BigInteger: fix some int64 issues
BigInteger: update comments and add test case
Merge branch 'int64-fix'

* int64-fix:
  BigInteger: update comments and add test case
  BigInteger: fix some int64 issues
Add Diffie-Hellman Test to BigInteger Testcase.
Link testDiffieHellmanKeyAgreement to corresponding bug ticket.
Remove randomness from testDiffieHellmanKeyAgreement.