Constant-time primitives

Master branch:

This package contains a number of primitives for writing constant-time algorithms.

Warning: at the moment it is not completely certain whether Julia provides constant-time guarantees for the operators the library is based on. It is also not clear if it is a good idea in general to write crypto in Julia. Use this library to prototype branchless algorithms and to control operations on secret data by wrapping it in ConstantTime.Value type.