The code was (as annotated in the source code) adopted from Tom's
LibCrypt (public domain code, [1]). It was neither optimised for speed
nor were any security features added.

[1] http://libtom.org/?page=features&newsitems=5&whatfile=crypt

R=rsc, berengarlehr, albert.strasheim, agl1
CC=golang-dev, schulze
https://golang.org/cl/2687042