Best Effort and Practice Activation Codes

Activation Codes are used in many different digital services and known by many different names including voucher, e-coupon and discount code. In this paper we focus on a specific class of ACs that are short, human-readable, fixed-length and represent value. Even though this class of codes is extensively used there are no general guidelines for the design of Activation Code schemes. We discuss different methods that are used in practice and propose BEPAC, a new Activation Code scheme that provides both authenticity and confidentiality. The small message space of activation codes introduces some problems that are illustrated by an adaptive chosen-plaintext attack (CPA-2) on a general 3-round Feis- tel network of size 2^(2n) . This attack recovers the complete permutation from at most 2^(n+2) plaintext-ciphertext pairs. For this reason, BEPAC is designed in such a way that authenticity and confidentiality are in- dependent properties, i.e. loss of confidentiality does not imply loss of authenticity.Comment: 15 pages, 3 figures, TrustBus 201

Format and Order Revealing Encryption

As more and more cloud services emerge so does the need for new methods for securing the data these services consume, especially since data leaks have become the norm rather than the exception. Since most cloud services require some kind of access to our private data in order to perform searches and provide services, new ways of securing our data in the cloud is needed. This dissertation examines the current state of the cryptographic world in order to try to and understand and resume what solutions currently exist for this particular type of problem. This work is motivated by a particular problem of data delegation to a cloud infrastructure. This problem involves the protection of sensitive data whilst it’s analysed by a third party. While there is no simple approach to solve this particular problem, this dissertation discusses three main approaches to tackle this problem. One approach attempts to define a new cryptographic scheme with a leakage profile that would allow a third party to only have access to some information of the plaintext but, at the same time, keep the plaintext safe from attackers. Another approach attempts to use already existing cryptographic schemes, such as, Format Preserving Encryption and Order Revealing Encryption to solve this particular problem. A final approach tries to solve this problem by utilising cryptographic tools, such as hash-functions and hash-based message authentication codes. An extended study was also conducted in many cryptographic schemes, both current and old cryptographic schemes. This study allowed for a better view of the cryptographic world and how these schemes could help us achieve a solution. For this dissertation, a prototype was also implemented of some recent cryptographic schemes. These prototype implementations allowed for a deeper understanding of how these schemes work and also allowed us to conduct some experiments while trying to combine two cryptographic schemes. The results of this dissertation show that that trying to solve a problem via creating a new cryptographic scheme is not an easy feat especially when one wants to define correctly the strict security requirements and also the work needed to understand the mathematical workings of similar schemes. Lastly we conclude that solving the problem with the help of already existing tools may be the easiest solution, but, it may also only work for a specific scenario and hence is of no use in other similar situations. A solution to the particular problem studied in this thesis is also presented at the end of this dissertation, although, it only applies to this specific problem and does not solve the more general problem of privacy of data delegation to the cloud.Com a explosão de serviços baseados na nuvem que ocorre nos dias de hoje, torna-se imperativo que os dados que são consumidos por este tipo de serviços sejam de alguma forma protegidos contra ataques ou roubos[Cen18]. O principal problema com este tipo de serviços é que, normalmente, estes serviços precisam de acesso aos dados para conseguirem fazer pesquisas e correlacionar dados de forma a que seja possível fornecer diversos serviços. Esta dissertação tem como objetivo estudar o mundo da criptografia de forma a perceber que tipo de garantias são oferecidas pelos esquemas criptográficos existentes nos dias de hoje para serviços baseados na nuvem. Este trabalho é motivado por um problema real de delegação de dados para a nuvem. Este problema envolve a proteção de dados sensíveis que precisam de ser analisados por entidades externas. Embora não haja uma abordagem simples para resolver este tipo de problemas, nesta dissertação iremos discutir três abordagens que, potencialmente, poderão resolver este problema. Uma abordagem tenta definir o que poderia ser a estrutura geral de um novo esquema criptográfico que pudesse lidar com o problema específico em análise. Numa outra abordagem iremos utilizar ferramentas existentes para tentar resolver o problema em questão. Iremos também tentar unir dois esquemas criptográficos existentes, de forma a tentar combater este problema em específico. Foi também realizado um estudo a vários esquemas criptográficos de forma a perceber quais as soluções que existem hoje em dia para problemas relacionados com a delegação de dados para entidades externas, como também, tentar perceber que esquemas criptográficos que ainda são resultados meramente teóricos mas que possam vir, no futuro, a ser úteis para combater esta problemática. Os resultados desta dissertação mostram que resolver um problema relacionado com criptografia nem sempre é fácil, uma vez que, a má utilização destes esquemas poderá levar a uma falha grave de segurança. Por fim, concluímos que, resolver um problema desta natureza através de ferramentas existentes é bastante mais fácil do que tentar desenvolver esquemas criptográficos novos, mas que irá perder o poder de poder ser aplicado a outros problemas semelhantes

