# mfer

Manifest file generator and checker.

# Build Status

[![Build Status](https://drone.datavi.be/api/badges/sneak/mfer/status.svg)](https://drone.datavi.be/sneak/mfer)

# Problem Statement

Given a plain URL, there is no standard way to safely and programmatically
download everything "under" that URL path. `wget -r` can traverse directory
listings if they're enabled, but every server has a different format, and
this does not verify cryptographic integrity of the files, or enable them to
be fetched using a different protocol other than HTTP/s.

Currently, the solution that people are using are sidecar files in the
format of `SHASUMS` checksum files, as well as a `SHASUMS.asc` PGP detached
signature. This is not checksum-algorithm-agnostic and the sidecar file is
not always consistently named.

Real issues I face:

- when I plug in an ExFAT hard drive, I don't know if any files on the
  filesystem are corrupted or missing
  - current ad-hoc solution are `SHASUMS`/`SHASUMS.asc` files
- when I want to mirror an HTTP archive, I have to use special tools like
  debmirror that understand the archive format
  - the debian repository metadata structure is hot garbage
- when I download a large file via HTTP, I have no way of knowing if the
  file content is what it's supposed to be

# Proposed Solution

A standard, a manifest file format, and a tool for generating same.

The manifest file would be called `index.mf`, and the tool for generating such would be called `mfer`.

The manifest file would do several important things:

- have a standard filename, so if given
  `https://example.com/downloadpackage/` one could fetch
  `https://example.com/downloadpackage/index.mf` to enumerate the full
  directory listing.
- contain a version field for extensibility
- contain structured data (protobuf, json, or cbor)
- provide an inner signed container, so that the manifest file itself can
  embed a signature and a public key alongside in a single file
- contain a list of files, each with a relative path to the manifest
- contain manifest timestamp
- contain ctime/mtime information for files so that file metadata can be
  preserved
- contain cryptographic checksums in several different algorithms for each
  file
  - probably encoded with multihash to indicate algo + hash
  - sha256 at the minimum
  - would be nice to include an IPFS/IPLD CIDv1 root hash for each file,
    which likely involves doing an ipfs file object chunking
  - maybe even including the complete IPFS/IPLD directory tree objects and
    chunklists?
    - this is because generating an `index.mf` does not imply publishing on
      ipfs at that time
  - maybe a bittorrent chunklist for torrent client compatibility? perhaps a
    top-level infohash for the whole manifest?

# Design Goals

- Replace SHASUMS/SHASUMS.asc files
- be easy to download/resume a whole directory tree published via HTTP
- be easy to use across protocols (given an HTTPS url, fetch manifest, then
  download file contents via bittorrent or ipfs)
- not strongly coupled to HTTP use case, should not require special hosting,
  content types, or HTTP headers being sent

# Non-Goals

- Manifest generation speed
  - likely involves IPFS chunking, bittorrent chunking, and several
    different cryptographic hash functions over the entirety of each and
    every file
- Small manifest file size (within reason)
  - 30MiB files are "small" these days, given modern storage/bandwidth
  - metadata size should not be used as an excuse to sacrifice utility (such
    as providing checksums over each chunk of a large file)

# Open Questions

- Should the manifest file include checksums of individual file chunks, or just for the whole assembled file?

  - If so, should the chunksize be fixed or dynamic?

- Should the manifest signature format be GnuPG signatures, or those from
  OpenBSD's signify (of which there is a good [golang
  implementation](https://github.com/frankbraun/gosignify)?

- Should the on-disk serialization format be proto3 or json?

# Tool Examples

- `mfer gen` / `mfer gen .`
  - recurses under current directory and writes out an `index.mf`
- `mfer check` / `mfer check .`
  - verifies checksums of all files in manifest, displaying error and
    exiting nonzero if any files are missing or corrupted
- `mfer fetch https://example.com/stuff/`
  - fetches `/stuff/index.mf` and downloads all files listed in manifest,
    optionally resuming any that already exist locally, and assures
    cryptographic integrity of downloaded files.

# Implementation Plan

## Phase One:

- golang module for reusability/embedding
- golang module client providing `mfer` CLI

## Phase Two:

- ES6 or TypeScript module for reusability/embedding
- ES6/TypeScript module client providing `mfer.js` CLI

# Hopes And Dreams

- `aria2c https://example.com/manifestdirectory/`
  - (fetches `https://example.com/manifestdirectory/index.mf`, downloads and
    checksums all files, resumes any that exist locally already)
- `mfer fetch https://example.com/manifestdirectory/`
- a command line option to zero/omit mtime/ctime, as well as manifest
  timestamp, and sort all directory listings so that manifest file
  generation is deterministic/reproducible
- URL format `mfer fetch https://exmaple.com/manifestdirectory/?key=5539AD00DE4C42F3AFE11575052443F4DF2A55C2`
  to assert in the URL which PGP signing key should be used in the manifest,
  so that shared URLs have a cryptographic trust root
- a "well-known" key in the manifest that maps well known keys (could reuse
  the http spec) to specific file paths in the manifest.
  - example: a `berlin.sneak.app.slideshow` key that maps to a json
    slideshow config listing what image paths to show, and for how long, and
    in what order

# Use Cases

## Web Images

I'd like to be able to put a bunch of images into a directory, generate a
manifest, and then point a slideshow client (such as an ambient display, or
a react app with the target directory in a query string arg) at that
statically hosted directory, and have it discover the full list of images
available at that URL.

## Software Distribution

I'd like to be able to download a whole tree of files available via HTTP
resumably by either HTTP or IPFS/BitTorrent without a .torrent file.

## Filesystem Archive Integrity

I use filesystems that don't include data checksums, and I would like a
cryptographically signed checksum file so that I can later verify that a set
of archive files have not been modified, none are missing, and that the
checksums have not been altered in storage by a second party.

## Filesystem-Independent Checksums

I would like to be able to plug in a hard drive or flash drive and, if there
is an `index.mf` in the root, automatically detect missing/corrupted files,
regardless of filesystem format.

# Collaboration

Please email [`sneak@sneak.berlin`](mailto:sneak@sneak.berlin) with your
desired username for an account on this Gitea instance.

I am currently interested in hiring a contractor skilled with the Go
standard library interfaces to specify this tool in full and develop a
prototype implementation.

# See also

## Prior art: Metalink

* https://wiki.mozilla.org/Metalink

* https://en.wikipedia.org/wiki/Metalink

* https://datatracker.ietf.org/doc/html/rfc5854

* https://www.rfc-editor.org/rfc/rfc6249.html