Webrev: http://cr.openjdk.java.net/~apetcher/8203228/webrev.00/
Please review the change for this leftover task from the X25519/X448 JEP. The current code uses BigInteger to convert the final result from a field element to a byte array that can be used to derive a key. Using branch-free operations instead of BigInteger will protect this secret from certain side-channel attacks.
The output conversion is done entirely by the asByteArray method of IntegerPolynomial, which is implemented by limbsToByteArray. For this change, I took the branch-free output conversion routine from the Poly1305 field and pushed it into the parent class. I had to generalize it a bit in order to deal with the peculiarities of the X25519/X448 fields and their representations. I also made addModPowerTwo branch free, because this was a relatively simple change once the rest was done. But this method is not used by X25519/X448.
In addition to running the full regression test suite, I also ran some benchmarks on X25519 and X448 to confirm that performance wasn't impacted. Not surprising, since this method is only called once per key generation/agreement operation.
