14 research outputs found
Cryptanalysis of Two McEliece Cryptosystems Based on Quasi-Cyclic Codes
We cryptanalyse here two variants of the McEliece cryptosystem based on
quasi-cyclic codes. Both aim at reducing the key size by restricting the public
and secret generator matrices to be in quasi-cyclic form. The first variant
considers subcodes of a primitive BCH code. We prove that this variant is not
secure by finding and solving a linear system satisfied by the entries of the
secret permutation matrix.
The other variant uses quasi-cyclic low density parity-check codes. This
scheme was devised to be immune against general attacks working for McEliece
type cryptosystems based on low density parity-check codes by choosing in the
McEliece scheme more general one-to-one mappings than permutation matrices. We
suggest here a structural attack exploiting the quasi-cyclic structure of the
code and a certain weakness in the choice of the linear transformations that
hide the generator matrix of the code. Our analysis shows that with high
probability a parity-check matrix of a punctured version of the secret code can
be recovered in cubic time complexity in its length. The complete
reconstruction of the secret parity-check matrix of the quasi-cyclic low
density parity-check codes requires the search of codewords of low weight which
can be done with about operations for the specific parameters
proposed.Comment: Major corrections. This version supersedes previuos one
LEDAkem: a post-quantum key encapsulation mechanism based on QC-LDPC codes
This work presents a new code-based key encapsulation mechanism (KEM) called
LEDAkem. It is built on the Niederreiter cryptosystem and relies on
quasi-cyclic low-density parity-check codes as secret codes, providing high
decoding speeds and compact keypairs. LEDAkem uses ephemeral keys to foil known
statistical attacks, and takes advantage of a new decoding algorithm that
provides faster decoding than the classical bit-flipping decoder commonly
adopted in this kind of systems. The main attacks against LEDAkem are
investigated, taking into account quantum speedups. Some instances of LEDAkem
are designed to achieve different security levels against classical and quantum
computers. Some performance figures obtained through an efficient C99
implementation of LEDAkem are provided.Comment: 21 pages, 3 table
Security and complexity of the McEliece cryptosystem based on QC-LDPC codes
In the context of public key cryptography, the McEliece cryptosystem
represents a very smart solution based on the hardness of the decoding problem,
which is believed to be able to resist the advent of quantum computers. Despite
this, the original McEliece cryptosystem, based on Goppa codes, has encountered
limited interest in practical applications, partly because of some constraints
imposed by this very special class of codes. We have recently introduced a
variant of the McEliece cryptosystem including low-density parity-check codes,
that are state-of-the-art codes, now used in many telecommunication standards
and applications. In this paper, we discuss the possible use of a bit-flipping
decoder in this context, which gives a significant advantage in terms of
complexity. We also provide theoretical arguments and practical tools for
estimating the trade-off between security and complexity, in such a way to give
a simple procedure for the system design.Comment: 22 pages, 1 figure. This paper is a preprint of a paper accepted by
IET Information Security and is subject to Institution of Engineering and
Technology Copyright. When the final version is published, the copy of record
will be available at IET Digital Librar
Folding Alternant and Goppa Codes with Non-Trivial Automorphism Groups
The main practical limitation of the McEliece public-key encryption scheme is
probably the size of its key. A famous trend to overcome this issue is to focus
on subclasses of alternant/Goppa codes with a non trivial automorphism group.
Such codes display then symmetries allowing compact parity-check or generator
matrices. For instance, a key-reduction is obtained by taking quasi-cyclic (QC)
or quasi-dyadic (QD) alternant/Goppa codes. We show that the use of such
symmetric alternant/Goppa codes in cryptography introduces a fundamental
weakness. It is indeed possible to reduce the key-recovery on the original
symmetric public-code to the key-recovery on a (much) smaller code that has not
anymore symmetries. This result is obtained thanks to a new operation on codes
called folding that exploits the knowledge of the automorphism group. This
operation consists in adding the coordinates of codewords which belong to the
same orbit under the action of the automorphism group. The advantage is
twofold: the reduction factor can be as large as the size of the orbits, and it
preserves a fundamental property: folding the dual of an alternant (resp.
Goppa) code provides the dual of an alternant (resp. Goppa) code. A key point
is to show that all the existing constructions of alternant/Goppa codes with
symmetries follow a common principal of taking codes whose support is globally
invariant under the action of affine transformations (by building upon prior
works of T. Berger and A. D{\"{u}}r). This enables not only to present a
unified view but also to generalize the construction of QC, QD and even
quasi-monoidic (QM) Goppa codes. All in all, our results can be harnessed to
boost up any key-recovery attack on McEliece systems based on symmetric
alternant or Goppa codes, and in particular algebraic attacks.Comment: 19 page
Green Bitcoin: Global Sound Money
Modern societies have adopted government-issued fiat currencies many of which
exist today mainly in the form of digits in credit and bank accounts. Fiat
currencies are controlled by central banks for economic stimulation and
stabilization. Boom-and-bust cycles are created. The volatility of the cycle
has become increasingly extreme. Social inequality due to the concentration of
wealth is prevalent worldwide. As such, restoring sound money, which provides
stored value over time, has become a pressing issue. Currently,
cryptocurrencies such as Bitcoin are in their infancy and may someday qualify
as sound money. Bitcoin today is considered as a digital asset for storing
value. But Bitcoin has problems. The first issue of the current Bitcoin network
is its high energy consumption consensus mechanism. The second is the
cryptographic primitives which are unsafe against post-quantum (PQ) attacks. We
aim to propose Green Bitcoin which addresses both issues. To save energy in
consensus mechanism, we introduce a post-quantum secure (self-election)
verifiable coin-toss function and novel PQ secure proof-of-computation
primitives. It is expected to reduce the rate of energy consumption more than
90 percent of the current Bitcoin network. The elliptic curve cryptography will
be replaced with PQ-safe versions. The Green Bitcoin protocol will help Bitcoin
evolve into a post-quantum secure network.Comment: 16 page
Monoidic Codes in Cryptography
International audienceAt SAC 2009, Misoczki and Barreto proposed a new class of codes, which have parity-check matrices that are quasi-dyadic. A special subclass of these codes were shown to coincide with Goppa codes and those were recommended for cryptosystems based on error-correcting codes. Quasi-dyadic codes have both very compact representations and allow for efficient processing, resulting in fast cryptosystems with small key sizes. In this paper, we generalize these results and introduce quasi-monoidic codes, which retain all desirable properties of quasi-dyadic codes. We show that, as before, a subclass of our codes contains only Goppa codes or, for a slightly bigger subclass, only Generalized Srivastava codes. Unlike before, we also capture codes over fields of odd characteristic. These include wild Goppa codes that were proposed at SAC 2010 by Bernstein, Lange, and Peters for their exceptional error-correction capabilities. We show how to instantiate standard code-based encryption and signature schemes with our codes and give some preliminary parameters