Hello,

I think this is a great idea.

As suggested by others, an even better default implementation would be:

class SecretKeysBackend:

    def get_signing_key(self):
        if isinstance(settings.SECRET_KEY, (list, tuple)):
            return settings.SECRET_KEY[0]
        else:
            return settings.SECRET_KEY

    def get_verification_keys(self):
        if isinstance(settings.SECRET_KEY, (list, tuple)):
            return settings.SECRET_KEY
        else:
            return [settings.SECRET_KEY]

Once Django is updated to take advantage of this feature, hat would make key 
rotation practical for every Django user!

(And it seems easier to adjust the semantics of SECRET_KEY than to introduce a 
SECRET_KEYS settings.)

Best regards,

-- 
Aymeric.



> On 10 Nov 2018, at 11:12, Andreas Pelme <andr...@pelme.se> wrote:
> 
> Hi,
> 
> settings.SECRET_KEY can be used for sessions, password resets, form wizards 
> and
> other cryptographic signatures via the signing APIs. Changing SECRET_KEY means
> that all of those will be invalidated and the users will be affected in weird
> ways without really knowing what happened. (Why am I logged out? Where did my
> form submission go? Why does not this password reset link work?). This is
> desirable in case the key is compromised and swift action must be taken.
> 
> There are other situations when it would be nice to change the SECRET_KEY when
> this sudden invalidation is not desirable:
> 
> - When someone leaves a project/company that had access to the production
>  system. After SSH keys/login credentials is revoked the developer could
>  potentially have a copy of the secret key. It is essentially a backdoor with
>  full remote access. It would be wise to rotate the key in those cases.
> 
> - Periodic and automatic rotations of keys to make it less useful in the
>  future.
> 
> The current situation of a single SECRET_KEY makes key rotation impractical. 
> If
> you run a busy site with active users 24/7, there is never a nice time to
> change the SECRET_KEY.
> 
> A solution for this problem would be sign new secrets with a new key while
> still allow signatures made with the old key to be considered valid at the 
> same
> time. Changing keys and having a couple of hours of overlap where signatures
> from both keys are accepted would mitigate most of the user facing problems
> with invalidating sessions, password reset links and form wizard progress.
> 
> You could do this today by implementing your own session backend, message
> storage backend and password reset token generator but that is cumbersome and
> does not work across reusable apps that directly use low level Django signing
> APIs unless they too provide hooks to provide your own secret.
> 
> I propose a pluggable project wide secret key backend
> (settings.SECRET_KEY_BACKEND maybe?) with an API something like:
> 
> class SecretKeyBackend:
>  def get_signing_key(self): …
>  def get_verification_keys(self): ...
> 
> The default (and backward compatible) backend would then be implemented as
> something like:
> 
> class SecretKeySettingsBackend:
>  def get_signing_key(self):
>    return settings.SECRET_KEY
>  def get_verification_keys(self):
>    return [settings.SECRET_KEY]
> 
> django.core.signing.Signer.{sign,unsign} would need to be updated to use this
> backend instead of directly using settings.SECRET_KEY.
> 
> That would solve the problem project wide and work across any third party
> application that uses django.core.signing directly.
> 
> This would open the door for third party secrets backend packages that
> retrieves keys from systems such as Hashicorp Vault, AWS Secrets Manager,
> Google Cloud KMS, Docker Secrets etc.
> 
> Having a method that retrieves the key would allow changes to secret key 
> during
> run time instead of relying on a hard coded setting would allow the key to
> change without restarting the server process.
> 
> Would something like this be worth pursuing? Could it be designed in som other
> way? I could not find any previous discussion/tickets on this and thought it
> would be a good idea to discuss it here before opening a ticket or making an
> attempt at a PR. :)
> 
> Cheers,
> 
> Andreas
> 
> 
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