Perpetual Powers of Tau

The Semaphore team is conducting phase 1 of a multi-party trusted setup ceremony based on the Zcash Powers of Tau ceremony. The goal is to securely generate zk-SNARK parameters for circuits of up to 2 ^ 28 (260+ million) constraints. This means that the process will generate twice as many minus one (530+ million) powers of tau.

As long as one party in the ceremony behaves honestly and is not comprimised, the entire setup is trustworthy.

Ceremony progress

Participant ID	Identity	GPG key	Attestation
0001	Koh Wei Jie	Keybase	0001_weijie_response
0002	Kobi Gurkan	Keybase	In progress

Procedure

There is a coordinator and multiple participants. The ceremony occurs in sequential rounds. Each participant performs one or more rounds at a time. The coordinator decides the order in which the participants act. There can be an indefinite number of rounds.

The ceremony starts with the coordinator generating an initial challenge file, and publishing it in a publicly accessible repository.

The first participant downloads challenge, runs a computation to produce a response file, and sends it to the coordinator.

The coordinator will then produce a new_challenge file, and publish it along with the response. The next selected participant will then download new_challenge and produce a response, and the process repeats indefinitely.

Whenever a new zk-SNARK project needs to perform a trusted setup, they can pick the latest response, verify the entire chain of challenges and responses up to the selected response, and finally apply a random beacon to it. Next, they can move on to phase 2 of the trusted setup which is circuit-specific and out of scope of phase 1.

To illustrate this process, consider a Coordinator, two participants (Alice and Bob), and a zk-SNARK project author Charlie:

1. Coordinator generates challenge_0 and publishes it.
2. Alice generates response_1 and publishes it.
3. Coordinator generates challenge_1 and publishes it.
4. Bob generates response_2 and and publishes it.
5. Coordinator generates challenge_2 and publishes it.
6. Charlie applies the random beacon to challenge_2 to finalise the setup.

The resulting public transcript should contain:

1. challenge_0
2. response_1
3. challenge_1
4. response_2
5. challenge_2
6. The final parameters

The random beacon

Zcash announced on their ceremony mailing list that they would use the hash of a specific Bitcoin block. They made this announcement before the block was mined. See:

https://github.com/ZcashFoundation/powersoftau-attestations/tree/master/0088

A similar process can be used for this ceremony. Note that mining difficulty has grown since 2018, so there is now slightly less entropy per Bitcoin block hash.

The transcript

The transcript can be fully verified as long as it is public and that there are no bugs in the code used to generate challenges and responses.

Participants can choose to be anonymous. If they choose to be publicly known, they should own a GPG keypair whose public key is known to be associated with their real-world identity, socially or via any other means.

Given the above, the transcript should contain all the challenge and response files, and the Blake2 hash of each file.

It should also contain an attestation for each response. This is a text file with:

• Blake2 hashes of the challenge received and the response generated
• A detailed description of the hardware and software that the participant used to generate the response.
• A detailed description of any security and anti-surveillance measures that the partcipant has used.

Additionally, it should contain each participant's GPG signature of their attestation, so as to assure the public that it was generated by the person who had claimed to have done so.

Logistics

Each challenge file is about 97G in size and each response file is about 49G. An Azure compute VM (Standard F4s with 4 vcpus and 8GB memory) takes about 24 hours to generate.

The coordinator is using Microsoft Azure compute VMs to generate new_challenge files, and Blob Storage to host challenges and responses.

Each participant can transfer their response to the coordinator via sftp. This process is semi-interactive as it this requires either the participant to provide their SSH public key in advance, or the coordinator to send them a private key. Alternatively, they can use any of the following interactive methods:

• BitTorrent
• IPFS
• Third-party large-file transfer services like MASV

Coordination

A mailing list has been set up to coordinate the ceremony:

https://groups.google.com/a/ethereum.org/forum/?hl=en#!forum/perpetual-powers-of-tau-ceremony-group

Instructions for each participant

First, set up a Linux machine and install Rust and Cargo following instructions here.

Download and compile the required source code:

git clone [email protected]:kobigurk/phase2-bn254.git --branch ppot_ceremony && \
cd phase2-bn254/powersoftau && \
cargo build --release

Download the challenge_nnnn_name file from the coordinator. The filename might be something like challenge_0004_alice. Rename it to challenge:

mv challenge_nnnn_name challenge

Run the computation with challenge in your working directory:

/path/to/phase2-bn254/powersoftau/target/release/compute_constrained

You will see this prompt:

Will contribute to accumulator for 2^28 powers of tau
In total will generate up to 8191 powers
Type some random text and press [ENTER] to provide additional entropy...

Make sure that it says 2^28 powers of tau, and then enter random text as prompted. You should try to provide as much entropy as possible from sources which are truly hard to replicate. See below for examples derived from Zcash's own ceremony.

The compuation will run for about 24 hours on a fast machine. Please try your best to avoid electronic surveillance or tampering during this time.

When it is done, you will see something like this:

Finihsing writing your contribution to `./response`...
Done!

Your contribution has been written to `./response`

The BLAKE2b hash of `./response` is:
        12345678 90123456 78901234 56789012 
        12345678 90123456 78901234 56789012 
        0b5337cd bb05970d 4045a88e 55fe5b1d 
        507f5f0e 5c87d756 85b487ed 89a6fb50 
Thank you for your participation, much appreciated! :)

Upload the response file to the coordinator's server using SFTP or rsync. We will provide you with options and guidance on how to do this.

Finally, document the process you used, following the template here: https://github.com/weijiekoh/perpetualpowersoftau/tree/master/0001_weijie_response

Sign it with your GPG key and post it to the mailing list.

Signing an attestation with an Ethereum account instead of GPG

If you wish to sign your attestation using an Ethereum account instead of GPG, please SHA256 hash your attestation and use an account publicly associated with your identity and store it in this Notary contract using the register(bytes32 _hash) function. Send the transaction hash to the coordinator, who will include it in the transcript.

This Notary contract is simply:

pragma solidity 0.5.11;

contract Notary {
    mapping (bytes32 => bool) public hashes ;
    
    function register(bytes32 _hash) public {
        hashes[_hash] = true;
    }
    
    function check(bytes32 _hash) public view returns (bool) {
        return hashes[_hash];
    }
}

Examples of entropy sources

1. /dev/urandom
2. The most recent Bitcoin block hash
3. Randomly mashing keys on the keyboard
4. Asking random people on the street for numbers
5. Geiger readings of radioactive material. e.g. a radioactive object, which can be anything from a banana to a Chernobyl fragment.
6. Environmental data (e.g. the weather, seismic activity, or readings from the sun)