A famous theorem of Tate asserts that two elliptic curves over the same finite field have the same number of points exactly when they are isogenous. It turns out, however, to be extremely difficult to construct an explicit isogeny; if one could, it would be a powerful tool in elliptic curve cryptography. Instead, we consider the distribution properties of random compositions of low degree isogenies, which can be tackled using Hecke L-functions. As an application, we give a proof that (in most cases) the cryptographic properties of an elliptic curve depend solely on the ground field and point count, but not crucially on the individual curve itself. (Joint work with David Jao, Waterloo, and Ramarathnam Venkatesan, Microsoft Research)