vi Εκτεταμένη Περίληψη viii

Let P be a point set of n points in R. Every point has a label/color (say black or white), but we do not know the labels. In particular, let B and W be the set of black and white points in P . Furthermore, let ∆ = diam(P ), and assume that there exist two parallel hyperplanes h, h′ in distance γ from each other, such that the slab between h and h′ does not contain an point of P , and the points...

A verifiably encrypted signature can convince the verifier that a given cipher-text is the encryption of a signature on a given message. It is often used as a building block to construct optimistic fair exchange. Recently, Gu et.al gave an ID-based verifiably encrypted signature scheme and claimed that their scheme was secure in random oracle model. Unfortunately, in this works, we show that th...

Assuming a cryptographically strong cyclic group G of prime order q and a random hash function H, we show that ElGamal encryption with an added Schnorr signature is secure against the adaptive chosen ciphertext attack, in which an attacker can freely use a decryption oracle except for the target ciphertext. We also prove security against the novel one-more-decyption attack. Our security proofs ...

In PODC 2003, Park et al. [32] first introduce a connection between fair exchange and sequential two-party multi-signature scheme and provide a novel method of constructing fair exchange protocol by distributing the computation of RSA signature. This approach avoids the design of verifiable encryption scheme at the expense of having cosigner store a piece of prime signer’s secret key. Dodis and...

The well-known Signed ElGamal scheme consists of ElGamal encryption with a non-interactive Schnorr proof of knowledge. While this scheme should be intuitively secure against chosen-ciphertext attacks in the random oracle model, its security has not yet been proven nor disproven so far, without relying on further non-standard assumptions like the generic group model. Currently, the best known po...

A wide variety of objectively practical cryptographic schemes can be constructed using only symmetric-key operations and linear operations. To formally study this restricted class of cryptographic algorithms, we present a new model called Linicrypt. A Linicrypt program has access to a random oracle whose inputs and outputs are field elements, and otherwise manipulates data only via fixed linear...

We introduce an efficient algorithmic framework for model selection in online learning, also known as parameter-free online learning. Departing from previous work, which has focused on highly structured function classes such as nested balls in Hilbert space, we propose a generic meta-algorithm framework that achieves online model selection oracle inequalities under minimal structural assumption...