RFR: 8368841: X25519 implementation differs from the specification in RFC 7748

Tue Feb 17 03:15:43 UTC 2026

Our implementation of X25519 does not zero the MSB of the `u` coordinate as required by RFC 7748. Currently the test `jdk/sun/security/ec/xec/TestXDH.java` zeroes the bit manually while parsing test vectors, which is likely why this went uncaught.

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Commit messages:
 - Changed XDHPublicKeyImpl to zero the MSB of  in accordance with RFC 7748

Changes: https://git.openjdk.org/jdk/pull/29750/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29750&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368841
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29750.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29750/head:pull/29750

PR: https://git.openjdk.org/jdk/pull/29750