RFR: 8271566: DSA signature length value is not accurate in P11Signature

[Martin Balao]

As described in JDK-8271566 [1], this patch proposal is intended to fix a problem that arises when using DSA keys that have a 256-bits (or larger) G parameter for signatures (either signing or verifying). There were some incorrect assumptions and hard-coded length values in the code before. Please note that, for example, the tuple (2048, 256) for DSA is valid according to FIPS PUB 186-4.

Beyond the specific issues in signatures, I decided to provide a broader solution and enable key parameter retrieval for other key types (EC, DH) when possible. This is, when the key is not sensitive. One thing that I should note here is that token keys (those that have the CKA_TOKEN attribute equal to 'true') are considered sensitive in this regard, at least by the NSS Software Token implementation. I don't have access to other vendor implementations but if there is any concern, we can adjust the constraint to NSS-only. However, I'm not sure which use-case would require to get private keys out of a real token, weakening its security. I'd be more conservative here and not query the values if not sure that it will succeed.

No regressions found in jdk/sun/security/pkcs11. A new test added: LargerDSAKey.

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[1] - https://bugs.openjdk.java.net/browse/JDK-8271566

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Commit messages:
 - 8271566: DSA signature length value is not accurate in P11Signature

Changes: https://git.openjdk.java.net/jdk/pull/4961/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4961&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271566
  Stats: 242 lines in 3 files changed: 169 ins; 6 del; 67 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4961.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4961/head:pull/4961

PR: https://git.openjdk.java.net/jdk/pull/4961