Git Commit Vanity Hash Solver

Neat tool to find a 'vanity' hash for a given git commit. Make all your commits hashes start with the prefix c0ffee, cafe, badc0de5 or whatever makes you happy!

Install

go install github.com/trichner/gitc0ffee@latest

Usage

# do a normal git commit
$ git commit -am '...'

# update the last commit with a vanity hash
$ gitc0ffee --update-ref --prefix c0ffee

Will this break git tooling?

Maybe. Not all tooling deals well with prefix collisions. Some tools just deal with short-revisions (7 characters) and may therefore break.

How fast is it?

• 6 character prefix: less than a second
• 8 character prefix: in the order of one or more minutes

Measured on a MacBook Pro 16' 2021 with an M1 Max. Slightly slower on an AMD Ryzen 7 5800X.

Why not use the GPU?

Using the GPU is a lot more effort and a lot less portable, since it takes less than a second to brute force the c0ffee prefix there is no need for anything fancier.

The solver implementation can easily be extended though and as a matter of fact there are already at least three available:

singlethreaded  - plain Go implementation of single thread brute force
concurrent      - concurrent version of the singlethreaded solver
native          - concurrent solver with hot-loop written in C,
                  slightly faster than 'concurrent' solver

# use:
gitc0ffee --solver <solver> ...

What prefix should I choose?

All even-length hexadecimal prefix will do ([0-9a-f]{0,40}), for cool inspiration see Hexspeak. Other ideas are repetions or sequences, e.g. 0001, 0002, ...

Note that the longer the prefix is, the longer cracking will take. Prefixes beyond 8 characters may not finish in useful time.

Implementation Details

Conceptually it roughly works as follows:

1. Get the latest commit digest (git rev-parse HEAD).
2. Parse the raw object (git cat-file -p <digest>).
3. Add an additional coffeesalt header to the commit object and tweak the salt value until a prefix collision is found. This is the actual brute-forcing.
4. Write the new commit object to the git store (git hash-object -t commit --stdin).
5. (optional) Update the current branch the new commit object (git update-ref HEAD <new digest>).

Previous Work & Inspirations

There are quite a few similar tools. Some are either a bit more on the proof-of-concept side or might need specific GPU features.

Wishlist

• use CPU accelerated assembly, see also Linux Implementation
• GPU accelerated solver - OpenCL or CUDA