Dynamic Threshold Cryptosystem without Group Manager

In dynamic networks with flexible memberships, group signatures and distributed signatures are an important problem. Dynamic threshold cryptosystems are best suited to realize distributed signatures in dynamic (e.g. meshed) networks. Without a group manager or a trusted third party even more flexible scenarios can be realized. Gennaro et al. showed, it is possible to dynamically increase the size of the signer group, without altering the public key. We extend this idea by removing members from the group, also without changing the public key. This is an important feature for dynamic groups, since it is very common, e.g. in meshed networks that members leave a group. Gennaro et al. used RSA and bi-variate polynomials for their scheme. In contrast, we developed a DL-based scheme that uses ideas from the field of proactive secret sharing (PSS). One advantage of our scheme is the possibility to use elliptic curve cryptography and thereby decrease the communication and computation complexity through a smaller security parameter. Our proposal is an efficient threshold cryptosystem that is able to adapt the group size in both directions. Since it is not possible to realize a non-interactive scheme with the ability to remove members (while the public key stays unchanged), we realized an interactive scheme whose communication efficency is highly optimized to compete with non-interactive schemes. Our contribution also includes a security proof for our threshold scheme

A Novel Adaptive Proactive Secret Sharing without a Trusted Party

A $(t+1,n)$ proactive secret sharing is to protect a secret in long-lived system by distributing it to a group of $n$ participants and refreshing their shares periodically in this fixed group, while any $t+1$ and more than $t+1$ shares can reconstruct the secret. In some environment, it needs to change not only the number of participants $n$ but also the threshold value $t$. An adaptive proactive secret sharing is to refresh the shares as $t$ and $n$ change. In this paper, we propose a novel adaptive proactive secret sharing scheme without a trusted party. Our proposed scheme is uniformly efficient and tolerates $t$ Byzantine faults in any single time interval, where the number of participants $n\geq 3t+1$. The threshold value $t$ and the number of participants $n$ can be changed arbitrarily in two adjacent intervals. We also prove that our proposed scheme is secure under the discrete logarithm intractability assumption

Strong Privacy Protection in Electronic Voting

We give suggestions for protection against adversaries with access to the voter's equipment in voting schemes based on homomorphic encryption. Assuming an adversary has complete knowledge of the contents and computations taking place on the client machine we protect the voter's privacy in a way so that the adversary has no knowledge about the voter's choice. Furthermore, an active adversary trying to change a voter's ballot may do so, but will end up voting for a random candidate. To accomplish the goal we assume that the voter has access to a secondary communication channel through which he can receive information inaccessible to the adversary. An example of such a secondary communication channel is ordinary mail. Additionally, we assume the existence of a trusted party that will assist in the protocol. To some extent, the actions of this trusted party are verifiable

Strong Privacy Protection in Electronic Voting

We give suggestions for protection against adversaries with access to the voter's equipment in voting schemes based on homomorphic encryption. Assuming an adversary has complete knowledge of the contents and computations taking place on the client machine we protect the voter's privacy in a way so that the adversary has no knowledge about the voter's choice. Furthermore, an active adversary trying to change a voter's ballot may do so, but will end up voting for a random candidate. To accomplish the goal we assume that the voter has access to a secondary communication channel through which he can receive information inaccessible to the adversary. An example of such a secondary communication channel is ordinary mail. Additionally, we assume the existence of a trusted party that will assist in the protocol. To some extent, the actions of this trusted party are verifiable

An implementation of the Paillier crypto system with threshold decryption without a trusted dealer

We consider the problem of securely generating the keys of the Paillier crypto system [11] with (t, n) threshold decryption, without a trusted dealer. Nishide and Sakurai [10] describe a solution, secure in the malicious model. We use their ideas to make a simpler solution for the semi-honest model, and further introduce a few optimisations. We implement the secure key generation protocol on a single computer, and consider its performance

A Framework for Efficient Adaptively Secure Composable Oblivious Transfer in the ROM

Oblivious Transfer (OT) is a fundamental cryptographic protocol that finds a number of applications, in particular, as an essential building block for two-party and multi-party computation. We construct a round-optimal (2 rounds) universally composable (UC) protocol for oblivious transfer secure against active adaptive adversaries from any OW-CPA secure public-key encryption scheme with certain properties in the random oracle model (ROM). In terms of computation, our protocol only requires the generation of a public/secret-key pair, two encryption operations and one decryption operation, apart from a few calls to the random oracle. In~terms of communication, our protocol only requires the transfer of one public-key, two ciphertexts, and three binary strings of roughly the same size as the message. Next, we show how to instantiate our construction under the low noise LPN, McEliece, QC-MDPC, LWE, and CDH assumptions. Our instantiations based on the low noise LPN, McEliece, and QC-MDPC assumptions are the first UC-secure OT protocols based on coding assumptions to achieve: 1) adaptive security, 2) optimal round complexity, 3) low communication and computational complexities. Previous results in this setting only achieved static security and used costly cut-and-choose techniques.Our instantiation based on CDH achieves adaptive security at the small cost of communicating only two more group elements as compared to the gap-DH based Simplest OT protocol of Chou and Orlandi (Latincrypt 15), which only achieves static security in the ROM