Add the required methods to SECP256r1 to conform to the EllipticCurve
virtual base class. Using this updated version of SECP256r1, support in
LibTLS is implemented.
These changes generalize the interface with an elliptic curve
implementation. This allows LibTLS to support elliptic curves generally
without needing the specifics of elliptic curve implementations.
This should allow for easier addition of other elliptic curves.
This implementation of the secp256r1 elliptic curve uses two techniques
to improve the performance of the operations.
1. All coordinates are stored in Jacobian form, (X/Z^2, Y/Z^3, Z), which
removes the need for division operations during point addition or
doubling. The points are converted at the start of the computation,
and converted back at the end.
2. All values are transformed to Montgomery form, to allow for faster
modular multiplication using the Montgomery modular multiplication
method. This means that all coordinates have to be converted into
this form, and back out of this form before returning them.
