CVE-2018-16868

This patchset implements mitigations for CVE-2018-16868 a Bleichenbacher-like attack that makes use of cache side-channel leakage. The mitigations are mostly implemented in Nettle, and GnuTLS has been changed to use a new side-channel silent fucntion exported from Nettle. Nettle >= 3.4.1 is now required.

Paper describing the attack: http://www.wisdom.weizmann.ac.il/~eyalro/project/cat/cat.pdf

Resolves #630 (closed)

Checklist

• Code modified for security issue
• Test suite updated with functionality tests
• Documentation updated / NEWS entry present (for non-trivial changes)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code