With the outbreak of e-mail message leakage events, such as the Hillary Clinton's Email Controversy, privacy and security of sensitive e-mail information have become users' primary concern. Encrypted email seems to be a viable solution for providing security, but it will greatly limit their operations. Public encryption with keyword search (PEKS) scheme is a popular technology to incorporate security protection and favorable operability functions together, which can play an important role in searching over encrypted email in a cloud server. In this paper, we propose a practical PEKS scheme named as public-key multi-keyword searchable encryption with hidden structures (PMSEHS). It could enable e-mail receivers to do the multi-keyword and boolean search in the large encrypted email database as fast as possible, without revealing more information to the cloud server. We also give comparative experiments, which demonstrate that our scheme has a higher efficiency in multi-keyword search for encrypted emails.

In Fast Software Encryption (FSE) 2015, while presenting a new idea (i.e., the design of stream ciphers with the small internal state by using a secret key, not only in the initialization but also in the keystream generation), Sprout was proposed. Sprout was insecure and an improved version of Sprout was presented in FSE 2017. We introduced Fruit stream cipher informally in 2016 on the web page of IACR (eprint) and few cryptanalysis were published on it. Fortunately, the main structure of Fruit was resistant. Now, Fruit-80 is presented as a final version which is easier to implement and is secure. The size of LFSR and NFSR in Fruit-80 is only 80 bits (for 80-bit security level), while for resistance to the classical time-memory-data tradeoff (TMDTO) attacks, the internal state size should be at least twice that of the security level. To satisfy this rule and to design a concrete cipher, we used some new design ideas. It seems that the bottleneck of designing an ultra-lightweight stream cipher is TMDTO distinguishing attacks. A countermeasure was suggested, and another countermeasure is proposed here. Fruit-80 is better than other small-state stream ciphers in terms of the initialization speed and area size in hardware. It is possible to redesign many of the stream ciphers and achieve significantly smaller area size by using the new idea.

For any bent function, it is very interesting to determine its dual function, because the dual function is also bent in certain cases. For k odd and gcd(n, k) = 1, it is known that the Coulter-Matthews bent function f(x) = T r (ax 3^k+1/2) is weakly regular bent over F3ⁿ, where a ∈ F₃ⁿ*, and T r (·) : F₃ⁿ → F₃ is the trace function. In this paper, we investigate the dual function of f (x), aiming to determine a universal formula. In particular, for two cases, we determine the formula explicitly: for the case of n = 3t + 1 and k = 2t + 1 with t ≥ 2, the dual function is given by Tr (- x^32t+1+^3t+1+2/a^32t+1+3t+1+1) - x^32t+1/a^-32t+3t+1 + x²/a^{-32t+1+3t+1+1}); and for the case of n = 3t + 2 and k = 2t + 1 with t≥2,the dual function is given by Tr (-x^32t+2+1/a^32t+2-3t+1+3 - x^{2.32t+1+3t+1+1}/a^32t+2+3t+1+3 + x²/a^{-32t+2+3t+1+3}) As a byproduct, we find two new classes of ternary bent functions with only three terms. Moreover, we also prove that in certain cases f (x) is regular bent.