Revert "minor performance improvement"

This reverts commit fd774760f95de86d9e235659fba0fe2869da4bba.
Revert "measure time of GetSharedSecret"

This reverts commit bce0a10acb752b5a40b0422f008d3b459ea5fbc0.
Revert "using constant time implementation made by CodesInChaos"

This reverts commit bf0aaa395ad2e8498c367edff5f584a7a65204cc.
added warning that the implementation is suscptible to timing attacks