[Cryptography-dev] Destroying keys and secrets…

[John Pacific]

Afaik, there is no reliable way to do this in Python.

I have taken some time to implement some OpenSSL methods that at least
clear the memory on private numbers before freeing it in BIGNUMBER
operations.

See the PR here: https://github.com/pyca/cryptography/pull/4072

However, as it comes for the Elliptic curve

On Feb 16, 2018 14:54, "Matt Bullock" <matt.s.b.42 at gmail.com> wrote:

> This is an area I've spent a fairly significant amount of time
> investigating. My conclusion was "no, there is no reliable way to do this",
> but I'd love to be proven wrong.
>
> The fundamental problem is that how exactly <Python implementation> stores
> variables in memory is not defined as part of the implementation
> requirements, and so even if you can figure out a way to wipe a value from
> memory on one implementation, there is no guarantee that it will work on
> any other implementation.
>
> With CPython, you /can/ technically use "ctypes" to read and write memory
> directly, and by introspecting a similar object (say, an arbitrary string
> with known value), you can reach in and overwrite data that is /probably/
> the data you're trying to wipe out.
>
> Unfortunately, this approach makes a lot of assumptions about how CPython
> actually structures variables in memory: assumptions that may or may not be
> valid today and may or may not be valid in the future (or the past, for
> that matter). /At best/ the only thing you can reasonably say is that this
> /probably/ works for the specific version of the specific implementation
> you have tested it on, and that it /might/ not have adverse side effects.
>
> I've also toyed with the idea of making a C extension analogous to the
> SecretKey structure from Java, which never lets the actual key material out
> of the structure, but in order to actually do anything with the key
> material it still needs to surface to Python at some point, which puts us
> back in the same position we started with. ..unless we were to re-implement
> all of pyca/cryptography in Cython, but that's a thought for a different
> time...
>
>    --Matt
>
> On Fri, Feb 16, 2018 at 1:25 PM Andrew Donoho <awd at ddg.com> wrote:
>
>> Gentlefolk,
>>
>>
>>
>>         Apparently, my Google-fu is weak and I come seeking advice.
>>
>>         Secret management is important. In particular, I want to make
>> sure that any secrets I decrypt are erased from memory before the storage
>> is reclaimed by the VM. In other environments, I would just dig into each
>> object until I get the pointer for the storage and then bang zeros, ones
>> and randomness into the block. Then garbage collection would proceed apace.
>>
>>
>>
>> Here’s an example from the cryptography documentation, <
>> https://cryptography.io/en/latest/hazmat/primitives/symmetric-encryption/
>> >:
>>
>> >>> import os
>> >>> from cryptography.hazmat.primitives.ciphers import Cipher,
>> algorithms, modes
>> >>> from cryptography.hazmat.backends import default_backend
>> >>> backend = default_backend()
>> >>> key = os.urandom(32)
>> >>> iv = os.urandom(16)
>> >>> cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=backend)
>> >>> encryptor = cipher.encryptor()
>> >>> ct = encryptor.update(b"a secret message") + encryptor.finalize()
>> >>> decryptor = cipher.decryptor()
>> >>> decryptor.update(ct) + decryptor.finalize()
>> 'a secret message’
>>
>>
>> The `key` above is a `bytes` object. It has storage somewhere. Even
>> though it is a read-only Python object, I can pierce the abstraction, if I
>> have to, with C.
>>
>> My question is: has someone else already done so and published the
>> handful of methods needed?
>>
>> If not, should this be an API added to cryptography?
>>
>>
>>
>> Anon,
>> Andrew
>> ____________________________________
>> Andrew W. Donoho
>> Donoho Design Group, L.L.C.
>> awd at DDG.com, +1 (512) 750-7596 <(512)%20750-7596>, twitter.com/adonoho
>>
>> Doubt is not a pleasant condition, but certainty is absurd.
>>     — Voltaire
>>
>>
>>
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>
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