What is the recommended way to add 2 points?
I see:
Create point: void static secp256k1_gej_set_xy(secp256k1_gej_t *r, const secp256k1_fe_t *x, const secp256k1_fe_t *y);
Add both points: void static secp256k1_gej_add(secp256k1_gej_t *r, const secp256k1_gej_t *a, const secp256k1_gej_t *b);
And then finally convert the jacobian point back into a cartesian value (0x04, x, y) as used in Bitcoin.
(using x/z^2 and y/z^3 as it says in group.h)
There’s currently no public API call to do so.
It could easily be added the way you say (you could keep one of both point affine even), unless the point you’re adding is a known multiple of the generator (in which case tweak_add should suffice). If so, pull requests welcome.
Closing. It’s been a few years and (a) we haven’t encountered a use case for this; (b) we’ve formed a policy of not exposing arbitrary EC math functionality, at risk of people using them to create novel cryptosystems in inefficient/insecure ways.
If you need to add points, the right way to do it is to encapsulate whatever you’re really doing into a module, fork the lib to add it under src/modules, and give a safe public API. If it’s generally useful, PRs are welcome.
