doas is a minimal replacement for the venerable sudo. It was initially written by Ted Unangst of the OpenBSD project to provide 95% of the features of sudo with a fraction of the codebase.
At the moment only linux with GLIBC or musl libc is supported and tested.
This is not an official port/project from OpenBSD!
As much as possible I've attempted to stick to doas as tedu desired it. As things stand it's essentially just code lifted from OpenBSD with PAM or shadow based authentication glommed on to it.
Compatibility functions in libopenbsd come from openbsd directly (strtonum.c, reallocarray.c, strlcpy.c, strlcat.c), from openssh (readpassphrase.c) or from sudo (closefrom.c).
The PAM and shadow authentication code does not come from the OpenBSD project.
In the Arch Extra repo.