GASH is designed for compiling circuit description files (written in gcdf language, which has somewhat similar syntax with C++) to binary circuits (in circ format, which has similar syntax with the And Inverter Graph-AIG circuit syntax).

Then, GASH garbles the circuit described by the circ file (if the machine is garbler), connects to peer, and perform Yao's protocol.

###Write code in gash-lang

gash-lang is a simple language with syntax similar to C. It consists of two parts, a main function, and a few directives.

func FuncName(Type name, ...) {

}
#Directives
...

What gash will do is first compile the function into a boolean circuit, and use the directives as hints to execute the circuits in Yao's GC framework. The directives include

• #definput Symbol Value. Sets the value of Symbol to Value. Value will be truncated if the symbol size is less than that of Value.
• #defip Ip. Sets the peer's ip address. The peer of the garbler is the evaluator, and vice versa. It is usually better for the evaluator to connect to the garbler.
• #defrole Role. Sets the role of the current program, the value could be GARBLER or EVALUATOR.
• #defport Port. Sets the peer's port used by the GC framework.
• #defotport Port. Sets the peer's port used by Oblivious Transfer.

####Data Type Gash currently has 1 generic data types intX, where 1<=X<=64. All integers are signed, thus the largest number we can represent is 1<<63.

###Control Flow Gash has a rather restricted control flow than C and other high-level programming language. This is because Gash is not turing complete. Every program/circuit that gash generates finishes in a finite number of steps. Here are the two restrictions of gash compared to C.

• No while and for loop.
• No else if.

However, there are (trivial) ways to achieve for loop with finite steps and else if in certain conditions. If you want to write

    for (int i = 0; i < 10; i++) {
        <doSomething i>
    }

where <doSomething i> is the code that you want to executes in the loop, parameterized by i. You can write it in gash-lang as such.

        <doSomething 0>
        <doSomething 1>
        ...
        <doSomething 9>
        <doSomething 10>

If you want to write

    if (<condition 0>) {
        <doSomething 0>    
    } else if (<condition 1>) {
        <doSomething 1>
    }
    ...
    else if (<condition n-1>) {
        <doSomething n-1>
    } else  {
        <doSomething n>
    }

You should nest the else ifs by writing it as follows.

    if (<condition 0>) {
        <doSomething 0>
    } else {
        if (<condition 1>) {
            <doSomething 1>
        } else {
            ...
                ...
                    ...
                        if (<condition n-1>) {
                            <doSomething n-1>
                        } else {
                            <doSomething n>
                        }
                    ...
                ...
            ...    
        }
    }

Alice holds her secret value a and Bob holds the secret value b, they want to determine whether a > b without telling each other her/his secret value. They decide to use GASH to solve the problem.

Suppose a=1000, Alice writes the following circuit description file alice.gcdf:

func millionaire(int32 a, int32 b) {
    int32 ret = 0;
    if (a > b) { ret = 1; }
    return ret;
}
#definput a 1000
#defrole GARBLER
#defport 47665

Suppose b=500 and Alice has ip address 178.56.1.9, Bob writes the following circuit description file bob.gcdf:

func millionaire(int32 a, int32 b) {
    int32 ret = 0;
    if (a > b) { ret = 1; }
    return ret;
}

#definput b 500
#defrole EVALUATOR
#defport 47665
#defip 178.56.1.9

Alice opens a terminal and does

gashlang -i alice.gcdf -o alice.circ -d alice.data

Bob opens a terminal and does

gashlang -i bob.gcdf -o bob.circ -d bob.data

GASH is tested on Fedora 28.

GASH requires g++ (v8), flex, bison, gmp, openssl, boost (program_options only), and gtest . Please install those packages and their headers before trying to compile.

For Fedora users, you may call

./install_dependency.sh

to install those dependencies.

After all dependencies are installed, run

make

at the root folder of gash to build all the libraries and tests.

It also requires CPUs that support SSE4 (since we will be using the flag -msse4). Most Intel and AMD CPUs support it (click here for an extended reference)

To run tests, go to gash/test, in there are several folders which contain different type of unit tests. Most users would like to go to the exec folder and run the test executables for different tests. For example, if you want to test the billionaire problem, go to gash/test/exec and run

./test_exec_billionaire

The result would look like the following

[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from EXECTest
[ RUN      ] EXECTest.Billionaire64
R:9e6da844e4c1a1fc 8d53a648ee96aa95

Connect() failed, try again later: Connection refused
Connect() failed, try again later: Connection refused
Connect() failed, try again later: Connection refused
connected
Receiving using 8 bytes
Receving using 64 selection bits
Sending 64 labels
Verifying 1oo2 OT
Verifying 1oo2 OT
OT Verification successful
OT Verification successful
7.856	4180	2101
------ OT sender ------
Send amount: 4181
Recv amount: 2101
8.032	2109	4198
------ OT receiver ------
Send amount: 2109
Recv amount: 4198
707:1
Evaluator's sockets closed!
707:1
Garbler's sockets closed!
[       OK ] EXECTest.Billionaire64 (182 ms)
[----------] 1 test from EXECTest (182 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (182 ms total)
[  PASSED  ] 1 test.
[       OK ] EXECTest.Billionaire64 (182 ms)
[----------] 1 test from EXECTest (182 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (182 ms total)
[  PASSED  ] 1 test.

Notice that there are two lines that say

707: 1
707: 1

The first line is the output of the evaluator, meaning that the value of wire 707 is 1. The second line is the output of the garbler. That is why the two lines are the same (since the output is always the same).