OSSL_PARAM behaviour for unknown keys

Dmitry Belyavsky beldmit at gmail.com
Mon Dec 14 19:20:29 UTC 2020

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
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.

After that the provided EVP object should be either in a consistent state
or not, assuming the upcoming operation.
And the possibility to find out whether the state is consistent and
suitable for the upcoming operation or not is a must and should be provided
by an API.

SY, Dmitry Belyavsky
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