Module ecdh 

Functions

bjj_is_on_curve

Check a*x^2 + y^2 == 1 + d*x^2*y^2

bjj_scalar_mul

scalar * point with full validation (on-curve + subgroup check).

derive_public_key_from_sk

PK = sk * Base8

derive_shared_secret_bjj

ECDH shared secret derivation. Returns X coordinate of sk * pk.