Elliptic curves over prime fields in Montgomery form.
Montgomery curves are introduced in (Section 10.3.1 of) Montgomery's ``Speeding the Pollard and Elliptic Curve Methods of Factorization'' and described in perhaps more detail in Bernstein and Lange's ``Montgomery curves and the Montgomery ladder'' and Bernstein, Birkner, Joye, Lange, and Peters's ``Twisted Edwards Curves''.
A Montgomery curve over a prime field of size
where