avail-srs

Introduction

For Polygon Avail project, we needed to have one publicly verifiable reference string, which can be used for constructing & verifying KZG polynomial commitment proofs, so we decided to make use of Filecoin's Powers of Tau, which also uses BLS12-381 curve.

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Polygon Avail: avail-docs.matic.today

Filecoin's Attestations: github.com/filecoin-project/phase2-attestations

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Filecoin keeps both phase1 & phase2 files here: trusted-setup.filecoin.io

We use challenge_19 of phase1.

This repository contains programs required for extracting out N( <= 2**27 )-many parameters from challenge_19, which are eventually converted into desired serialised format, can be used by Polygon Avail Validators/ Light Clients. During extraction & serialisation, program also asserts public parameters for correctness by running some testcases.

We use N = 1 << 16 for constructing out reference string

If you follow steps below, at last you must have extracted.data & serialised_pp.data.

file name	significance	sha256
challenge_19	downloaded phase1 powers-of-tau file, with N ( = 2 ** 27 ) many parameters	7f311127652a83e3499e7d5e6c9a3dd78f6cb4bd27ea9ce8c1af3818a97adc8f
extracted.data	contains N ( = 65536 ) many parameters extracted from challenge_19, in compressed form	eee6430020c96dccccc95ca7b433025e70b58359f400ddf06e4aba37a212afd6
serialised_pp.data	serialised reference strings, ready to be used by Avail validators/ light clients	3857bb2ec085d4cb8a201c6e40a108870b817f539deadcc3e1e755138f715b10

Download

We serve aforementioned 3 static files from https://avail-srs.matic.today. Download them using

wget -v https://avail-srs.matic.today/{challenge_19, extracted.data, serialised_pp.data}

After download, make sure you match SHA256 hash with provided one, in above table.

Requirements

• Make sure you've rust toolchain installed. You may take a look at: rustup.rs
• You also need to have wget, UNIX program.

Usage

• Download phase1 powers-of-tau file

wget -v https://trusted-setup.filecoin.io/phase1/challenge_19

sha256sum challenge_19 # match with 👆 table

• Run parameter extractor, must generate extracted.data

pushd powersoftau
cargo run <absolute-path-to-challenge_19>
sha256sum extracted.data # match with 👆 table
popd

• Finally serialise into desired format, must generate serialised_pp.data

pushd srs
cargo run <absolute-path-to-extracted.data>
sha256sum serialised_pp.data # match with 👆 table
popd

Extra

Downloaded challenge_19 file has 1 << 27 parameters, stored in uncompressed form.

In uncompressed form G1 points: 96 bytes while G2 points: 192 bytes

num_of_tau_powers = 1 << 27
num_of_g1_tau_powers = (num_of_tau_powers << 1) - 1

File layout looks like

0           | hash                  <64                   bytes>
...
64          | g1 tau powers         <96 * ((2 ** 28) - 1) bytes>
...
25769803744 | g2 tau powers         <192 * (2 ** 27)      bytes>
...
51539607520 | alpha tau powers      <96 * (2 ** 27)       bytes>
...
64424509408 | beta tau powers       <96 * (2 ** 27)       bytes>
...
77309411296 | beta in g2            <192                  bytes>
...

challenge_19 file size must be 77309411488 bytes.

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After parameter extraction, generated extracted.data holds 1 << 16 parameters, stored in compressed form.

In uncompressed form G1 points: 48 bytes while G2 points: 96 bytes

File layout looks like

0           | hash                  <64             bytes>
...
64          | g1 tau powers         <48 * (2 ** 16) bytes>
...
3145792     | g2 tau powers         <48 * (2 ** 16) bytes>
...
9437248     | alpha tau powers      <48 * (2 ** 16) bytes>
...
12582976    | beta tau powers       <48 * (2 ** 16) bytes>
...
15728704    | beta in g2            <96             bytes>
...

extracted.data file size must be 15728800 bytes.

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After serialisation step, reference string holding file serialised_pp.data has everything an Avail Validator/ Light Client wants to have for proof generation/ verification.

It holds commit key & opening key in compressed form. Use following code snippet for deserialising reference string from byte array.

let mut data: Vec<u8> = Vec::new();

let fd = OpenOptions::new().read(true).open("serialised_pp.data")?;
fd.read_to_end(&mut data)?;

let degree = 1 << 10; // say 1024 for this case

let pp = PublicParameters::from_bytes(&data[..]).unwrap();
let (proving_key, verification_key) = pp.trim(degree).unwrap(); // create/ verify proofs !

Acknowledgement

• We make use of Filecoin hosted phase1 powers of tau file ( read challenge_19 )
• We also use slightly modified https://github.com/arielgabizon/powersoftau, for extracting out parameters. Check tree for more info.
• Test cases run when serialising extracted parameters to compressed byte array, are taken from here