We demonstrate how to prevent differential attacks on multivariate public key cryptosystems using the Plus (+) method of external perturbation. In particular, we prescribe adding as few as 10 Plus polynomials to the Perturbed Matsumoto-Imai (PMI) cryptosystem when g=1 and r=6, where θ is the Matsumoto-Imai exponent, n is the message length, g = gcd(θ,n), and r is the internal perturbation dimension; or as few as g+10 when g ≠ 1. The external perturbation does not significantly decrease the efficiency of the system, and in fact has the additional benefit of resolving the problem of finding the true plaintext among several preimages of a given ciphertext. We call this new scheme the Perturbed Matsumoto-Imai-Plus (PMI+) cryptosystem.

Recently Landau and Diffie gave in a series of articles in the Notices of the American Mathematical Society and in the American Mathematical Monthly excellent expositions on how the theory of multivariable polynomials are used in cryptography. However they covered only half of the story. They covered only the theory of polynomials in symmetric or secret cryptography. There is another half of the story, namely the story about the theory of multivariable polynomials in asymmetric or public key cryptosystems. We give an overview of the families of public key cryptosystems, which have been developed in the last ten years.

Sflash is a fast multivariate signature scheme. Though the first version Sflash-v1 was flawed, a second version, Sflash-v2 was selected by the Nessie Consortium and was recommended for implementation of low-end smart cards. Very recently, due to the security concern, the designer of Sflash recommended that Sflash-v2 should not be used, instead a new version Sflash-v3 is proposed, which essentially only increases the length of the signature. The Sflash family of signature schemes is a variant of the Matsumoto and Imai public key cryptosystem. The modification is through the Minus method, namely given a set of polynomial equations, one takes out a few of them to make them much more difficult to solve. In this paper, we attack the Sflash-v3 scheme by combining an idea from the relinearization method by Kipnis and Shamir, which was used to attack the Hidden Field Equation schemes, and the linearization method by Patarin. We show that the attack complexity is less than 2^{80}, the security standard required by the Nessie Consortium.

Though the multivariable cryptosystems first suggested by Matsumoto and Imai was defeated by the linearization method of Patarin due to the special properties of the Matsumoto-Imai (MI) cryptosystem, many variants and extensions of the MI system were suggested mainly by Patarin and his collaborators. In this paper, we propose a new variant of the MI system, which was inspired by the idea of “perturbation”. This method uses a set of r (a small number) linearly independent linear functions z_i=∑_{j=1}^n α_{ij}x_j + β_i, i=1,..,r, over the variables x_i , which are variables of the MI system. The perturbation is performed by adding random quadratic function of z i to the MI systems. The difference between our idea and a very similar idea of the Hidden Field Equation and Oil-Vinegar system is that our perturbation is internal, where we do not introduce any new variables, while the Hidden Field Equation and Oil-Vinegar system is an “external” perturbation of the HFE system, where a few extra (external) new variables are introduced to perform the perturbation. A practical implementation example of 136 bits, its security analysis and efficiency analysis are presented. The attack complexity of this perturbed Matsumoto-Imai cryptosystem is estimated.

The first author constructed a q-parameterized spherical category $\sC$ over C(q) in [Liu15], whose simple objects are labelled by all Young diagrams. In this paper, we compute closed-form expressions for the fusion rule of $\sC$, using Littlewood-Richardson coefficients, as well as the characters (including a generating function), using symmetric functions with infinite variables.