29, 2023
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