Cubic and symmetric compositions over rings

We consider generalized symmetric compositions over a ring k on the one hand, and unital algebras with multiplicative cubic forms on the other. Given a primitive sixth root of unity in k, we construct functors between these categories which are equivalences if 3 is a unit in k. This extends to arbitrary base rings, and with new proofs, results of Elduque and Myung on non-degenerate symmetric compositions and separable alternative algebras of degree 3 over fields. It also answers a problem posed in “The Book of Involutions” [Knus et al.: American Mathematical Society Colloquium Publications, vol. 44. American Mathematical Society, Providence, RI (1998), 34.26].