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MPCLib Offline Garbling

Introduction

Garbling offline is a new functionality for MPCLib that allows garbling several circuits during the offline phase of an MPC Protocol to reduce the computation time of a circuit during the online phase.

The circuits are garbled and then persisted, so they can be later loaded into memory and finally evaluated. For now, the circuits can be persisted using in MySQL databases or the file system.

How it works

Three new MAMBA instructions have been added to MPCLib framework: OGC, LOADGC and EGC. Notice that these instructions are only supported for full-threshold setups.

OGC

This instruction is in charge of garbling and persisting the circuits for each player. It receives two arguments:

  • Circuit number. More information about circuits and each of their identification numbers could be found here
  • Number of times the circuit will be garbled

For example:

less_eq_circuit=66001
number_of_circuits=2
OGC(less_eq_circuit, number_of_circuits)

It will persist the garbled circuit depending on the selected storage type.

  • For MySQL databases all the garbled circuits will be stored in the database specified in the storage config file, notice that the database system should be already set up.

  • For filesystem an output file in JSON format will be generated, following this nomenclature: <circuit number>-<iteration>-P<player-number>.json. It will contain the information of the garbled circuit and will be located in the Garbled_Circuits directory.

A code example can be found here.

LOADGC

This instruction is an intermediate operation between OGC and EGC. It receives two arguments: the circuit number and the number of circuits to be loaded into memory.

It will read the garbled circuits from the selected storage system and store then in memory, so they can later be consumed by the EGC instruction.

A code example can be found here

EGC

This instruction evaluates a garbled circuit. It receives one argument: the circuit number. It will only evaluate one circuit at a time.

As in a queue, the first garbled circuit that had been loaded in memory using LOADGC a will be the one to be evaluated.

Make sure to load as many circuits as you need, otherwise an exception will be thrown.

A code example can be found here

Automated Circuit Creation

We have incorporated an automated mechanism to invoke OGC. It can be parametrized directly on the.mpc file here.

As an alternative, the user can invoke an interactive Script to avoid the direct manipulation of the .mpc file, it can be invoked via ./Scripts/gc-gen.sh. The user has to follow the instructions provided by the Script together with the inputs it requests. The script can be found here.

Choicebits files

In order to evaluate a circuit that has been garbled in a different program execution, an additional config file per player in the /Data directory is required as well: choicebits-P$playernumber.

These files can be found in the pre-configured folders for full threshold setups in Auto-Test-Data (subdirectories from 18 to 21). They are also generated when running Setup.x, only for full-threshold set-ups as well.

If your choicebits files are empty, new choicebits will be generated and persisted in the files once you run any mpc program.

Why?

The choicebits generated in aBitFactory's initialization method need to be the same for both the garbling and the evaluation process. If garbling process wants to be executed in a separated program from its evaluation, the choicebits used for garbling the circuit have to be persisted.

Testing Mode

We know how painful and time consuming can be to produce Garbled Circuits for testing. Just take a look at the experimentation in Trhough the Looking Glass. For that reason, we have incorporated a series of system parameters in config.h that allow us to keep reusing (a) Circuit(s). Note that if you change the system parameters, you will need to recompile FANNG-MPC. The parameters are the following:

ignore_share_db_count:

When activated (set to 1), it does not transition the status of the circuits extracted from the DB to used. This basically means that you will be reusing the circuits every time you invoke LOADGC. We remind the reader that the entries are updated via a parallel thread that is spawned via the read method from Garbled_Circuit_Storage.cpp. You can edit the pramater by directly changing it in the config.h as follows:

    #define ignore_share_db_count 1  //change this to 0 to deactivate

reuse_circuit_zero:

Similarly, We have added a system parameter called reuse_circuit_zero to the config.h file for testing the EGC instruction. When this parameter is activated (set to 1), the system only uses the first circuit on the circuit pile. In case the pile is empty, FANNG-MPC will generate and place 1 circuit on the pile to be reused. You can edit it as follows:

    #define reuse_circuit_zero 1 //change this to 0 to deactivate