On the Equivalent Keys in Multivariate Cryptosystems

The number of equivalent keys in multivariate cryptosystem is closely related to the scheme security. This study analyzes the structure of the private key space in some multivariate schemes. The result gives the lower bounds on the number of equivalent keys of some variants of the hidden field equation （HFE） scheme including plus, minus-plus, embedding, and internal perturbation. This method estimates the number of invertible transformations which maintain the form of the central map invariant. Furthermore,a formal proof shows that the two modifications of fixing and embedding are equivalent in security analyses of multivariate schemes. Also this paper corrects previous proofs in Wolf’s work on the number of equivalent keys in HFEv,the unbalanced oil and vinegar （UOV） scheme, and the stepwise triangular systems （STS）.

Cryptanalysis on an image block encryption algorithm based on spatiotemporal chaos

An image block encryption scheme based on spatiotemporal chaos has been proposed recently. In this paper, we analyse the security weakness of the proposal. The main problem of the original scheme is that the generated keystream remains unchanged for encrypting every image. Based on the flaws, we demonstrate a chosen plaintext attack for revealing the equivalent keys with only 6 pairs of plaintext/ciphertext used. Finally, experimental results show the validity of our attack.

CRYPTANALYSIS OF AN EXTENDED MQ SIGNATURE SYSTEM HTTM

Since Multivatriate Quadratic(MQ)-based Public Key Cryptosystem(MPKC)has been one of the hot research fields of post-quantum cryptography,it becomes important to analyze the security of new MPKC schemes.Wang et al.proposed a novel multivariate signature scheme with Hash-based Tame Transformation and Minus(HTTM)in 2011.For this extended MQ-based signature,we can transform it into an SFLASH variant by splitting and merging HT transformation,and solve an equivalent private key corresponding to the public key of HTTM.Thus,the adversary can forge legitimate signature for any message by using this equivalent private key.

A New Feistel-Type White-Box Encryption Scheme

The white-box attack is a new attack context in which it is assumed that cryptographic software is implemented on an un-trusted platform and all the implementation details are controlled by the attackers. So far, almost all white-box solutions have been broken. In this study, we propose a white-box encryption scheme that is not a variant of obfuscating existing ciphers but a completely new solution. The new scheme is based on the unbalanced Feistel network as well as the ASASASA （where ＂A＂ means affine, and ＂S＂ means substitution） structure. It has an optional input block size and is suitable for saving space compared with other solutions because the space requirement grows slowly （linearly） with the growth of block size. Moreover, our scheme not only has huge white-box diversity and white-box ambiguity but also has a particular construction to bypass public white-box cryptanalysis techniques, including attacks aimed at white-box variants of existing ciphers and attacks specific to the ASASASA structure. More precisely, we present a definition of white-box security with regard to equivalent key, and prove that our scheme satisfies such security requirement.