1. A method implemented by a computing device, the method comprising: providing two distinct primes p and l; providing j a j-invariant of a supersingular elliptic curve E over a finite field F q of degree at most 2 over a prime field of characteristic p; determining univariate polynomial φ l (x,j) modulo p by computing the l+1 distinct subgroups of order l and computing the j-invariants (j 1 ,. .. , j l+1 ) of the l+1 supersingular elliptic curves that are l-isogenous to E; based on information associated with a plurality of values of j, determining bivariate polynomial φ l (x, y) modulo p via interpolation; and using the bivariate polynomial in a cryptosystem.