RFR: 8281561: Disable http DIGEST mechanism with MD5 by default [v3]

[Michael McMahon]

> Hi,
> 
> Could I get the following change reviewed please, which is to disable the MD5 message digest algorithm by default in the HTTP Digest authentication mechanism? The algorithm can be opted into by setting a new system property "http.auth.digest.reEnabledAlgs" to include the value MD5. The change also updates the Digest authentication implementation to use some of the more secure features defined in RFC7616, such as username hashing and additional digest algorithms like SHA256 and SHA512-256.
> 
> - Michael

Michael McMahon has updated the pull request incrementally with one additional commit since the last revision:

  update after second review round

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7688/files
  - new: https://git.openjdk.java.net/jdk/pull/7688/files/b765d412..3d5ef143

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7688&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7688&range=01-02

  Stats: 160 lines in 12 files changed: 59 ins; 25 del; 76 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7688.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7688/head:pull/7688

PR: https://git.openjdk.java.net/jdk/pull/7688