Thank you very much for your kind answer !

Kaleb J. Himes wrote:

Thanks for your question, please checkout the sections of code wrapped by the setting HAVE_COMP_KEY (Short for HAVE COMPRESSED KEY).

I'm studying too Apple VAS encryption. I have tried to follow your hint, but I have not understood so much and I have not been able to solve the problem.
It seems to me HAVE_COMP_KEY option affects mainly keys import/export while the function
EC_POINT_set_compressed_coordinates(const EC_GROUP *group, EC_POINT *p, const BIGNUM *x, int y_bit, BN_CTX *ctx)
calculates y value for a given x value (as square roots of x^3 + a*x + b) and then internally calls EC_POINT_set_affine_coordinates()
I'm not able to find some equivalent function within WolfSSL.