genSTARK
This library is intended to help you quickly and easily generate STARKbased proofs of computation using JavaScript. The goal is to take care of as much boilerplate code as possible, and let you focus on the specifics of your computations.
Background
A STARK is a novel proofofcomputation scheme that allows you to create an efficiently verifiable proof that a computation was executed correctly. The scheme was developed by EliBen Sasson and team at TechnionIsrael Institute of Technology. STARKs do not require an initial trusted setup, and rely on very few cryptographic assumptions. See references for more info.
Disclaimer
DO NOT USE THIS LIBRARY IN PRODUCTION. At this point, this is a researchgrade library. It has known and unknown bugs and security flaws.
Install
$ npm install @guildofweavers/genstark save
Usage
Here is a trivial example of how to use this library. In this example, the computation is just adding 2 to the current value at each step. That is: x_{n+1} = x_{n} + 2.
import { instantiateScript } from '@guildofweavers/genstark';
// define a STARK for this computation
const fooStark = instantiateScript(Buffer.from(`
define Foo over prime field (2^32  3 * 2^25 + 1) {
secret input startValue: element[1];
// define transition function
transition 1 register {
for each (startValue) {
init { yield startValue; }
for steps [1..63] { yield $r0 + 2; }
}
}
// define transition constraints
enforce 1 constraint {
for all steps { enforce transition($r) = $n; }
}
}`));
// create a proof that if we start computation at 1, we end up at 127 after 64 steps
const assertions = [
{ register: 0, step: 0, value: 1n }, // value at first step is 1
{ register: 0, step: 63, value: 127n } // value at last step is 127
];
const proof = fooStark.prove(assertions, [[1n]]);
// verify that if we start at 1 and run the computation for 64 steps, we get 127
const result = fooStark.verify(assertions, proof);
console.log(result); // true
There are a few more sophisticated examples in this repository:
 Demo STARKs  demonstration of how to use various features of this library.
 MiMC STARK  basically the same as Vitalik Buterin's MiMC tutorial.
 Rescue STARKs  various STARKs based on Rescue hash function (e.g. proof of hash preimage, Merkle proof).
 Poseidon STARKs  various STARKs based on Poseidon hash function (e.g. proof of hash preimage, Merkle proof).
 Library examples  demonstration of how to use AirAssembly libraries from within AirScript.
When you run the examples, you should get a nice log documenting each step. Here is an example output of running 128bit MiMC STARK for 2^{13} steps:
Starting STARK computation
Set up evaluation context in 146 ms
Generated execution trace in 52 ms
Computed execution trace polynomials P(x) in 7 ms
Lowdegree extended P(x) polynomials over evaluation domain in 83 ms
Serialized evaluations of P(x) and S(x) polynomials in 92 ms
Built evaluation merkle tree in 87 ms
Computed composition polynomial C(x) in 574 ms
Combined P(x) and S(x) evaluations with C(x) evaluations in 50 ms
Computed lowdegree proof in 231 ms
Computed 48 evaluation spot checks in 2 ms
STARK computed in 1327 ms

Proof serialized in 8 ms; size: 94.58 KB

Proof parsed in 6 ms

Starting STARK verification
Set up evaluation context in 9 ms
Computed positions for evaluation spot checks in 1 ms
Decoded evaluation spot checks in 1 ms
Verified evaluation merkle proof in 4 ms
Verified transition and boundary constraints in 52 ms
Verified lowdegree proof in 14 ms
STARK verified in 85 ms

STARK security level: 96
API
You can find complete API definitions in genstark.d.ts. Here is a quick overview of the provided functionality:
Defining a STARK
The simplest way to create a STARK for a computation is to instantiate it from AirScript source like so:
const myStark = new instantiateScript(source, options, logger);
where:

source
is the AirScript source code. If this parameter is aBuffer
, it is expected to contain AirScript code in UTF8 format. If the parameter is a string, it is expected to be a path to a file containing AirScript code. 
options
is an optional parameter with additional STARK options. 
logger
is an optional logger. The default logger prints output to the console, but it can be replaced with anything that complies with the Logger interface.
STARK options
When provided, STARK options parameter should have the following form:
Property  Description 

extensionFactor?  Number by which the execution trace is "stretched." Must be a power of 2 at least 2x of the constraint degree, but cannot exceed 32. This property is optional, the default is smallest power of 2 that is greater than 2 * constraint degree. 
exeQueryCount?  Number of queries of the execution trace to include into the proof. This property is optional; the default is 80; the max is 128. 
friQueryCount?  Number of queries of the columns of low degree proof to include into the proof. This property is optional; the default is 40; the max is 64. 
hashAlgorithm?  Hash algorithm to use when building Merkle trees for the proof. Currently, can be one of the following values: sha256 , blake2s256 . This property is optional; the default is sha256 . 
wasm  A flag indicating whether to use WebAssembly optimizations. This proper is optional, the default is true . 
Note: WASMoptimization is available for certain finite fields and hash functions. If the field or the hash function you are using does not support WASMoptimization, a warning will be printed and its JavaScript equivalents will be used. In general, WASM optimization can speed up STARK proof time by 2x  5x.
You can also instantiate a STARK from AirAssembly source, or even from an AirSchema object:
const myStark = new instantiate(source, component, options, logger);
where:

