RFR 8203228: Branch-free output conversion for X25519 and X448

[Anthony Scarpino]

The change looks fine to me

Tony

On 06/25/2018 05:49 AM, Adam Petcher wrote:
> It would be nice to get this X25519/X448 enhancement into JDK 11. If 
> anyone has some time to review this in the next day or so, I would 
> appreciate it.
> 
> 
> On 5/15/2018 2:42 PM, Adam Petcher wrote:
>> 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.
>>
>