[Python-ideas] Secure string disposal (maybe other inmutable seq types too?)

Stephan Houben stephanh42 at gmail.com
Sat Jun 23 09:57:50 EDT 2018

Would it not be much simpler and more secure to just disable core dumps?

/etc/security/limits.conf on Linux.

If the attacker can cause and read a core dump, the game seems over anyway
since sooner or later he will catch the core dump at a time the string was
not yet deleted.


Op za 23 jun. 2018 02:32 schreef Ezequiel Brizuela [aka EHB or qlixed] <
qlixed at gmail.com>:

> As all the string in python are immutable, is impossible to overwrite the
> value or to make a "secure disposal" (overwrite-then-free) of a string
> using something like:
> >>> a = "something to hide"
> >>> a =  "x"*len(a)
> This will lead on the process memory "something to hide" and "x" repeated
> len(a) times.
> - Who cares? Why is this relevant?
>   Well if you handle some sensitive information like CC numbers,
> Passwords, PINs, or other kind of information you wanna minimize the chance
> of leaking any of it.
> - How this "leak" can happen?
>   If you get a core/memory dump of an app handling sensitive information
> you will get all the information on that core exposed!
> - Well, so what we can do about this?
>   I propose to make the required changes on the string objects to add an
> option to overwrite the underlying buffer. To do so:
>   * Add a wiped as an attribute that is read-only to be set when the
> string is overwrited.
>   * Add a wipe() method that overwrite the internal string buffer.
> So this will work like this:
> >>> pwd =getpass.getpass('Set your password:') # could be other sensitive
> data.
> >>> encrypted_pwd = crypt.crypt(pwd)  # crypt() just as example.
> >>> pwd.wiped  # Check if pwd was wiped.
> False
> >>> pwd.wipe()  # Overwrite the underlying buffer
> >>> pwd.wiped  # Check if pwd was wiped.
> True
> >>> print(pwd)  # Print noise (or empty str?)
> >>> del pwd  # Now is in hands of the GC.
> The wipe method immediately overwrite the underlying string buffer,
> setting wiped as True for reference so if the string is further used this
> can be checked to confirm that the change was made by a wipe and not by
> another procedure. Also initially the idea is to use unicode NULL datapoint
> to overwrite the string, but this could be change to let the user
> parametrize it over wipe() method.
> An alternative to this is to add a new exception "WipedError" that could
> be throw where the string is accessed again, but I found this method too
> disruptive to implement for a normal/standard string workflow usage.
> Quick & Dirty FAQ:
> - You do it wrong!, the correct code to do that in a secure way is:
> >>> pwd = crypt.crypt(getpass.getpass('Set your password'))
> Don't you know that fool?
>   Well no, the code still generate a temporary string in memory to pass to
> crypt. But now this string is lying there and can't be accessed for an
> overwrite with wipe()
> - Why not create a new type like in C# or Java?
>   I see that this tend to disrupt the usual workflow of string usage. Also
> the idea here is not to offer secure storage of string in memory because
> there is already a few mechanism to achieve with the current Python base. I
> just want to have the hability to overwrite the buffer.
> - Why don't use one of the standard algorithms to overwrite like DoD5220
> or MIL-STD-414?
>   This kind of standard usually are oriented for usage on persistent
> storage, specially on magnetic media for where the data could be "easily"
> recoverd. But this could ve an option that could be implemented adding the
> option to plug a function that do the overwrite work inside the wipe method.
> - This is far beyond of the almost implementation-agnostic definition of
> the python lang. How about to you make a module with this functionality and
> left the lang as is?
>   Well I already do it:
> https://github.com/qlixed/python-memwiper/
>   But i hit a lot of problems in the road, I was working on me free time
> over the last year on this and make it "almost" work, but that is not
> relevant to the proposal.
>   I think that this kind of security things needs to be tackled from
> within the language itself specially when the lang have GC. I firmly
> believe that the security and protections needs to be part of the "with
> batteries" offer of Python. And I think that this is one little thing that
> could help a lot to secure our apps.
>   Let me know what do you think!
> ~ Ezequiel (Ezekiel) Brizuela [ aka Qlixed ] ~
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