187 lines
7.2 KiB
Markdown
187 lines
7.2 KiB
Markdown
# mfer
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Manifest file generator and checker.
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# Build Status
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[](https://drone.datavi.be/sneak/mfer)
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# Problem Statement
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Given a plain URL, there is no standard way to safely and programmatically
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download everything "under" that URL path. `wget -r` can traverse directory
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listings if they're enabled, but every server has a different format, and
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this does not verify cryptographic integrity of the files, or enable them to
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be fetched using a different protocol other than HTTP/s.
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Currently, the solution that people are using are sidecar files in the
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format of `SHASUMS` checksum files, as well as a `SHASUMS.asc` PGP detached
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signature. This is not checksum-algorithm-agnostic and the sidecar file is
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not always consistently named.
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Real issues I face:
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- when I plug in an ExFAT hard drive, I don't know if any files on the
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filesystem are corrupted or missing
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- current ad-hoc solution are `SHASUMS`/`SHASUMS.asc` files
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- when I want to mirror an HTTP archive, I have to use special tools like
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debmirror that understand the archive format
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- the debian repository metadata structure is hot garbage
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- when I download a large file via HTTP, I have no way of knowing if the
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file content is what it's supposed to be
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# Proposed Solution
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A standard, a manifest file format, and a tool for generating same.
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The manifest file would be called `index.mf`, and the tool for generating such would be called `mfer`.
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The manifest file would do several important things:
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- have a standard filename, so if given
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`https://example.com/downloadpackage/` one could fetch
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`https://example.com/downloadpackage/index.mf` to enumerate the full
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directory listing.
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- contain a version field for extensibility
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- contain structured data (protobuf, json, or cbor)
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- provide an inner signed container, so that the manifest file itself can
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embed a signature and a public key alongside in a single file
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- contain a list of files, each with a relative path to the manifest
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- contain manifest timestamp
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- contain ctime/mtime information for files so that file metadata can be
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preserved
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- contain cryptographic checksums in several different algorithms for each
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file
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- probably encoded with multihash to indicate algo + hash
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- sha256 at the minimum
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- would be nice to include an IPFS/IPLD CIDv1 root hash for each file,
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which likely involves doing an ipfs file object chunking
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- maybe even including the complete IPFS/IPLD directory tree objects and
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chunklists?
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- this is because generating an `index.mf` does not imply publishing on
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ipfs at that time
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- maybe a bittorrent chunklist for torrent client compatibility? perhaps a
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top-level infohash for the whole manifest?
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# Design Goals
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- Replace SHASUMS/SHASUMS.asc files
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- be easy to download/resume a whole directory tree published via HTTP
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- be easy to use across protocols (given an HTTPS url, fetch manifest, then
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download file contents via bittorrent or ipfs)
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- not strongly coupled to HTTP use case, should not require special hosting,
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content types, or HTTP headers being sent
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# Non-Goals
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- Manifest generation speed
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- likely involves IPFS chunking, bittorrent chunking, and several
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different cryptographic hash functions over the entirety of each and
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every file
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- Small manifest file size (within reason)
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- 30MiB files are "small" these days, given modern storage/bandwidth
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- metadata size should not be used as an excuse to sacrifice utility (such
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as providing checksums over each chunk of a large file)
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# Open Questions
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- Should the manifest file include checksums of individual file chunks, or just for the whole assembled file?
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- If so, should the chunksize be fixed or dynamic?
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- Should the manifest signature format be GnuPG signatures, or those from
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OpenBSD's signify (of which there is a good [golang
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implementation](https://github.com/frankbraun/gosignify)?
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- Should the on-disk serialization format be proto3 or json?
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# Tool Examples
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- `mfer gen` / `mfer gen .`
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- recurses under current directory and writes out an `index.mf`
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- `mfer check` / `mfer check .`
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- verifies checksums of all files in manifest, displaying error and
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exiting nonzero if any files are missing or corrupted
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- `mfer fetch https://example.com/stuff/`
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- fetches `/stuff/index.mf` and downloads all files listed in manifest,
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optionally resuming any that already exist locally, and assures
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cryptographic integrity of downloaded files.
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# Implementation Plan
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## Phase One:
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- golang module for reusability/embedding
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- golang module client providing `mfer` CLI
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## Phase Two:
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- ES6 or TypeScript module for reusability/embedding
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- ES6/TypeScript module client providing `mfer.js` CLI
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# Hopes And Dreams
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- `aria2c https://example.com/manifestdirectory/`
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- (fetches `https://example.com/manifestdirectory/index.mf`, downloads and
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checksums all files, resumes any that exist locally already)
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- `mfer fetch https://example.com/manifestdirectory/`
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- a command line option to zero/omit mtime/ctime, as well as manifest
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timestamp, and sort all directory listings so that manifest file
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generation is deterministic/reproducible
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- URL format `mfer fetch https://exmaple.com/manifestdirectory/?key=5539AD00DE4C42F3AFE11575052443F4DF2A55C2`
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to assert in the URL which PGP signing key should be used in the manifest,
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so that shared URLs have a cryptographic trust root
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- a "well-known" key in the manifest that maps well known keys (could reuse
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the http spec) to specific file paths in the manifest.
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- example: a `berlin.sneak.app.slideshow` key that maps to a json
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slideshow config listing what image paths to show, and for how long, and
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in what order
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# Use Cases
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## Web Images
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I'd like to be able to put a bunch of images into a directory, generate a
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manifest, and then point a slideshow client (such as an ambient display, or
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a react app with the target directory in a query string arg) at that
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statically hosted directory, and have it discover the full list of images
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available at that URL.
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## Software Distribution
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I'd like to be able to download a whole tree of files available via HTTP
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resumably by either HTTP or IPFS/BitTorrent without a .torrent file.
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## Filesystem Archive Integrity
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I use filesystems that don't include data checksums, and I would like a
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cryptographically signed checksum file so that I can later verify that a set
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of archive files have not been modified, none are missing, and that the
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checksums have not been altered in storage by a second party.
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## Filesystem-Independent Checksums
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I would like to be able to plug in a hard drive or flash drive and, if there
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is an `index.mf` in the root, automatically detect missing/corrupted files,
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regardless of filesystem format.
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# Collaboration
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Please email [`sneak@sneak.berlin`](mailto:sneak@sneak.berlin) with your
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desired username for an account on this Gitea instance.
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I am currently interested in hiring a contractor skilled with the Go
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standard library interfaces to specify this tool in full and develop a
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prototype implementation.
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# See also
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## Prior art: Metalink
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* https://wiki.mozilla.org/Metalink
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* https://en.wikipedia.org/wiki/Metalink
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* https://datatracker.ietf.org/doc/html/rfc5854
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* https://www.rfc-editor.org/rfc/rfc6249.html |