## Monday, October 17, 2016

### Supersingular isogeny Diffie-Hellman

Supersingular isogeny Diffie-Hellman (SIDH) is a relatively recent proposal for a post-quantum secure key-exchange method which I will briefly outline here.

Most post-quantum cryptography is based on lattices, codes, multivariate quadratics or hashes. See Gustavo's post for more.

A fifth category seems to slowly establish itself: Isogeny-based crypto.

These schemes are based on the difficulty of finding an isogeny between two supersingular elliptic curves. Isogenies are specific rational maps between elliptic curves which must also be a group homomorphism for the group of points on the curve.

The original proposal [Stolbunov et al., 2006] was to use the problem of finding isogenies between ordinary elliptic curves but this system was shown to be breakable with a quantum computer [Childs et al., 2010]. After that it was proposed to use supersingular elliptic curves instead [De Feo et al., 2011].

SIDH currently has significantly worse performance than lattice based key-exchange schemes but offers much smaller key-sizes. Compared to Frodo it is 15 times slower but the key size is only one twentieth. Compared to NewHope it is over 100 times slower at less than one third of the keysize. This can be relevant in scenarios like IOT where cryptographic computations require orders of magnitude less energy then actually transmitting data via the radio.

Although finding isogenies between curves is difficult, Vélu's formulas allow calculating an isogeny with a given finite subgroup as its kernel. All such isogenies are identical up to isomorphism.

Now, starting with a public curve $E$ that is a system parameter we have both parties, Alice and Bob generate an isogeny for kernels $\langle r_a \rangle, \langle r_b\rangle$ respectively. Let $r_a, r_b$ be any generators of a subgroup for now. This gives us two isogenies
$$\phi_a: E \rightarrow E_a\\ \phi_b: E \rightarrow E_b.$$
Now we  would like to exchange $E_a, E_b$ between the partners and somehow derive a common $E_{ab}$ using the kernels we have used. Unfortunately, $r_a$ does not even lie on $E_b$ so we have a problem.

The solution that was proposed by De Feo et al. is to use 4 more points $P_a, P_b, Q_a, Q_b$ on $E$ as public parameters, two for each party. This allows constructing
$$r_a = m_aP_a + n_aQ_a\\ r_b = m_bP_b + n_bQ_b$$ using random integers $m_a, n_a, m_b, n_b$ appropriate for the order.
Now, after calculating the isogenies $\phi_a, \phi_b$ the parties not only exchange the curves $E_a, E_b$ but also $\phi_a(P_b), \phi_a(Q_b)$ and $\phi_b(P_a), \phi_b(Q_a)$.
Looking at the example of Alice she can now calculate
$$m_a\phi_b(P_a)+n_a\phi_b(Q_a) = \phi_b(m_aP_a + n_aQ_a) = \phi_b(r_a)$$ and Bob can perform the analogous computation. Constructing another isogeny using this $\langle \phi_b(r_a) \rangle$ and $\langle \phi_a(r_b) \rangle$ respectively gives Alice and Bob two curves $E_{ba}, E_{ab}$ which are isomorphic and their j-invariant can be used as a common secret.

I will leave you with this wonderfully formula laden image from the De Feo et al. paper showing the protocol.

1. Well, there are algorithms that are currently assumed to be the best for finding isogenies. They have a complexity of $O(p^{1/4})$ classical and $O(p^{1/6})$ post-quantum. However SIDH is not very old maybe somebody will soon find a much better attack. :)