source
is eitherBuffer
with AirAssembly source code, a string that is a path to a file with AirAssembly source code, or anAirSchema
object. 
component
is a name of the component within AirAssembly source from which the STARK is to be instantiated. If this parameter is omitted thedefault
component will be instantiated. 
options
is an optional parameter with additional STARK options. 
logger
is an optional logger. The default logger prints output to the console, but it can be replaced with anything that complies with the Logger interface.
Generating proofs
Once you have a Stark
object, you can start generating proofs using Stark.prove()
method like so:
const proof = myStark.prove(assertions, inputs?, seed?);
where:

assertions
is an array of Assertion objects (also called boundary constraints). These assertions specify register values at specific steps of a valid computation. At least 1 assertion must be provided. 
inputs
is an array of values initializing all declared inputs. This parameter must be provided only if the STARK requires inputs. 
seed
is an array of seed values for initializing execution trace. This parameter must be provided only if the STARK requires initialization from seed values.
Inputs
Handling of inputs deserves a bit more explanation.
If you've instantiated a STARK from AirScrip source, you don't need to worry about the seed
parameter as AirScript does not support explicit trace initialization. However, STARKs instantiated from AirScript source always require inputs
parameter. The shape of this parameter will depend on how you've defined your inputs (see AirScript documentation for more info).
For example, if your input declaration looks like this:
public input foo: element[1];
your inputs
array will need to contain a single element which will be an array of values for input foo
. For example:
[[1n, 2n]]
If you've declared more than one input, like so:
public input foo: element[1];
public input bar: element[1];
your inputs
array will need to contain an array of values for each declared input. For example:
[[1n, 2n], [3n, 4n]]
Here, the [1n, 2n]
values are assigned to input foo
and [3n, 4n]
values are assigned to input bar
.
If you've declared nested inputs like so:
public input foo: element[1];
public input bar: element[1][1];
your inputs
object may look like so:
[[1n, 2n], [[3n, 4n], [5n, 6n]]]
Here, for each value of foo
, we need to provide a list of value for bar
.
AirAssembly inputs
If you've instantiated a STARK from AirAssembly source, you may need to provide values for both inputs
and seed
parameters based on what is required by AirAssembly declaration. The inputs
parameter requirements are defined via input registers, while seed
parameter requirements are defined via trace initializer.
Verifying proofs
Once you've generated a proof, you can verify it using Stark.verify()
method like so:
const result = myStark.verify(assertions, proof, publicInputs?);
where:

assertions
is the same array of Assertion objects that was passed to theprove()
method. 
proof
is the proof object that was generated by theprove()
method. 
publicInputs
is an array of values for initializing all declared public inputs.
Verifying a proof basically attests to something like this:
If you start with some set of inputs (known to the prover), and run the computation for the specified number of steps, the execution trace generated by the computation will satisfy the specified assertions.
Assertions
Assertions (or boundary constraints) are objects that specify the exact value of a given mutable register at a given step. An assertion object has the following form:
interface Assertion {
register: number; // index of a mutable register
step : number; // step in the execution trace
value : bigint; // value that the register should have at the specified step
}
Performance
Some very informal benchmarks run on Intel Core i57300U @ 2.60GHz (single thread):
STARK  Field Size  Degree  Registers  Steps  Proof Time  Proof Size 

MiMC  128 bits  3  1  2^{13}  1.3 sec  95 KB 
MiMC  128 bits  3  1  2^{17}  23 sec  147 KB 
MiMC  256 bits  3  1  2^{13}  11.5 sec  108 KB 
MiMC  256 bits  3  1  2^{17}  230 sec  165 KB 
Merkle Proof (Rescue, d=8)  128 bits  5  8  2^{8}  300 ms  60 KB 
Merkle Proof (Rescue, d=16)  128 bits  5  8  2^{9}  600 ms  72 KB 
Merkle Proof (Poseidon, d=8)  128 bits  8  12  2^{9}  900 ms  74 KB 
Merkle Proof (Poseidon, d=16)  128 bits  8  12  2^{10}  1.8 sec  84 KB 
STARKs in the above examples have security parameters set to provide ~96 bits security.
Note 1: Rescue and Poseidon hash function instantiations are not really "applestoapples"  refer to here and here for exact parameters.
Note 2: Currently, STARKs in 128bit fields are able to take advantage of WebAssembly optimization, and thus, are much faster than STARKs in 256bit fields.
References
This library is originally based on Vitalik Buterin's zkSTARK/MiMC tutorial. Other super useful resources:
 STARKs whitepaper: Scalable, transparent, and postquantum secure computational integrity
Vitalik Buterin's blog series on zkSTARKs:
 STARKs, part 1: Proofs with Polynomials
 STARKs, part 2: Thank Goodness it's FRIday
 STARKs, part 3: Into the Weeds
StarkWare's STARK Math blog series:
 STARK Math: The Journey Begins
 Arithmetization I
 Arithmetization II
 Low Degree Testing
 A Framework for Efficient STARKs
Other STARK libraries:
License
MIT © 2020 Guild of Weavers