OSSL_PARAM behaviour for unknown keys

Kurt Roeckx kurt at roeckx.be
Mon Dec 14 21:10:05 UTC 2020

On Mon, Dec 14, 2020 at 08:20:29PM +0100, Dmitry Belyavsky wrote:
> Dear Kurt,
> On Mon, Dec 14, 2020 at 3:59 PM Kurt Roeckx <kurt at roeckx.be> wrote:
> > Hi,
> >
> > doc/man3/OSSL_PARAM.pod current says:
> > Keys that a I<setter> or I<responder> doesn't recognise should
> > simply be ignored. That in itself isn't an error.
> >
> > The intention of that seems to be that you just pass all the data
> > you have, and that it takes data it needs. So you can pass it data
> > that it doesn't need because it's only used in case some other parameter
> > has some specific value. For example, depending on the DRBG mode
> > (HMAC, CTR, HASH) you have different parameters, and you can just
> > pass all the parameters for all the modes.
> >
> > I think for behaviour for a setter is not something that we want,
> > it makes it complicated for applications to check that it will
> > behave properly. I think that in general, if the applications
> > wants to set something and you don't understand it, you should
> > return an error. This is about future proofing the API. For
> > instance, a new version supports a new mode to work in and that
> > needs a new parameter. If it's build against a version that knows
> > about it, but then runs against a version that doesn't know about
> > it, everything will appear to work, but be broken. If we return
> > an error, it will be clear that it's not supported.
> >
> > An alternative method of working is that the application first
> > needs to query that it's supported. And only if it's supported
> > it should call the function. But we don't have an API to query for
> > that. You might be able to ask for which keys you can set, but it
> > doesn't cover which values you can set. I hope we at least return
> > an error for a known key with an unknown value. But it's my
> > understanding that we currently don't always return all supported
> > keys, and that the supported keys can depend on one of the set
> > parameters.
> >
> > I suggest that we change the return value to indicate that all
> > parameters have been used or not. For instance return 1 in case
> > all used, return 2 in case not all used.
> >
> >
> From my GOST implementor's experience, the provider can get a lot of
> parameters.
> Some of them are supported, some of them are not.
> The particular provider is the only subsystem that knows which parameters
> are supported and which are necessary for the operations.
> So the caller can provide some unsupported parameters, some supported and
> some totally wrong for the provider.
> These are the cases that must be distinguishable on the caller side.

If I understand you correctly, what you're saying is that it's
sometimes ok to ignore some parameters. For instance, if you try
to create an RSA object, and you pass it CRT parameters, and the
implementation doesn't do anything with them, it can ignore them
if it wants to.

I would say that the provider should know what those parameters
mean, so that it's not an "unknown key", it just ignores them,
at which points it can say that it understands all the parameters.

Some might argue that they don't want to use something that
doesn't make use of the CRT parameters, but then they probably
shouldn't be using that provider to begin with.

> After that the provided EVP object should be either in a consistent state
> or not, assuming the upcoming operation.

The object should always be in a consistent state. I would prefer
that in case of failure the object is not created (or modified).
Which brings us to some other open points about the API we have. We
should not introduce new APIs where you can modify the state of the
object, so it can not be in a non-consistent state. It's much more
simple to get things correct in that case. But as long as we have
to support old APIs where it can be modified, the prefered
consistent state is to not mofify the object on error. Some APIs make
this very hard, so the other acceptable state is that you can free
the object. With an API that doesn't allow modification, either
you get a complete object, or you get no object.


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