Sum Demo

Description and guide for computing sum of inputs with secure MPC.

Secure summation protocol

The implementation of the following protocol may be found in jiff/demos/sum/mpc.js lines 23 through 36.

Input: arbitrary number of parties P1,...Pn with inputs x1,...xn

Each party Pi does the following: - secret shares their input xi to all other parties - iteratively uses secret addition protocol to add all of the shares they received together to get a total sum - reconstructs output sum in final opening step

Note on the code

The compute function in mpc.js executes once for every single party. In line 29 of mpc.js, the parties' shares are created. It is important to note that the variable created in that line, shares, is not just the secret shares belonging to a single parties' inputs but rather includes all shares that that party has received. The for loop in line 31 of mpc.js loops through a single party's shares of all of those parties' inputs.

Note also that the parties have to iteratively use secret addition instead of doing a single sum of their shares of x1,...,xn because in JIFF the secret addition protocol is a binary operation. I.e. if you have shares a, b, and c that you want to add, then you can't do

var output = a.sadd(b,c);

but instead have to do

var d = a.sadd(b);
var output = d.sadd(c);

Legal inputs

This instantiation of summation only supports positive integer inputs. For an implementation that supports fixed-point numbers, see the fixedpoint-sum demo.

Running Demo

1. Running a server:

   node demos/sum/server.js

2. Either open browser based parties by going to http://localhost:8080/demos/sum/client.html in the browser, or a node.js party by running

   node demos/sum/party.js <input> [<party count> [<computation_id> [<party id>]]]]'

3. Running tests: run the following. Note that you do not need to have the server running when running the tests; they run the server on their own.

   npm run-script test-demo -- demos/sum/test.js

File structure

The demo consists of the following parts:

1. Server script: server.js
2. Web Based Party: Made from the following files:
   • client.html: UI for the browser.
   • client.js: Handlers for UI buttons and input validations.
3. Node.js-Based Party:
   • party.js: Main entry point. Parses input from the command line and initializes the computation.
4. The MPC protocol: Implemented in mpc.js. This file is used in both the browser and node.js versions of the demo.
5. test.js: mocha unit tests.
6. Documentation:
   • This README.md file.