mfer/docs/FORMAT.md

.mf File Format Specification

Version 1.0

Overview

An .mf file is a binary manifest that describes a directory tree of files, including their paths, sizes, and cryptographic checksums. It supports optional GPG signatures for integrity verification and optional timestamps and file permissions for metadata preservation.

Nothing goes in the 1.0 manifest that 1.0 does not read or write: no field is reserved or kept for later use.

File Structure

An .mf file consists of two parts, concatenated:

1. Magic bytes (8 bytes): the ASCII string ZNAVSRFG
2. Outer message: a Protocol Buffers serialized MFFileOuter message

There is no length prefix or version byte between the magic and the protobuf message. The protobuf message extends to the end of the file.

See mfer/mf.proto for exact field numbers and types.

Outer Message (MFFileOuter)

The outer message contains:

Field	Number	Type	Description
version	101	enum	Must be VERSION_ONE (1)
compressionType	102	enum	Compression of innerMessage; must be COMPRESSION_ZSTD (1)
size	103	int64	Uncompressed size of innerMessage (corruption detection)
sha256	104	bytes	SHA-256 hash of the compressed innerMessage (corruption detection)
uuid	105	bytes	Random v4 UUID; must match the inner message UUID
innerMessage	199	bytes	Zstd-compressed serialized MFFile message
signature	201	bytes (optional)	GPG signature (ASCII-armored or binary)
signer	202	bytes (optional)	Full GPG key ID of the signer
signingPubKey	203	bytes (optional)	Full GPG signing public key

SHA-256 Hash

The sha256 field (104) covers the compressed innerMessage bytes. This allows verifying data integrity before decompression.

Compression

The innerMessage field is compressed with Zstandard (zstd). Implementations must enforce a decompression size limit to prevent decompression bombs. The reference implementation limits decompressed size to 256 MB. It writes zstd frames with a window of at most 8 MiB, the largest window the zstd format recommends decoders support, and refuses frames that ask for a larger one. It also refuses an inner message whose file entries, hashes, timestamps and MIME types, counted at 176, 112, 64 and 16 bytes each, add up to more than 8 times its size.

Inner Message (MFFile)

After decompressing innerMessage, the result is a serialized MFFile (referred to as the manifest):

Field	Number	Type	Description
version	100	enum	Must be VERSION_ONE (1)
files	101	repeated MFFilePath	List of files in the manifest
uuid	102	bytes	Random v4 UUID; must match outer UUID
createdAt	201	Timestamp (optional)	When the manifest was created

File Entries (MFFilePath)

Each file entry contains:

Field	Number	Type	Description
path	1	string	Relative file path (see Path Rules)
size	2	int64	File size in bytes
hashes	3	repeated MFFileChecksum	At least one hash required
mimeType	301	string (optional)	MIME type
mtime	302	Timestamp (optional)	Modification time
ctime	303	Timestamp (optional)	Change time (inode metadata change)
mode	304	uint32	Permission bits (see File Mode)

File Mode

mode holds a file's Unix permission bits, the nine rwx bits, so it is never above 0777 (octal); the setuid, setgid and sticky bits are never recorded. Writers record 0000 unless whoever creates the manifest asks for permissions. 0000, the proto3 default, means no mode was recorded: readers never check or apply it.

The reference implementation records modes when gen or freshen is given --include-permissions. check fails a file whose permission bits differ from a recorded mode other than 0000. fetch sets a recorded mode other than 0000 on each file it writes, and refuses a manifest that records a mode above 0777 before it requests any file.

Path Rules

All path values must satisfy these invariants:

• UTF-8: paths must be valid UTF-8
• Forward slashes: use / as the path separator (never \)
• Relative only: no leading /
• No parent traversal: no .. path segments
• No empty segments: no // sequences
• No trailing slash: paths refer to files, not directories

Implementations must validate these invariants when reading and writing manifests. Paths that violate these rules must be rejected.

Hash Format (MFFileChecksum)

Each checksum is a single bytes multiHash field containing a multihash-encoded value. Multihash is self-describing: the encoded bytes include a varint algorithm identifier followed by a varint digest length followed by the digest itself.

The 1.0 implementation writes SHA-256 multihashes (0x12 algorithm code). Implementations must be able to verify SHA-256 multihashes at minimum.

Signature Scheme

Signing is optional. When present, the signature covers a canonical string constructed as:

ZNAVSRFG-<UUID>-<SHA256>

Where:

• ZNAVSRFG is the magic bytes string (literal ASCII)
• <UUID> is the hex-encoded UUID from the outer message
• <SHA256> is the hex-encoded SHA-256 hash from the outer message (covering compressed data)

Components are separated by hyphens. The signature is produced by GPG over this canonical string and stored in the signature field of the outer message.

Deterministic Serialization

By default, manifests are generated deterministically:

• File entries are sorted by path in lexicographic byte order
• createdAt is omitted unless explicitly requested
• mode is 0000 unless explicitly requested

This ensures that two independent runs over the same directory tree produce byte-identical .mf files (assuming file contents and metadata have not changed).

MIME Type

The recommended MIME type for .mf files is application/octet-stream. The .mf file extension is the canonical identifier.

Reference

• Proto definition: mfer/mf.proto
• Reference implementation: git.eeqj.de/sneak/mfer