10 research outputs found
On the Structure of Bispecial Sturmian Words
A balanced word is one in which any two factors of the same length contain
the same number of each letter of the alphabet up to one. Finite binary
balanced words are called Sturmian words. A Sturmian word is bispecial if it
can be extended to the left and to the right with both letters remaining a
Sturmian word. There is a deep relation between bispecial Sturmian words and
Christoffel words, that are the digital approximations of Euclidean segments in
the plane. In 1997, J. Berstel and A. de Luca proved that \emph{palindromic}
bispecial Sturmian words are precisely the maximal internal factors of
\emph{primitive} Christoffel words. We extend this result by showing that
bispecial Sturmian words are precisely the maximal internal factors of
\emph{all} Christoffel words. Our characterization allows us to give an
enumerative formula for bispecial Sturmian words. We also investigate the
minimal forbidden words for the language of Sturmian words.Comment: arXiv admin note: substantial text overlap with arXiv:1204.167
Lattice Blind Signatures with Forward Security
Blind signatures play an important role in both electronic cash and
electronic voting systems. Blind signatures should be secure against various
attacks (such as signature forgeries). The work puts a special attention to
secret key exposure attacks, which totally break digital signatures. Signatures
that resist secret key exposure attacks are called forward secure in the sense
that disclosure of a current secret key does not compromise past secret keys.
This means that forward-secure signatures must include a mechanism for
secret-key evolution over time periods.
This paper gives a construction of the first blind signature that is forward
secure. The construction is based on the SIS assumption in the lattice setting.
The core techniques applied are the binary tree data structure for the time
periods and the trapdoor delegation for the key-evolution mechanism.Comment: ACISP 202
Matrix PRFs: Constructions, Attacks, and Applications to Obfuscation
We initiate a systematic study of pseudorandom functions (PRFs) that are
computable by simple matrix branching programs; we refer to these objects as
“matrix PRFs”. Matrix PRFs are attractive due to their simplicity, strong
connections to complexity theory and group theory, and recent applications in
program obfuscation.
Our main results are:
* We present constructions of matrix PRFs based on the conjectured hardness of
some simple computational problems pertaining to matrix products.
* We show that any matrix PRF that is computable by a read-c, width w
branching program can be broken in time poly(w^c); this means that any matrix
PRF based on constant-width matrices must read each input bit omega(log
lambda) times. Along the way, we simplify the “tensor switching lemmas”
introduced in previous IO attacks.
* We show that a subclass of the candidate local-PRG proposed by Barak et al.
[Eurocrypt 2018] can be broken using simple matrix algebra.
* We show that augmenting the CVW18 IO candidate with a matrix PRF provably
immunizes the candidate against all known algebraic and statistical zeroizing
attacks, as captured by a new and simple adversarial model
Directly revocable ciphertext-policy attribute-based encryption from lattices
Attribute-based encryption (ABE) is a promising type of cryptosystem achieving fine-grained access control on encrypted data.
Revocable attribute-based encryption (RABE) is an extension of ABE that provides revocation mechanisms when user\u27s attributes change, key exposure, and so on.
In this paper, we propose two directly revocable ciphertext-policy attribute-based encryption (DR-ABE) schemes from lattices, which support flexible threshold access policies on multi-valued attributes, achieving user-level and attribute-level user revocation, respectively.
Specifically, the revocation list is defined and embedded into the ciphertext by the message sender
to revoke a user in the user-level revocable scheme or revoke some attributes of a certain user in the attribute-level revocable scheme.
We also discuss how to outsource decryption and reduce the workload for the end user.
Our schemes are proved to be secure in the standard model, assuming the hardness of the learning with errors (LWE) problem
A Survey on Continuous Time Computations
We provide an overview of theories of continuous time computation. These
theories allow us to understand both the hardness of questions related to
continuous time dynamical systems and the computational power of continuous
time analog models. We survey the existing models, summarizing results, and
point to relevant references in the literature
Tight Cell-Probe Lower Bounds for Dynamic Succinct Dictionaries
A dictionary data structure maintains a set of at most keys from the
universe under key insertions and deletions, such that given a query , it returns if is in the set. Some variants also store values
associated to the keys such that given a query , the value associated to
is returned when is in the set.
This fundamental data structure problem has been studied for six decades
since the introduction of hash tables in 1953. A hash table occupies bits of space with constant time per operation in expectation. There has
been a vast literature on improving its time and space usage. The
state-of-the-art dictionary by Bender, Farach-Colton, Kuszmaul, Kuszmaul and
Liu [BFCK+22] has space consumption close to the information-theoretic optimum,
using a total of bits, while supporting all operations in
time, for any parameter . The term is referred to as the wasted bits per key.
In this paper, we prove a matching cell-probe lower bound: For
, any dictionary with wasted bits per key
must have expected operational time , in the cell-probe model with
word-size . Furthermore, if a dictionary stores values of
bits, we show that regardless of the query time, it must have
expected update time. It is worth noting that this is the first
cell-probe lower bound on the trade-off between space and update time for
general data structures.Comment: 35 page