A bcrypt library for Lua
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A Lua wrapper for OpenBSD's bcrypt.


lua >= 5.1


Many of the files in this repository have been taken from OpenBSD's tree. You should consult individual file headers for specific licensing information. More broadly, everything here is compatible with the ISC license.


$ 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 ) )

Security concerns

Lua will keep plaintext passwords around in memory as part of its string interning mechanism. As far as I'm aware, there's nothing I can do about this.


If you would like to automatically tune the number of rounds to your hardware, you can include a function like:

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

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

        rounds = rounds + 1

This function returns the largest load factor such that bcrypt.digest( str, work ) takes less than t milliseconds (assuming your CPU isn't dodgy).

Note that this will take at least 2 * SAMPLES * t ms to evaluate.


Some operating systems do not provide a method for reliably getting random data from inside a chroot. One workaround for this is to chroot after initialising lua-bcrypt, for example by using lua-setuid.

local setuid = require( "setuid" )
local bcrypt = require( "bcrypt" )

assert( setuid.chroot( "." ) )
assert( not io.open( "/etc/passwd", "r" ) )

print( bcrypt.digest( "adsf", 5 ) )

There are also operating system specific workarounds. On non-bleeding-edge (earlier than 3.17) Linux kernels, you can run:

mkdir /path/to/chroot/dev
mknod -m 644 /path/to/chroot/dev/urandom c 1 9

I have included a test script in test-chroot.lua.