gitoid crate

This crate implements GitOids, Git Object Identifiers, in Rust. The crate is created and maintained by the OmniBOR project, and is intended primarily for that project's use cases.

Usage

The key type of this crate is GitOid, which is parameterized over two traits: HashAlgorithm and ObjectType. Both of these are sealed traits, which means they are only implementable by types found in the gitoid crate itself. To use the GitOid type, you must provide these type parameters like so:

use gitoid::{GitOid, Sha256, Blob};

fn main() {
    let id = GitOid::<Sha256, Blob>::from_str("hello, world");
    println!("{}", id);
}

If you intend to use just a specific instantiation of the GitOid type, you can make this a bit cleaner with a type alias:

use gitoid::{Sha256, Blob};

type GitOid = gitoid::GitOid<Sha256, Blob>;

fn main() {
    let id = GitOid::from_str("hello, world");
    println!("{}", id);
}

Design

This crate is designed to limit the size of the GitOid type in memory, and to place as much work as possible at compile time. To that end, the GitOid type uses a generic array under the hood to ensure the storage buffer is exactly sized to the number of bytes required to store the hashes output by the chosen hash algorithm. The hash algorithm and object type information are also wired up at compile time through method calls on the GitOid type, so they can be accessible at runtime without actually being stored on a per-GitOid-basis.

Git Compatibility

This crate actually diverges from Git's handling of object identifiers in two meaningful ways.

1. The in-memory representation of GitOIDs is different in the gitoid crate and in git itself. In Git, the relevant type is called object_id, and is defined as follows:

   struct object_id {
       unsigned char hash[GIT_MAX_RAWSZ];
       int algo;	/* XXX requires 4-byte alignment */
   };
   

   This type contains a buffer, sized to hold a number of bytes equal to the maximum needed by the largest hash supported by Git (currently 32 bytes as required by SHA-256), along with an integer which is used to indicated the selected hash algorithm. This is ineffecient in the case of hash algorithms whose hash output is smaller than 32 bytes (like SHA-1), and also means that algorithm selection is delegated to runtime. It also doesn't, at the type level or in the embedded data, distinguish between the four types of objects supposed for identification by Git: blobs (files), commits, tags, and trees (directories). The object types are handled by standard formatting rules for producing the input to the hash function which produces the hash (this is what we'll call the "GitOID hash construction") instead.

   So this representation is less space efficient than it could be and omits some information (object type) in favor of an implicit type based on the construction of the input to the hash function.

   In the gitoid crate, by comparison, the only thing we store at runtime is a buffer sized exactly to the number of bytes needed to store the hash output by the chosen hash function, and we use zero-cost compile time features to encode the hash algorithm and object type.

   We do not currently implement handling for object types besides blob, because that's all we need for the OmniBOR project, and would love to add support for tree, commit, and tag in the future.

2. The Git project talks about Git Object IDs being done either with the SHA-1 hash algorithm or with SHA-256, but that's actually not quite true. The SHA-1 algorithm is known to be broken, with the ability for attackers to instigate collisions, and to limit the impact of this breakage, Git by default uses a variant of SHA-1 called SHA-1CD (short for "SHA-1 with Collision Detection). This algorithm checks data being hashed for the presence of some collision-generating vectors of data, and if those are detected, it modifies the hashing in a way that stops the collision from happening.

   For Git's purposes, this white lie is tolerable, because the IDs are never intended for use outside of Git, but for the purpose of OmniBOR we care about being completely accurate about the construction used since IDs are intended to be independently reprodicible by anyone.

   In this crate, we therefore distinguish between the sha1 algorithm and the sha1cd algorithm. This is reflected in the gitoid-scheme URLs generated when using the GitOid type.

Minimum Supported Rust Version (MSRV)

This crate does not maintain a Minimum Supported Rust Version, and generally tracks the latest Rust stable version.

License

This crate is Apache 2.0 licensed.