crypto: fix non-multiple of 8 in SubtleCrypto.deriveBits

[panva]

A WPT update made me look into this.

From the Node.js docs:

The Node.js implementation requires that length, when a number, is a multiple
of 8.

This was never true, instead the implementation returned the closest full byte length.

At the moment the browser implementations do the following

• Chromium aligns with this PR (and the updated WPTs)
• Firefox throws DataError
• Safari aligns with Node.js prior to this PR

There's no interop on this in the first place and there's a pending decision around disallowing truncation in ECDH/X25519/X448 altogether in a future spec update.

Given that this is in my opinion a semver-major PRs that contain breaking changes and should be released in the next major version. change I would rather we only have to do one, i.e. disallow truncation when the spec changes in a major, or fix the implementation with this PR in a major. We've got time to figure out what to do in time for v24.x but i'm opening this to ping @nodejs/crypto and @nodejs/web-standards

[nodejs-github-bot]

Review requested:

• @nodejs/crypto

[panva]

@jasnell do you remember what lead to the current implementation which follows neither the docs nor the spec?

[jasnell]

@panva ... sorry! I just spotted this one!

do you remember what lead to the current implementation which follows neither the docs nor the spec?

I think it's just a bug.

[github-actions]

Failed to start CI

   ⚠  Something was pushed to the Pull Request branch since the last approving review.
   ✘  Refusing to run CI on potentially unsafe PR

https://github.com/nodejs/node/actions/runs/11860937753

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/63558/

[panva]

Firefox has updated their implementation, i think it's alright to follow suit with this. Question is whether major or not?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.42%. Comparing base (b02cd41) to head (af40b70).
Report is 207 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #55296      +/-   ##
==========================================
- Coverage   88.42%   88.42%   -0.01%     
==========================================
  Files         654      654              
  Lines      187852   187862      +10     
  Branches    36134    36136       +2     
==========================================
+ Hits       166102   166109       +7     
- Misses      14989    15001      +12     
+ Partials     6761     6752       -9     

Files with missing lines	Coverage Δ
lib/internal/crypto/diffiehellman.js	97.59% <100.00%> (+0.06%)	⬆️

... and 35 files with indirect coverage changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no suggestions.

[jasnell]

I still consider this a bug fix and not a major change.