oblivious hokey-pokey

01 Jan 2024

While reading some recent work by Gilad Asharov, et al. (Secure Statistical Analysis on Multiple Datasets: Join and Group-By, CCS 2023), I noticed that the paper’s Protocol 3.4, used for computing oblivious inner joins, bore a striking resemblance to the Hokey-Pokey, with only slight modifications:

\[\begin{align*} &\ \ \mathbf{k}L && \text{You put your left foot in,}\\ &\ \ \mathbf{k}L || \mathbf{k}R && \text{You put your left foot out,}\\ \vec f=&\,\left[\mathbf{k}L || \mathbf{k}R || \mathbf{k}L\right] && \text{You put your left foot in,}\\ \vec g=&\,\mathsf{Sort}(\{A, \mathbf{0}^n, -A\} \text{ on } \vec f) && \text{And you shake it all about.}\\ \vec h=&\,\mathsf{PrefixSum}(\vec g) && \text{You do the hokey pokey,}\\ \vec f'=&\,\mathsf{Invert}(\vec h) && \text{And you turn yourself around,}\\ &\,\mathsf{Output}\ \vec f' && \text{That's what it's all about!} \end{align*}\]

This works if we assume \(\text{put left foot out} \approx \text{put right foot in}\); that is, \(\mathsf{put}(L)=-\mathsf{put}(R)=-\mathsf{put}(-L)\) so the function \(\mathsf{put}(\cdot)\) is odd.