Fix and document the verifier

bin/verify

@@ -1,5 +1,9 @@
 #! /bin/sh
 
-make install_verifier build_electionguard_java
+echo "This script verifies a ballot or a whole election created in a Bulletin Board application"
+echo
+echo "For requirements to run this script, please read:"
+echo "https://github.com/decidim/decidim-bulletin-board/blob/develop/verifier/README.md"
+echo
 
 cd verifier && node src/index.js "$@"

bulletin_board/server/.rubocop.yml

@@ -28,5 +28,3 @@ Rails/HasManyOrHasOneDependent:
 
 FactoryBot/FactoryAssociationWithStrategy:
   Enabled: false
-
-

verifier/README.md

@@ -1,13 +1,76 @@
-- Common Verifiers
-  - Chained hashes
-  - Content hashes
-  - Signed data is properly signed
-  - IATs (???)
-  - Election checks
-    - number of trustees >= quorum > 1
-    - start date < end_date
-    - client name and pretty name (???)
-    - at least 1 contest
-    - each contest has at least 2 candidates
-    - vote_variation, number_elected and votes_allowed (???)
-  - Print pretty results
+Verifying the election
+======================
+
+This directory contains code for verifying the election. 
+
+Running the verifier requires (at the moment) several dependencies:
+
+- Java v8 or higher
+- Node.JS v12 or higher
+- Python 3.8.11 (higher might work, no promises here)
+
+To start, please clone this repository:
+
+```bash
+git clone https://github.com/decidim/decidim-bulletin-board.git
+cd decidim-bulletin-board
+```
+
+In order to run the verifier, you need to build the verifier jar, and then run it:
+
+**First, be sure to install the electionguard-java submodule:**
+
+```bash
+git submodule update --init --recursive
+```
+
+This step is only required once, or when the submodule is updated. Be sure that the folder `voting_schemes/electionguard/verifier/electionguard-java ` is empty. This step installs a [Gradle](https://gradle.org/) script to build the JAR, however it shouldn't be necessary to have "gradle" installed. If interested, just install it with `apt install gradle` or similar.
+
+**Second, install all the dependencies by running:**
+
+```bash
+make install_verifier
+```
+
+**Finally, install the electionguard python package**
+
+```bash
+make install_electionguard_python_wrapper
+```
+
+If you see a **BUILD SUCCESSFUL** message and no errors, you are ready to run the verifier.
+
+Running the verifier
+--------------------
+
+The verification process involves two things:
+
+- checking a file, which may contain a ballot or the whole encrypted election (depending on what you want to verify). The process of verifying a ballot spoils the ballot itself, so it can't be send to the real election afterwards (the user will need to vote again and generate a new ballot).
+- Connection to the Bulletin Board API, which contains all the signed transactions for the election. This is done by providing a URL to the API, no extra requirements are necessary. This step is only needed when verifying the a single ballot, when verifying the whole election, the bulletin board URL is not needed as all the necessary information is contained in the TAR file.
+
+### Verifying a single ballot
+
+The verifier can be run with the following command:
+
+```bash
+bin/verify FILE_TO_VERIFY BULLETIN_BOARD_URL
+```
+
+- The `FILE_TO_VERIFY` can be downloaded from the Decidim application, just before casting a vote (a file ending in `.txt`).
+- The `BULLETIN_BOARD_URL` is the URL of the Bulletin Board API, for example `https://bulletinboard.example.com/api/`
+
+### Verifying the whole election
+
+The verifier can be run with the following command:
+
+```bash
+bin/verify_election ELECTION_TAR_FILE
+```
+- The `ELECTION_TAR_FILE` is the TAR file downloaded from the Decidim application, just after the election is finished and the results published (a file ending in `.tar`). Look for the "Election log" link to find it.
+
+For help and examples, run the script with the `--help` modifier:
+
+```bash
+bin/verify --help
+```
+