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check and fetch --require-signature compared the required fingerprint with the first key in the manifest's embedded public key block, while gpg accepted a good signature by any key in that block. Loading a signed manifest now refuses one whose embedded block holds more than one primary key, counted as gpg reads the block, or whose signer field is not the primary key fingerprint gpg reports for the signature. --require-signature compares with the signer field, which loading has checked. Signing names and embeds the key gpg reports it signed with, so a key ID matching several keys still writes a manifest that loads. docs/FORMAT.md states what a verifier checks. Model: opus-5-5
173 lines
7.8 KiB
Markdown
173 lines
7.8 KiB
Markdown
# .mf File Format Specification
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Version 1.0
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## Overview
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An `.mf` file is a binary manifest that describes a directory tree of files,
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including their paths, sizes, and cryptographic checksums. It supports optional
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GPG signatures for integrity verification and optional timestamps and file
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permissions for metadata preservation.
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Nothing goes in the 1.0 manifest that 1.0 does not read or write: no field is
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reserved or kept for later use.
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## File Structure
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An `.mf` file consists of two parts, concatenated:
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1. **Magic bytes** (8 bytes): the ASCII string `ZNAVSRFG`
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2. **Outer message**: a Protocol Buffers serialized `MFFileOuter` message
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There is no length prefix or version byte between the magic and the protobuf
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message. The protobuf message extends to the end of the file.
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See [`mfer/mf.proto`](../mfer/mf.proto) for exact field numbers and types.
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## Outer Message (`MFFileOuter`)
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The outer message contains:
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| Field | Number | Type | Description |
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| ----------------- | ------ | ---------------- | ------------------------------------------------------------------------ |
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| `version` | 101 | enum | Must be `VERSION_ONE` (1) |
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| `compressionType` | 102 | enum | Compression of `innerMessage`; must be `COMPRESSION_ZSTD` (1) |
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| `size` | 103 | int64 | Uncompressed size of `innerMessage` (corruption detection) |
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| `sha256` | 104 | bytes | SHA-256 hash of the **compressed** `innerMessage` (corruption detection) |
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| `uuid` | 105 | bytes | Random v4 UUID; must match the inner message UUID |
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| `innerMessage` | 199 | bytes | Zstd-compressed serialized `MFFile` message |
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| `signature` | 201 | bytes (optional) | GPG signature (ASCII-armored or binary) |
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| `signer` | 202 | bytes (optional) | Fingerprint of the signing key |
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| `signingPubKey` | 203 | bytes (optional) | Full GPG signing public key |
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### SHA-256 Hash
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The `sha256` field (104) covers the **compressed** `innerMessage` bytes. This
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allows verifying data integrity before decompression.
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## Compression
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The `innerMessage` field is compressed with
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[Zstandard (zstd)](https://facebook.github.io/zstd/). Implementations must
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enforce a decompression size limit to prevent decompression bombs. The reference
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implementation limits decompressed size to 256 MB. It writes zstd frames with a
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window of at most 8 MiB, the largest window the zstd format recommends decoders
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support, and refuses frames that ask for a larger one. It also refuses an inner
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message whose file entries, hashes, timestamps and MIME types, counted at 176,
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112, 64 and 16 bytes each, add up to more than 8 times its size.
