$ luarocks install bcrypt

local bcrypt = require( "bcrypt" )

-- Bigger numbers here will make your digest exponentially harder to compute
local log_rounds = 9

local digest = bcrypt.digest( "password", log_rounds )
assert( bcrypt.verify( "password", digest ) )

function bcrypt.tune( t )
	local SAMPLES = 10
	local rounds = 5

	while true do
		local total = 0

		for i = 1, SAMPLES do
			local start = os.clock()
			bcrypt.digest( "asdf", rounds )
			local delta = os.clock() - start

			total = total + delta
		end

		if ( total / SAMPLES ) * 1000 >= t then
			return rounds - 1
		end

		rounds = rounds + 1
	end
end