A unified framework for trapdoor-permutation-based sequential aggregate signatures

We give a framework for trapdoor-permutation-based sequential aggregate signatures (SAS) that unifies and simplifies prior work and leads to new results. The framework is based on ideal ciphers over large domains, which have recently been shown to be realizable in the random oracle model. The basic idea is to replace the random oracle in the full-domain-hash signature scheme with an ideal cipher. Each signer in sequence applies the ideal cipher, keyed by the message, to the output of the previous signer, and then inverts the trapdoor permutation on the result. We obtain different variants of the scheme by varying additional keying material in the ideal cipher and making different assumptions on the trapdoor permutation. In particular, we obtain the first scheme with lazy verification and signature size independent of the number of signers that does not rely on bilinear pairings. Since existing proofs that ideal ciphers over large domains can be realized in the random oracle model are lossy, our schemes do not currently permit practical instantiation parameters at a reasonable security level, and thus we view our contribution as mainly conceptual. However, we are optimistic tighter proofs will be found, at least in our specific application.https://eprint.iacr.org/2018/070.pdfAccepted manuscrip

On generalized Feistel networks

We prove beyond-birthday-bound security for the well-known types of generalized Feistel networks, including: (1) unbalanced Feistel networks, where the $n$-bit to $m$-bit round functions may have $n\ne m$; (2) alternating Feistel networks, where the round functions alternate between contracting and expanding; (3) type-1, type-2, and type-3 Feistel networks, where $n$-bit to $n$-bit round functions are used to encipher $kn$-bit strings for some $k\ge2$; and (4) numeric variants of any of the above, where one enciphers numbers in some given range rather than strings of some given size. Using a unified analytic framework we show that, in any of these settings, for any $\varepsilon>0$, with enough rounds, the subject scheme can tolerate CCA attacks of up to $q\sim N^{1-\varepsilon}$ adversarial queries, where $N$ is the size of the round functions\u27 domain (the size of the larger domain for alternating Feistel). This is asymptotically optimal. Prior analyses for generalized Feistel networks established security to only $q\sim N^{0.5}$ adversarial queries

Improved quantum attack on Type-1 Generalized Feistel Schemes and Its application to CAST-256

Generalized Feistel Schemes (GFS) are important components of symmetric ciphers, which have been extensively researched in classical setting. However, the security evaluations of GFS in quantum setting are rather scanty. In this paper, we give more improved polynomial-time quantum distinguishers on Type-1 GFS in quantum chosen-plaintext attack (qCPA) setting and quantum chosen-ciphertext attack (qCCA) setting. In qCPA setting, we give new quantum polynomial-time distinguishers on $(3d-3)$-round Type-1 GFS with branches $d\geq3$, which gain $d-2$ more rounds than the previous distinguishers. Hence, we could get better key-recovery attacks, whose time complexities gain a factor of $2^{\frac{(d-2)n}{2}}$. In qCCA setting, we get $(3d-3)$-round quantum distinguishers on Type-1 GFS, which gain $d-1$ more rounds than the previous distinguishers. In addition, we give some quantum attacks on CAST-256 block cipher. We find 12-round and 13-round polynomial-time quantum distinguishers in qCPA and qCCA settings, respectively, while the best previous one is only 7 rounds. Hence, we could derive quantum key-recovery attack on 19-round CAST-256. While the best previous quantum key-recovery attack is on 16 rounds. When comparing our quantum attacks with classical attacks, our result also reaches 16 rounds on CAST-256 with 128-bit key under a competitive complexity