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## Inner Message (`MFFile`)
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After decompressing `innerMessage`, the result is a serialized `MFFile`
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(referred to as the manifest):
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| Field | Number | Type | Description |
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| ----------- | ------ | --------------------- | ------------------------------------- |
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| `version` | 100 | enum | Must be `VERSION_ONE` (1) |
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| `files` | 101 | repeated `MFFilePath` | List of files in the manifest |
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| `uuid` | 102 | bytes | Random v4 UUID; must match outer UUID |
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| `createdAt` | 201 | Timestamp (optional) | When the manifest was created |
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## File Entries (`MFFilePath`)
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Each file entry contains:
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| Field | Number | Type | Description |
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| ---------- | ------ | ------------------------- | ----------------------------------- |
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| `path` | 1 | string | Relative file path (see Path Rules) |
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| `size` | 2 | int64 | File size in bytes |
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| `hashes` | 3 | repeated `MFFileChecksum` | At least one hash required |
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| `mimeType` | 301 | string (optional) | MIME type |
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| `mtime` | 302 | Timestamp (optional) | Modification time |
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| `ctime` | 303 | Timestamp (optional) | Change time (inode metadata change) |
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| `mode` | 304 | uint32 | Permission bits (see File Mode) |
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## File Mode
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`mode` holds a file's Unix permission bits, the nine `rwx` bits, so it is never
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above `0777` (octal); the setuid, setgid and sticky bits are never recorded.
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Writers record `0000` unless whoever creates the manifest asks for permissions.
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`0000`, the proto3 default, means no mode was recorded: readers never check or
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apply it.
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The reference implementation records modes when `gen` or `freshen` is given
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`--include-permissions`. `check` fails a file whose permission bits differ from
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a recorded mode other than `0000`. `fetch` sets a recorded mode other than
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`0000` on each file it writes, and refuses a manifest that records a mode above
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`0777` before it requests any file.
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## Path Rules
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All `path` values must satisfy these invariants:
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- **UTF-8**: paths must be valid UTF-8
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- **Forward slashes**: use `/` as the path separator (never `\`)
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- **Relative only**: no leading `/`
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- **No parent traversal**: no `..` path segments
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- **No empty segments**: no `//` sequences
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- **No trailing slash**: paths refer to files, not directories
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Implementations must validate these invariants when reading and writing
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manifests. Paths that violate these rules must be rejected.
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## Hash Format (`MFFileChecksum`)
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Each checksum is a single `bytes multiHash` field containing a
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[multihash](https://multiformats.io/multihash/)-encoded value. Multihash is
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self-describing: the encoded bytes include a varint algorithm identifier
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followed by a varint digest length followed by the digest itself.
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The 1.0 implementation writes SHA-256 multihashes (`0x12` algorithm code).
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Implementations must be able to verify SHA-256 multihashes at minimum.
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## Signature Scheme
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Signing is optional. When present, the signature covers a canonical string
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constructed as:
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```
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ZNAVSRFG-<UUID>-<SHA256>
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```
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Where:
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- `ZNAVSRFG` is the magic bytes string (literal ASCII)
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- `<UUID>` is the hex-encoded UUID from the outer message
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- `<SHA256>` is the hex-encoded SHA-256 hash from the outer message (covering
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compressed data)
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Components are separated by hyphens. The signature is produced by GPG over this
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canonical string and stored in the `signature` field of the outer message. The
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signing key's public key goes in `signingPubKey` and its fingerprint, in hex, in
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`signer`.
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A verifier accepts a signed manifest only if `signingPubKey` holds exactly one
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primary key, `signature` is one good signature over the canonical string made by
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that key (or one of its subkeys), and `signer` is that key's fingerprint. The
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reference implementation refuses to load a manifest that fails these checks;
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`check` and `fetch` given `--require-signature` then compare the required
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fingerprint with `signer`.
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## Deterministic Serialization
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By default, manifests are generated deterministically:
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- File entries are sorted by `path` in **lexicographic byte order**
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- `createdAt` is omitted unless explicitly requested
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- `mode` is `0000` unless explicitly requested
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This ensures that two independent runs over the same directory tree produce
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byte-identical `.mf` files (assuming file contents and metadata have not
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changed).
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## MIME Type
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The recommended MIME type for `.mf` files is `application/octet-stream`. The
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`.mf` file extension is the canonical identifier.
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## Reference
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- Proto definition: [`mfer/mf.proto`](../mfer/mf.proto)
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- Reference implementation:
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[git.eeqj.de/sneak/mfer](https://git.eeqj.de/sneak/mfer)
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