## Oberseminar über Kryptographie

Dozent | Zeit | Raum | Erstmals am |
---|---|---|---|

Prof. A. May Prof. Eike Kiltz |
freitags, 10.45-12.00 | NA 5/64 | 10.04.15 |

Datum | Vortragende Person | Titel | Raumnummer | Beginn |
---|---|---|---|---|

10.04 | Manuel Fersch | Quantum Secure MACs | 5/64 | 10.45 Uhr |

08.05 | Yida Xu | Private Information Retrieval | 5/64 | 10.45 Uhr |

10.06 | Benedikt Auerbach | Optimal Structure-Preserving Signatures in Asymetric Bilinear Groups | 5/64 | 11.00 Uhr |

26.06 | Marie-Sarah Lacharité | Revisiting the security model for BGLS aggregate signatures | 5/64 | 11.00 Uhr |

19.06 | Gregor Leander | Some observation on Simon | 5/64 | 11.00 Uhr |

03.07 | Jorge Villar | Equivalences and Black-Box Separations of Matrix Diffie-Hellman Problems | 5/64 | 11.00 Uhr |

10.07 | Jiaxin Pan | Structure-Preserving Signatures from Standard Assumptions, Revisited | 5/64 | 11.00 Uhr |

17.07 | Felix Heuer | Standard Security Does Imply Security Against Selective Opening for Markov Distributions | 5/64 | 11.00 Uhr |

## Quantum Secure MACs

###### Once quantum computers with sufficient computing power can be built, many encryption and authentication schemes that are secure in the world of classical computing can be broken easily.In my master's thesis, I show how to construct MACs that are secure against quantum adversaries, even if there is quantum interaction with the attacker. This is a stronger security notion than the one used in post-quantum cryptography, where the adversary can use a quantum computer for its computations but the interaction with the cryptographic scheme is always classical. In order to achieve this kind of security, I use techniques introduced by Dan Boneh and Mark Zhandry in their paper 'Secure Signatures and Chosen Ciphertext Security in a Quantum Computing World'.

## Private Information Retrieval

###### In this presentation we give a introduction of the private information retrieval. Firstly, we briefly introduce the intention and requirement of PIR. We will also give a introduction of the development of PIR problem by introducing historical significant PIR from Information theoretic PIR with database replication to computational PIR without database replication as well as the later researches of trading off between communication complexity and computational complexity. Then we’ll learn Cachin’s modeling of PIR. Later we give a specified definition of correctness and privacy. We define the privacy of PIR through the indistinguishability of queries. We then construct a novel PIR under phi-hiding assumption and prove it secure.

## Revisiting the security model for BGLS aggregate signatures

###### Aggregate signature schemes combine multiple users' digital signatures on various messages into one fixed-length signature. I'll present the elegant, pairing-based aggregate signature scheme of Boneh, Gentry, Lynn, and Shacham (BGLS), in which anyone can aggregate the signatures in any order. In the original security model, the forger is given one public key to target. In real life, however, a forger knows all signers' public keys. Could the forger have an advantage when it can choose which signer(s) to target? How is the tightness of the security reduction affected by the forger having more power? Using diagrams to illustrate security reductions, I'll compare different types of forgers. Finally, I'll explore how the revised security model affects the recommended group sizes.

## Some observations on Simon

###### I will talk about the NSA block ciphers Simon and Speck. Actually only about Simon. I will explain some initial ideas on how to analyze their resistance against linear and differential attacks.

## Equivalences and Black-Box Separations of Matrix Diffie-Hellman Problems

###### In this talk we provide new algebraic tools to study the relationship between different Matrix Diffie-Hellman (MDDH) Problems. Namely, we provide a purely algebraic criterion to decide whether there exists a generic black-box reduction, and in many cases, when the answer is positive we also build an explicit reduction with the following properties: it only makes a single oracle call, it is tight and it makes use only of operations in the base group. It is well known that two MDDH problems described by matrices with a different number of rows are separated by an oracle computing certain multilinear map. Thus, we put the focus on MDDH problems of the same size. Then, we show that MDDH problems described with a different number of parameters are also separated (meaning that a successful reduction cannot reduce the amount of randomness in the problem instance). When comparing MDDH problems of the same size and number of parameters, we show that surprisingly they are equivalent or incomparable. This suggests that a complete classification into equivalence classes can be done. We give some positive and negative results about equivalence, in particular solving the open problem of whether the Linear and the Cascade MDDH problems are reducible to each other.

## Structure-Preserving Signatures from Standard Assumptions, Revisited

###### Structure-preserving signatures (SPS) are pairing-based signatures where all the messages, signatures and public keys are group elements, with numerous applications in public-key cryptography. We present new, simple and improved SPS constructions under standard assumptions via a conceptually different approach. Our constructions significantly narrow the gap between existing constructions from standard assumptions and optimal schemes in the generic group model. This is a joint work with Eike Kiltz and Hoeteck Wee. The paper will be presented at this year CRYPTO and the full version is on eprint (Report 2015/604)