Add length check for JWK key import

[devgianlu]

The PR adds length validation of JWK fields provided during key import to match WPT tests.

See WICG/webcrypto-secure-curves#33 (comment)

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Clone of WICG/webcrypto-secure-curves#38

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[devgianlu]

If there's interest in merging this I can join the WG.

[devgianlu]

@twiss I am now a memeber of the WG, so this requires only one other review I guess.

[twiss]

Great! Sorry, I started writing a review of this at some point but never got around to submitting it. See below.

[twiss]

I think this text is a bit in an awkward middle state where it has a lot of detailed steps, but still doesn't quite state exactly what to do; particularly this step is a bit handwavy in referring to RFC8037. I'm not opposed to being handwavy, but then I'd like to be more succinct as well. The step before this could also be argued to cover this already. Perhaps we can just make that one a bit more explicit by stating something like

                                  If |jwk| does not meet the requirements of
                                  the JWK private key format described in Section 2
                                  of [[RFC8037]], or the encoded key material does not
                                  represent an Ed25519 key of the correct length,
                                  then [= exception/throw =] a {{DataError}}.

and call it a day? (And similarly below.)

[twiss]

Btw, we might want to say something similar for the spki and pkcs8 formats.
If we have to add (more) detailed parsing steps for all of them, I think it'll get a bit unwieldy.

[devgianlu]

I agree it's a bit handwavy, but the spec is generally pretty explicit about the length of things, and I feel like it would a still ambigous when the JWK contains both public and private key material. For example, if the JWK is a private key you need to check both the public and private keys indipendently. So does the WPT test (see Bad key length: importKey(jwk).

Perhaps we can change interpreted to decoded: Let jwkPublic be the x field of jwk decoded according to Section 2 of [RFC8037].

There is no need to have length checks for SPKI and PKCS8 becuse the length is already encoded in the format so parsing will fail for invalid length data. Raw has already a very specific length check.