SHA-1 collision attacks are not detected

Summary

gitoxide uses SHA-1 hash implementations without any collision detection, leaving it vulnerable to hash collision attacks.

Details

gitoxide uses the sha1_smol or sha1 crate, both of which implement standard SHA-1 without any mitigations for collision attacks. This means that two distinct Git objects with colliding SHA-1 hashes would break the Git object model and integrity checks when used with gitoxide.

The SHA-1 function is considered cryptographically insecure. However, in the wake of the SHAttered attacks, this issue was mitigated in Git 2.13.0 in 2017 by using the sha1collisiondetection algorithm by default and producing an error when known SHA-1 collisions are detected. Git is in the process of migrating to using SHA-256 for object hashes, but this has not been rolled out widely yet and gitoxide does not support SHA-256 object hashes.

PoC

The following program demonstrates the problem, using the two SHAttered PDFs:

use sha1_checked::{CollisionResult, Digest};

fn sha1_oid_of_file(filename: &str) -> gix::ObjectId {
    let mut hasher = gix::features::hash::hasher(gix::hash::Kind::Sha1);
    hasher.update(&std::fs::read(filename).unwrap());
    gix::ObjectId::Sha1(hasher.digest())
}

fn sha1dc_oid_of_file(filename: &str) -> Result {
    // Matches Git’s behaviour.
    let mut hasher = sha1_checked::Builder::default().safe_hash(false).build();
    hasher.…