mfer

Manifest file generator and checker.

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?

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.

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.