Bug in Unary Encoding (UE) implementation

[Samuel-Maddock]

Recent work on empirical LDP auditing [1], has highlighted a small bug in the _perturb() implementation of UEClient where the users true value was not properly retained with probability $p$. This results in UEClient perturbation failing to guarantee $\varepsilon$-LDP, particularly for small values of $\varepsilon < 1$ (See Figure 3 in [1]).

As of fc622e3, this has been fixed and will be rolled out in version 1.2.0. It is recommended to update the package if you are currently using UEClient/UEServer.

[1] "Revealing the True Cost of Local Privacy: An Auditing Perspective", Héber H. Arcolezi and Sébastien Gambs, https://arxiv.org/abs/2309.01597