Very large cardinals around the rank-into-rank area potentially have applications in cryptography. Rank-into-rank cardinals produce self-distributive algebras which may be used as platforms for authentication schemes.

Recall that a rank-into-rank embedding is an elementary embedding $j:V_{\lambda}\rightarrow V_{\lambda}$. Let $\mathcal{E}_{\lambda}$ denote the collection of all rank-into-rank embeddings $j:V_{\lambda}\rightarrow V_{\lambda}$. Define a binary operation $*$ on $\mathcal{E}_{\lambda}$ by letting $j*k=\bigcup_{\alpha<\lambda}j(k|_{V_{\alpha}})$. Then $(\mathcal{E}_{\lambda},*)$ satisfies the self-distributivity identity $j*(k*l)=(j*k)*(j*l)$. If $\gamma$ is a limit ordinal with $\gamma<\lambda$, then define a congruence $\equiv^{\gamma}$ on $(\mathcal{E}_{\lambda},*)$ by letting
$j\equiv^{\gamma}k$ if and only if $j(x)\cap V_{\gamma}=k(x)\cap V_{\gamma}$ whenever $x\in V_{\gamma}$. Then [my question and answer][1] shows that the algebra $(\mathcal{E}_{\lambda}/\equiv^{\gamma},*)$ is locally finite and hence computable.


In the paper, [Using shifted conjugacy in braid-based cryptography][2], Dehornoy proposes an authentication scheme that can use any self-distributive algebra as a platform.  An authentication scheme is a cryptosystem that allows one to prove that she knows some secret information without giving away that secret information. In that paper, Dehornoy proposes that shifted conjugacy may be a good platform as a platform for his authentication scheme (shifted conjugacy has later been shown to be [insecure][3]).


Dehornoy in [2] also spends two pages discussing the classical Laver tables which are up-to-isomorphism the single element generated subalgebras of $(\mathcal{E}_{\lambda}/\equiv^{\gamma},*)$. Dehornoy remarks that the combinatorial complexity of the Laver tables and the fact that the classical Laver tables produce extremely fast growing functions suggests that the classical Laver tables or similar structures may be a good platform for his authentication scheme. However, the classical Laver tables are currently a very insecure platform for Dehornoy's authentication scheme. First of all, $A_{48}$ is the largest Laver table which has even been computed. This means that Dehornoy's authentication scheme provides at most 48 bits of security. Furthermore, the homomorphisms between the classical Laver tables allow one to easily write computer programs that break Dehornoy's authentication scheme for the classical Laver table. My generalized Laver tables [which you can compute online here][4] are not a secure platform for Dehornoy's authentication scheme either.

Although Dehornoy's authentication scheme is insecure for the classical and generalized Laver tables, it may be possible to use some other self-distributive algebra similar to $\mathcal{E}_{\lambda}$ as a secure platform for Dehornoy's authentication scheme. Another possibility is that we improve upon Laver table computation algorithms so that we are able to compute $A_{n}$ for $n>48$ and then use $A_{n}$ as a platform. These line of investigation have barely been researched, but I have recently proposed a [polymath project][5] to promote an investigation into these lines or research.


  [1]: http://mathoverflow.net/questions/210821/does-the-critical-sequence-for-subalgebras-of-elementary-embeddings-with-finitel
  [2]: http://www.math.unicaen.fr/~dehornoy/Papers/Dhk.pdf
  [3]: http://arxiv.org/pdf/0708.1768.pdf
  [4]: http://boolesrings.org/jvanname/lavertables-computation-generalized-finalmatrixandapplicationcalculator/
  [5]: http://mathoverflow.net/a/244330/22277