RFR: 8368841: X25519 implementation differs from the specification in RFC 7748 [v6]

[Ben Perez]

> 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.

Ben Perez has updated the pull request incrementally with one additional commit since the last revision:

  Changed params comparison

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29750/files
  - new: https://git.openjdk.org/jdk/pull/29750/files/028e5ba0..82416735

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29750&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29750&range=04-05

  Stats: 4 lines in 1 file changed: 2 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