char-custom

通常のハッシュの３倍

overview

This is a Rust tool that recalculates the hashes of all of the commit ids in a repository using SHA-256 instead of SHA-1.

technical info

Git calculates the hash of a commit by first concatenating the following and then hashing:

1. Commit source tree SHA-1 (subtrees, blobs)
2. Parent SHA-1 hash
3. Author info
4. Committer info
5. Commit message
6. A string containing "commit <size of 1-5 in bytes>NULL"
7. SHA-1 performed on 1-6

This corresponds to, in git commands:

1. git cat-file commit HEAD
6. printf "commit %s\0" $(git cat-file commit HEAD | wc -c
7. pipe to sha1sum

That's all right, but we need to replace the hashes with SHA-256. That appears in three locations: the source tree, the parent commit hash, and the hash of the concatenated view above. The last one is easy: just use sha256sum. The parent commit hash can be similarly replaced in a recursive fashion. However, the git commands don't expose a slot where you can insert a sha256sum pipe after tree construction--we have to do that ourselves.

Tree formation is given in the git source code.