function md5_sumhexa(k)
    local md5_core = require "md5.core"
    k = md5_core.sum(k)
    return (string.gsub(k, ".", 
        function (c) 
            return string.format("%02x", string.byte(c)) 
        end
        )
    )
end
print(md5_sumhexa("Hello World!"))