mfer

mfer is a reference implementation library and thin wrapper command-line utility written in Go and first published in 2022 under the WTFPL (public domain) license. It specifies and generates .mf manifest files over a directory tree of files to encapsulate metadata about them (such as cryptographic checksums or signatures over same) to aid in archiving, downloading, and streaming, or mirroring. The manifest files' data is serialized with Google's protobuf serialization format. The structure of these files can be found in the format specification which is included in the project repository.

The current version is pre-1.0 and while the repo was published in 2022, there has not yet been any versioned release. SemVer will be used for releases.

This project was started by @sneak to scratch an itch in 2022 and is currently a one-person effort, though the goal is for this to emerge as a de-facto standard and be incorporated into other software. A compatible javascript library is planned.

Build Status

Participation

The community is as yet nonexistent so there are no defined policies or norms yet. Primary development happens on a privately-run Gitea instance at https://git.eeqj.de/sneak/mfer and issues are tracked there.

Changes must always be formatted with a standard go fmt, syntactically valid, and must pass the linting defined in the repository (presently only the golangci-lint defaults), which can be run with a make lint. The main branch is protected and all changes must be made via pull requests and pass CI to be merged. Any changes submitted to this project must also be WTFPL-licensed to be considered.

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?

• 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 with your desired username for an account on this Gitea instance.

Links

• Repo: https://git.eeqj.de/sneak/mfer
• Issues: https://git.eeqj.de/sneak/mfer/issues

Authors

• @sneak <sneak@sneak.berlin>

License

• WTFPL