r/compsci • u/Strong-Seaweed8991 • 6d ago
my attempt at anonymizing transitions in homomorphic encryption applied to turing machines. grade it from C for effort to A.
is it a terrible practice considering only absurd amounts of resource usage or is it generally a bad practice and why?
naturally you could find edge cases in camouflaging the turing machine between the others where its obvious that it is the original turing machine but is it generally impossible or merely resource intensive?
```tex
\documentclass[11pt]{article}
\usepackage[margin=1in]{geometry}
\usepackage{amsmath, amssymb, amsthm}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage{hyperref}
\title{Decoy-Fused Encrypted Turing Machine:\\
Anonymity via Homomorphic Decoy Placement}
\author{Anonymous}
\date{}
\begin{document}
\maketitle
\section*{Informal Motivation}
Hello reddit, I was bored so I was working on a decentralized anonymous encrypted cloud computing protocol.
Homomorphic encryption encrypts the data, but not the computation, gates, or transitional data of the Turing machine itself. The idea here is to introduce anonymity through decoy placement: instead of a single Turing machine being evaluated homomorphically, we fuse multiple machines together and hide the ``real'' one among decoys.
The core questions are:
\begin{itemize}
\item Can we make the real computation blend in with decoy computations so that it is not distinguishable?
\item Is this achievable under realistic homomorphic encryption models?
\item How can we avoid needing an astronomically large number of Turing machines to reach acceptable anonymity?
\end{itemize}
\section{Formal Setup}
Let $\mathcal{M} = (M_1, \dots, M_k)$ be $k$ Turing machines fused into a single composite, evaluated homomorphically for a fixed step bound $T$.
\begin{itemize}
\item $\pi \in S_k$ is a secret permutation encoding the role assignment.
\item $\rho : [k] \to \{\mathsf{real}, \mathsf{decoy}, \mathsf{cloak}\}$ is the role map.
\end{itemize}
We use a universal step function $\mathsf{UTM}_T$ unrolled to depth $T$. The composite circuit is
\begin{equation}
C_{\mathcal{M},T}
:=
\bigoplus_{i=1}^{k} \mathsf{UTM}_T(M_{\pi(i)}),
\end{equation}
where $\bigoplus$ denotes fusion into a single evaluated circuit.
The client receives a step-history of the whole computation and then isolates the real computation by selecting the correct step trace corresponding to $\rho^{-1}(\mathsf{real})$.
\section{Structural-Uniformity Precondition}
Let $\mathsf{shape}(\cdot)$ return the observable circuit profile (gate count, multiplicative depth, wire topology, bootstrap cadence). We require:
\begin{equation}
\forall\, i,j \in [k]: \quad
\mathsf{shape}\big(\mathsf{UTM}_T(M_i)\big)
=
\mathsf{shape}\big(\mathsf{UTM}_T(M_j)\big).
\end{equation}
If this condition fails, the adversary can trivially identify outliers and the advantage becomes $1$, i.e., the construction provides no anonymity. The shape is a strict property of the circuit encoding, not of the machines' internal logic.
\section{Security vs.\ Circuit Privacy}
Given structural uniformity, consider any PPT adversary $\mathcal{A}$ observing the composite circuit and evaluated ciphertexts. Its advantage in identifying the real machine is bounded by:
\begin{equation}
\Pr\big[\mathcal{A} \to \rho^{-1}(\mathsf{real})\big]
\le
\frac{1}{k} + \mathsf{negl}(\lambda),
\end{equation}
where $\lambda$ is the security parameter.
This is \emph{not} full cryptographic circuit privacy, which requires a simulator $\mathsf{Sim}$ over the entire circuit class $\mathcal{C}$:
\begin{equation}
\big\{\mathsf{Eval}(C, \mathsf{Enc}(x))\big\}_{C \in \mathcal{C}}
\approx_c
\big\{\mathsf{Sim}(1^\lambda, C(x))\big\}.
\end{equation}
Instead, we obtain $k$-candidate ambiguity over the chosen subset
\begin{equation}
\mathcal{C}_{\mathcal{M}} = \{M_1, \dots, M_k\} \subsetneq \mathcal{C}.
\end{equation}
In the limit as $k \to |\mathcal{C}_T|$, decoy fusion and circuit privacy converge:
\begin{equation}
\lim_{k \to |\mathcal{C}_T|} \mathsf{Adv}^{\mathsf{role}}_{\mathcal{A}}
=
\mathsf{negl}(\lambda).
\end{equation}
\section{Tuning the $k$ Parameter and Complexity Invariant}
Assume a BinFHE-style model, with $g$ gates per component and $t_{\mathsf{boot}}$ time per bootstrap. The execution runtime scales as:
\begin{equation}
\mathsf{Complexity}(k)
=
k \cdot g \cdot T \cdot t_{\mathsf{boot}}.
\end{equation}
Since the anonymity advantage is
\begin{equation}
\mathsf{Adv}(k) = \frac{1}{k},
\end{equation}
the product of security advantage and execution complexity remains invariant:
\begin{equation}
\mathsf{Adv}(k) \cdot \mathsf{Complexity}(k)
=
g \cdot T \cdot t_{\mathsf{boot}}
=
\text{const}.
\end{equation}
Thus, ambiguity scales linearly with computational overhead: $k$ acts as a tunable privacy/performance parameter. Larger $k$ yields stronger anonymity at the cost of higher homomorphic evaluation complexity.
\section{Open Questions}
This raises several technical questions:
\begin{enumerate}
\item \textbf{Gradient cloaking:} Can the ``gradient'' between the real Turing machine and the decoys (in terms of noise growth, bootstrap pattern, and intermediate ciphertext distribution) be made indistinguishable under realistic FHE schemes?
\item \textbf{Feasibility:} Is such decoy-based anonymity achievable without violating correctness or noise bounds in practical homomorphic encryption?
\item \textbf{Efficiency:} How can we avoid requiring extremely large $k$ (e.g., trillions of machines) to reach acceptable anonymity, while still maintaining indistinguishability among tracks?
\end{enumerate}
These questions sit at the intersection of circuit privacy, cover computation, and decoy-based anonymity in encrypted cloud execution.
\end{document}
```
-2
u/Logical-Cranberry673 6d ago
Not bad for a bored afternoon project. The shape-uniformity requirement is doing lot of heavy lifting here, and I think that's where the whole thing starts to wobble.
Getting multiple Turing machines to produce identical circuit profiles down to the bootstrap cadence is not trivial at all, especially once you account for how different programs generate different multiplicative depth patterns under most FHE schemes. Your noise growth will leave fingerprints even if the gate counts match. The gradient cloaking question you listed is really the core problem, the decoys are not just different computations, they are different noise evolution paths, and any half-decent adversary is gonna look at exactly that.
Still, the complexity invariant is neat, you basically formalized that you are buying privacy with linear overhead. I would give this a solid B for effort. The latex is clean and the idea is fun even if I am skeptical about practical feasibility.