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  1. JDK
  2. JDK-8295010

Reduce if required in EC limbs operations




      There is maximum adding operations limit for EC limbs add and subtraction in the JDK implementation.

      The design is error prone as the code must be carefully checked so that the limit cannot reach. If a reach is noticed, a reduce operation would have to be hard-coded additionally. In the FieldGen.java implementation, the limit can only be 1 or 2 as the implementation code is only able to handle 2. But the FieldGen.java does not really check if 2 really works for the specific filed generation parameters.

      The design impact the performance as well. Because of this limit, the maximum limb size is 28 bits for 2 max adding limit. Otherwise there are integer (long) overflow issues. For example for 256 bits curves, 10 limbs is required for 28-bit limb size; and 9 limbs for 29-bit size. By reducing 1 limbs from 10 to 9, the Signature performance could improved by up to 20%.

      In the IntegerPolynomial class description, it is said "All IntegerPolynomial implementations allow at most one addition before multiplication. Additions after that will result in an ArithmeticException." It's too strict to follow without exam the code very carefully. Indeed, the implementation does not really follow the spec, and 2 addition may be allowed.

      It would be nice if there is no addition limit, and then we are free from these issues. It is doable by reducing the limbs if required for EC limbs operations.


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