# How are isogeny graphs made and how are they helpful to crypto?

I don't understand how the shapes of isogeny graphs are determined. While Alice and Bob do walk on it and don't backtrack, are they actually relevant to crypto?

Also, I was told that supersingular isogeny graphs are expander graphs. Similarly, how is the shape of an expander graph determined

• I highly recommend eprint.iacr.org/2018/593 for an overview of isogeny graphs in cryptography.
– djao
Sep 10, 2018 at 23:01
• @djao thank you so much for this paper! It ascends to slightly daunting depths very soon, but is immensely helpful. Is there a visual demonstration of the encryption process of SIDH in regards to an isogeny graph? Apr 10, 2019 at 8:48

Let an elliptic curve $E(F_p)$ have the Frobenius discriminant $D_\pi$, and $\left(\frac{D_\pi}{l}\right)$ be a Kronecker symbol for some $l$-degree isogeny. If $\left(\frac{D_\pi}{l}\right)=-1$, then there are no $l$-degree isogenies; if $\left(\frac{D_\pi}{l}\right)=1$, then two $l$-degree isogenies exist; if $\left(\frac{D_\pi}{l}\right)=0$, then $1$ or $l+1$ $l$-degree isogenies exist.
Therefore, if $\left(\frac{D_\pi}{l}\right)=1$. then $l$-degree isogenies of elliptic curves form branchless cycles, and changing direction in a cycle means switching to dual isogenies.
• @edlothia, It is easy to create a $2n$-regular graph, by choosing the $n$ $l_i$ that $\left(\frac{D_\pi}{l_i}\right)=1$. Sep 11, 2018 at 18:21