ca3e29e802
- Write complete .mf format specification (FORMAT.md) - Fill in all design question answers in TODO.md - Mark completed implementation items in TODO.md - Bump VERSION from 0.1.0 to 1.0.0 in Makefile - Update README to reference FORMAT.md and reflect 1.0 status
143 lines
6.1 KiB
Markdown
143 lines
6.1 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
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optional GPG signatures for integrity verification and optional timestamps
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for metadata preservation.
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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) | Full GPG key ID of the signer |
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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.
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This allows verifying data integrity before decompression.
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## Compression
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The `innerMessage` field is compressed with [Zstandard (zstd)](https://facebook.github.io/zstd/).
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Implementations must enforce a decompression size limit to prevent
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decompression bombs. The reference implementation limits decompressed size to
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256 MB.
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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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Field 304 (`atime`) has been removed from the specification. Access time is
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volatile and non-deterministic; it is not useful for integrity verification.
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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 compressed data)
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Components are separated by hyphens. The signature is produced by GPG over
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this canonical string and stored in the `signature` field of the outer
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message.
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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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- `atime` is never included (field removed from schema)
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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`.
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The `.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: [git.eeqj.de/sneak/mfer](https://git.eeqj.de/sneak/mfer)
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