8386600: Fix comparison checks in DHasKEM

[rm-gh-8]

Backporting JDK-8386600: Fix comparison checks in DHasKEM.

This PR fixes the X25519/X448 all-zero shared secret validation in DHasKEM.

For parity with Oracle JDK.

Ran related tests on linux-x64, linux-aarch64, macos-aarch64 and windows-x64:

make test TEST=test/jdk/javax/net/ssl
make test TEST=test/jdk/sun/security

Results:

windows-x64-specific-test.log
windows-x64-specific-2-test.log
macos-aarch64-specific-test.log
macos-aarch64-specific-2-test.log
linux-x64-specific-test.log
linux-x64-specific-2-test.log
linux-aarch64-specific-test.log
linux-aarch64-specific-2-test.log

---

• I confirm that I make this contribution in accordance with the OpenJDK Interim AI Policy.

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8386600 needs maintainer approval

Integration blocker

 ⚠️ Dependency #665 must be integrated first

Issue

• JDK-8386600: Fix comparison checks in DHasKEM (Bug - P3 - Requested)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk25u-dev.git pull/667/head:pull/667
$ git checkout pull/667

Update a local copy of the PR:
$ git checkout pull/667
$ git pull https://git.openjdk.org/jdk25u-dev.git pull/667/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 667

View PR using the GUI difftool:
$ git pr show -t 667

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk25u-dev/pull/667.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back rmesde! A progress list of the required criteria for merging this PR into pr/665 will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[mlbridge]

Webrevs

• 00: Full (ad35a03e)

[rm-gh-8]

/approval request for backport of JDK-8386600: Fix comparison checks in DHasKEM.

This PR fixes the X25519/X448 all-zero shared secret validation in DHasKEM.

For parity with Oracle JDK.

Low Risk — the change fixes a dead condition, but since the all-zero check never executed before, there is no risk of regression to existing handshake behavior.

[openjdk]

@rm-gh-8
8386600: The approval request has been created successfully.