Update crypto.md to correct description of decipher.setAuthTag

[jbuhacoff]

Calling decipher.setAuthTag after decipher.update will result in an error like Unsupported state or unable to authenticate data. The example code in CCM mode is correct, but to demonstrate the mistake in the documentation you can take the same example and move the setAuthTag call to in between update and final you will see the error.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[mscdex]

That's not quite correct since only CCM mode needs to have setAuthTag() before update(). For GCM and OCB modes you can have setAuthTag() any time before final() (in fact, we already have tests for this).

[jbuhacoff]

Right, that should be documented.

I suggested changing it to mention update instead of final because I think that would work in all cases. If that's not desirable, we should at least mention that for CCM it actually needs to be called before decipher.update().

[mscdex]

I think adding a note about the differences between the modes with regard to calling setAuthTag() would be best. For users of the GCM and OCB modes, it could be beneficial to call setAuthTag() at a later point in time, so it's best to continue to let them know they can do that.

[jbuhacoff]

Updated the pull request to address comments by @mscdex, please check it.

[mscdex]

I would just modify this sentence instead to clarify, something like:

The `decipher.setAuthTag()` method must be called before
[`decipher.update()`][] for `CCM` mode or before [`decipher.final()`][] for
`GCM` and `OCB` modes. `decipher.setAuthTag()` can only be called once.

[jbuhacoff]

Great, thanks! I just pushed a correction with this advice.

[mscdex]

/cc @nodejs/crypto to make sure this is the intended behavior

[bnoordhuis]

Thanks, that's correct. GCM and OCB are online AEADs, CCM is not.

[addaleax]

Landed in d135b50, thanks for the PR! 🎉