EcKdf documentation unclear

[nwalfield]

The documentation for EcKdf says:

In the current version of this crate, only the null KDF is
supported, which takes no shared data. Therefore 'a can always be inferred
'static.

But there are two methods: null and sha256. sha256's signature is:

fn sha256() -> Self

That is, it doesn't have an elegant way to fail fast. Is sha256 supported? Is the error returned later? Is the above comment out of date?

Thanks!

[hug-dev]

Thanks for the bug report!

Seems that both the documentation and the code might be wrong 😓 And it was me who created the PR here #239

Should we do the following:

• update the documentation to remove the paragraph you cited
• modify the sha256 function so that it is passed the shared data as a Option<&'a [u8]>?

@freedge maybe you also have opinions here since you wanted this to be added in the first place :)

[Jakuje]

One more thing to add:

• test coverage

[nwalfield]

It would be great if sha256 could be fixed and other variants added. I'm personally particularly interested in the CKD_[SHA224|SHA256|SHA384|SHA512]_KDF_SP800 variants, which the OpenPGP protocol uses.