## CryptoDB

### Akın Ünal

#### ORCID: 0000-0002-8929-0221

#### Publications

**Year**

**Venue**

**Title**

2023

EUROCRYPT

Worst-Case Subexponential Attacks on PRGs of Constant Degree or Constant Locality
★
Abstract

In this work, we will give new attacks
on the pseudorandomness of algebraic pseudorandom number generators (PRGs)
of polynomial stretch.
Our algorithms apply to a broad class of PRGs
and are in the case of general local PRGs faster than currently known attacks.
At the same time, in contrast to most algebraic attacks,
subexponential time and space bounds will be proven for our attacks
without making any assumptions of the PRGs or assuming any further conjectures.
Therefore, we yield in this text the first subexponential distinguishing attacks on PRGs
from constant-degree polynomials and close current gaps in the
subexponential cryptoanalysis of lightweight PRGs.
Concretely, against PRGs $F : \mathbb{Z}_q^{n} \rightarrow \mathbb{Z}_q^{m}$
that are computed by polynomials of degree $d$ over a field $\mathbb{Z}_q$
and have a stretch of $m = n^{1+e}$
we give an attack with space and time complexities
$n^{O(n^{1 - \frac{e}{d-1}})}$ and noticeable advantage
$1 - {O(n^{1 - \frac{e}{d-1}}/{q})}$.
If $q$ lies in $O(n^{1 - \frac{e}{d-1}})$, we give a second attack with
the same space and time complexities
whose advantage is at least $q^{-O(n^{1 - \frac{e}{d-1}})}$.
If $F$ is of constant \emph{locality} $d$ and $q$ is constant,
we construct a third attack that has a space and time complexity of
$\exp(O(n^{1 - \frac{e'}{(q-1)d-1}}))$ and noticeable advantage
$1-O(n^{-\frac{e'}{(q-1)d-1}})$ for every constant $e' < e$.

2022

TCC

The Price of Verifiability: Lower Bounds for Verifiable Random Functions
Abstract

Verifiable random functions (VRFs) are a useful extension of pseudorandom functions for which it is possible to generate a proof that a certain image is indeed the correct function value (relative to a public verification key). Due to their strong soundness requirements on such proofs, VRFs are notoriously hard to construct, and existing constructions suffer either from complex proofs (for function images), or rely on complex and non-standard assumptions.
In this work, we attempt to explain this phenomenon. We show that for a large class of pairing-based VRFs, it is not possible to obtain short proofs and a reduction to a simple assumption simultaneously. Since the class of "consecutively verifiable" VRFs we consider contains in particular the VRF of Lysyanskaya and that of Dodis-Yampolskiy, our results explain the large proof size, resp. the complex assumption of these VRFs.

2020

EUROCRYPT

Impossibility Results for Lattice-Based Functional Encryption Schemes
📺
Abstract

Functional Encryption denotes a form of encryption where a master secret key-holder can control which functions a user can evaluate on encrypted data.
Learning With Errors (LWE) (Regev, STOC'05) is known to be a useful cryptographic hardness assumption which implies strong primitives such as, for example, fully homomorphic encryption (Brakerski et al., ITCS'12) and lockable obfuscation (Goyal et al., Wichs et al., FOCS'17). Despite its strength, however, there is just a limited number of functional encryption schemes which can be based on LWE. In fact, there are functional encryption schemes which can be achieved by using pairings but for which no secure instantiations from lattice-based assumptions are known: function-hiding inner product encryption (Lin, Baltico et al., CRYPTO'17) and compact quadratic functional encryption (Abdalla et al., CRYPTO'18). This raises the question whether there are some mathematical barriers which hinder us from realizing function-hiding and compact functional encryption schemes from lattice-based assumptions as LWE.
To study this problem, we prove an impossibility result for function-hiding functional encryption schemes which meet some algebraic restrictions at ciphertext encryption and decryption. Those restrictions are met by a lot of attribute-based, identity-based and functional encryption schemes whose security stems from LWE. Therefore, we see our results as important indications why it is hard to construct new functional encryption schemes from LWE and which mathematical restrictions have to be overcome to construct secure lattice-based functional encryption schemes for new functionalities.

#### Coauthors

- Nicholas Brandt (1)
- Dennis Hofheinz (1)
- Julia Kastner (1)
- Akın Ünal (3)