Finding the subgroup in isogeny-based cryptography
While there is a sub-exponential attack to compute isogenies on ORDINARY elliptic curves (the basis for the Rostovev and Stulbunov paper that you reference) there is not (yet at least) a sub-exponential attack to compute isogenies on SUPERSINGULAR elliptic curves.  The cryptosystem proposed by DeFeo, Jao, and Plut back in 2011 is based on Supersingular rather than ordinary elliptic curves.  Microsoft Research recently published a paper on this cryptosystem.  The paper contains a good overview of research on this problem to date. An preprint of Microsoft's paper is found here.