secp256k1: Refactor constant time selection function.#3711
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dajohi
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jholdstock
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The various functions for comparing in constant time are no longer limited to only being used in mod n scalars, so move them into their own file.
This introduces a new function for selecting between two uint64s in constant time and modifies code that manually performs the selection to make use of the function to improve readability. The function gets inlined, so there is no loss of performance. The order of the condition is also reversed to match the typical hardware intrinsic which also resembles the ternary operator.
This adds a test for the new constantTimeSelect64 function to help ensure proper behavior.
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This requires #3708.This introduces a new function for selecting between two uint64s in constant time and modifies code that manually performs the selection to make use of the function to improve readability along with tests to ensure proper functionality. The function gets inlined, so there is no loss of performance.
The order of the condition is also reversed to match the typical hardware intrinsic which also resembles the ternary operator.
It also separates out the various other functions for comparing in constant time to a separate file since they are no longer limited to only being used in mod n scalars.