# [DRAFT] Random Permutations With Constant Space

## Summary

You want to iterate over a random ordering of items without storing or needing to lookup already-used items. We can borrow an algorithm from cryptography called the Feistel Network to generate random permutations in constant space and time. Applications include generating unique gift card codes or credit card numbers for your customers without exposing already-generated codes/numbers, and efficiently implementing the fizzlefade effect from Wolfenstein 3D.

• Understand what a Feistel Network is.
• Use Feistel Networks to generate random permutations on small domains.
This is a draft post, still writing it

## Introduction

A permutation is a specific order of a set of items. For example, the set could be the numbers ${1, 2, 3, 4, 5}$ in any order. One particular permutation of the set is $[5, 4, 3, 2, 1]$ A permutation does not repeat items (whereas a combination does repeat items). Sometimes we would like to generate a random permutation. Consider these three examples:

1. You are writing a card game and want to shuffle the cards before dealing them out. There are 52 cards in a deck so the set of items is ${1, 2, \dots, 52}$.

2. You are an online merchant and want to generate gift card numbers for customers, for example 14-digit gift card numbers. The set of all possible numbers contains $10^{14} \approx 2^{47}$ numbers but we still want to be certain we don’t give out the same gift card number to multiple customers. We’d like to generate gift card numbers without looking up a database of existing gift card numbers. Gift card numbers must be assigned as randomly as possible, or else an attacker who obtains a big block of gift card numbers could guess older/newer numbers and steal them. How guessable a gift card number is should be configurable.

3. You are create a retro computer game and want to incorporate the fizzlefade effect, where you fill in the screen with red pixels, one pixel at a time, until the screen is completely red. You want the effect to be perfect and never attempt to fill in a pixel more than once! Modern screen resolutions can be 3840 x 2160 pixels, i.e. approximately $8.3 * 10^6 \approx 2^{23}$ pixels.

For the first card example we can merely create an in-memory array of 52 numbers, shuffle them using the Fisher-Yates algorithm in $\mathcal{O}(n)$ time and $\mathcal{O}(n)$ space, then start dealing out cards. This is easy because of how small a deck of cards is. However, attempting the same with our gift card example, where each gift card is represented by a 64-bit integer (8 bytes per gift card number), would require $2^{47}$ numbers $*$ $8 = 2^{3}$ bytes per number $\approx$ 1.1 petabytes! Computers today can contain that much RAM but what a waste of memory and time!

## Problem

Given an unordered set of items, we would like to iterate over random permutations of those items in:

• $\mathcal{O}(1)$ space, i.e. do not increase the amount of space needed with the size of the set of items, and
• $\mathcal{O}(1)$ time, i.e. take a constant amount of time to generate a permutation that does not increase with the size of the set of items.

An attacker who is able to ask for many consecutive random permutations (e.g. gift card codes) should not be able to easily guess older and newer gift card codes. The output of our solution should seem indistinguishable from randomness.

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## Solution

A Feistel network is an algorithm that can permute an input integer in its domain. Concretely, given an n-bit integer with input domain $[0, 2^n)$, a Feistel network promises to map that input to another output integer in the same domain, such that the mapping is bijective. Each input maps to one and only one output, and vice-versa (each output maps to one and only one input). This is the definition of a permutation. See the Appendix “Demonstration that a Feistel Network permutes an input integer”.

TODO

https://github.com/asimihsan/permutation-iterator-rs

## Appendix

### Demonstration that a Feistel Network permutes an input integer

Recall that the XOR operator $\oplus$ applied to two binary digits returns 1 if they are different, 0 if they are not. Hence by definition for any input integer $A$, $A \oplus A = 0$.

Hence consider some input integer $P$ and split it into two pieces $A$ and $B$, where $P = A \mathbin\Vert B$ ($A$ and $B$ concatenated together give P). $A$ and $B$ can have any non-zero lengths.

Let’s demonstrate a 2-round Feistel network. This applies the Feistel round function twice. At the end we swap the parts.

$$\begin{array}{lll} \textrm{Encryption, at the start:} \\\\ L_0 &=& A, \\ R_0 &=& B \\\\ \textrm{After the first round:} \\\\ L_1 &=& R_0, \\ R_1 &=& L_0 \oplus f(R_0, K_0) \\\\ \textrm{After the second round:} \\\\ L_2 &=& R_1, \\ R_2 &=& L_1 \oplus f(R_1, K_1) \\\\ \textrm{After the rounds are done, swap to produce result C} \\\\ C &=& R_2 \mathbin\Vert L_2 \end{array}$$

Given output $C$, apply the process in reverse, ensuring that the Feistel functions and round keys are applied in reverse.

$$\begin{array}{rcl} \textrm{Decryption, at the start:} \\\\ T_0 &=& R_2\\ U_0 &=& L_2 \\\\ \textrm{After the first round:} \\\\ T_1 &=& U_0 \\ U_1 &=& T_0 \oplus f(U_0, K_1) \\\\ \textrm{After the second round:} \\\\ T_2 &=& U_1 \\ U_2 &=& T_1 \oplus f(U_1, K_0) \\\\ \textrm{After the rounds are done, swap to produce result Q:} \\\\ Q &=& U_2 \mathbin\Vert T_2 \\\\ \end{array}$$

Now we simplify $Q$ (the encrypted-then-decrypted value) and demonstrate it is equal to $M$ (the original input).

$$\begin{array}{rcl} Q &=& \overbrace{T_1 \oplus f(U_1, K_0)}^{U_2} \mathbin\Vert \overbrace{U_1}^{T_2} \\ &=& U_0 \oplus f(T_0 \oplus f(U_0, K_1), K_0) \mathbin\Vert T_0 \oplus f(U_0, K_1) \\ &=& L_2 \oplus f(R_2 \oplus f(L_2, K_1), K_0) \mathbin\Vert R_2 \oplus f(L_2, K_1) \\ &=& R_1 \oplus f(L_1 \oplus \overbrace{f(R_1, K_1) \oplus f(R_1, K_1)}^{0}, K_0) \mathbin\Vert L_1 \oplus \overbrace{f(R_1, K_1) \oplus f(R_1, K_1)}^{0} \\ &=& R_1 \oplus f(\overbrace{L_1 \oplus 0}^{L_1}, K_0) \mathbin\Vert \overbrace{L_1 \oplus 0}^{L_1} \\ &=& R_1 \oplus f(L_1, K_0) \mathbin\Vert L_1 \\ &=& L_0 \oplus \overbrace{f(R_0, K_0) \oplus f(R_0, K_0)}^{0} \mathbin\Vert R_0 \\ &=& L_0 \mathbin\Vert R_0 \\ &=& A \mathbin\Vert B \\ &=& P\qquad_{\square} \end{array}$$

This applies to any number of Feistel rounds.