combine_witness_signatures

combine_witness_signatures

combine_witness_signatures is a GitHub Action for combining signatures on checkpoints cosigned by known witnesses.

This action would be used by a serverless witness distributor.

Overview

The serverless log creates a checkpoint file as part of the integration step. This file is signed by the log and commits to the contents of the log at the point at which it was created.

Witnesses are entities which work to help prevent the possibility of a log undertaking an undetected split-view attack, they do this by verifying consistency of the log checkpoints they see, and cosigning those they find to be consistent. Entities which rely on the contents of the log can thereby trust that the view of the log they see has also been seen by at least the set of witnesses which have cosigned the checkpoint they hold.

A distributor makes these cosigned checkpoints available to log clients.

This GitHub Action implements a serverless distributor. For more details on witnessing strategies as well as witness implementation(s), see the witness package.

Operation

A YAML formatted config file must be created and checked-in to the repo. An example config file is shown below:

MaxWitnessSignatures: 5

Witnesses:
  - can-I-get-a-witness+b97a1e56+AZkpOlwZwR+wwasAENZwIa98ufmWmzlq0Tx0XN7voU6X
  - witness-over-here+29c4e8f4+AUbwUCBUM2sDdHeiKUrp6LnMErE7GEz0iH+0WbgbJZxx
  - wolsey-bank-alfred+0336ecb0+AVcofP6JyFkxhQ+/FK7omBtGLVS22tGC6fH+zvK5WrIx

Logs:
  - ID: test
    PublicKey: github.com/AlCutter/serverless-test/log+28035191+AVtQ/9lW+g90rQY3+pODJvMQ8X/tTvh/EuvCDLSmUk4S
}

PRs containing cosigned checkpoint files under the distributor's .../logs/<logID>/incoming directory are raised by witnesses, validated, and merged.

Once these PRs are merged, this action:

1. is triggered on pushes to master
2. attempts to combine the checkpoints present for a given log with the ones from the incoming directory
3. produces one or more files containing checkpoints with merged signatures.

The output files are named checkpoint.0, checkpoint.1, etc. and contain the largest checkpoint seen which has at least the number of witness cosignatures specified by the file name. checkpoint.0 will always have the largest checkpoint seen, regardless of whether or not it's been cosigned by witnesses.

Usage

Inputs

To use this PR with your log, create a .github/workflows/distributor_master.yaml file with the following contents:

on:
  push:
    branches:
      # This is the name of the primary branch, which may be `main` for newer repos.
      - master

jobs:
  combine_witness_sigs:
    runs-on: ubuntu-latest
    name: Combine witness signatures
    steps:
    - uses: actions/checkout@v2
    - name: Combine witness signatures
      id: combine_witness_signatures
      uses: AlCutter/trillian-examples/serverless/deploy/github/distributor/combine_witness_signatures@serverless_distributor
      with:
          distributor_dir: './distributor'
          config: './distributor/config.json'
    - uses: stefanzweifel/git-auto-commit-action@v4
      with:
        commit_user_name: Serverless Bot
        commit_user_email: actions@github.com
        commit_author: Serverless Bot <actions@github.com>
        commit_message: Automatically merge witness signatures