Re: Secure tables in Lua?

[David Given <dg@...>]

Dr. Markus Walther wrote:
[...]
> My question therefore to the list: how would one go about replicating 
> this in (extended?) Lua, perhaps first for tables?
>
> My initial thoughts were to use a metatable approach with a new 
> write-only field mt.__secure (or perhaps __protected) guarding R/W 
> access to protected entries of main table t. mt.__secure would hold the 
> key k to unlock t[k] (perhaps generalized to a table of such keys k_i).
> A new builtin name() generates fresh keys.

Nah, you don't need any of that.

function create_secure_table()
  local data = {}
  local mt = {
    __index = function(t, k) return data[k] end,
    __newindex = function(t, k, v) data[k] = v end
  }
  local t = {
    newkey = function() return {} end
  }
  setmetatable(t, mt)
  return t
end

create_secure_table() returns the table. t:newkey() will return a new 
unprintable key. Accessing the table via t[k] should work normally. You 
can't table-walk to find existing keys because they're all safely hidden 
in an upvalue.

In fact, since that the overall purpose of this (if I've understood it 
correctly!) is to hide internal representation, none of this is 
necessary in Lua: hiding data in upvalues like this is far easier than 
fiddling about with tables, and considerably faster. In the above 
example, there's no way the user can get from the opaque object t to the 
table data that actually contains the keys (without using the debug 
library, but if you give that to users, you lose all pretence of 
security anyway).

--
David Given
dg@cowlark.com