Section:
New Results
Discrete Logarithm computations in
finite fields with the NFS algorithm
The best discrete logarithm record computations in prime fields and
large characteristic finite fields are obtained with Number Field
Sieve algorithm (NFS) at the moment. This algorithm is made of four
steps:
-
polynomial selection;
-
relation collection (with a sieving technique);
-
linear algebra (computing the kernel of a huge matrix, of
millions of rows and columns);
-
individual discrete logarithm computation.
The two more time consuming steps are the relation collection step and
the linear algebra step.
The polynomial selection is quite fast but is very important since it
determines the complexity of the algorithm. Selecting better
polynomials is a key to improve the overall running-time of the NFS
algorithm.
A. Guillevic and F. Morain have written a chapter
[18] on discrete logarithm computations for a
book on pairings.
Breaking a MNT curve using DL computations
There is a reduction between an elliptic curve defined over
and a finite extension of degree (aka embedding
degree) of the base field, using pairing computations. In brief, one
can transport the discrete logarithm problem from to
. If is relatively small, this yields a DLP much
easier to solve than directly on . To give some highlight on
current easyness, A. Guillevic, F. Morain and E. Thomé (from CARAMBA
EPC in LORIA) computed a discrete log on a curve of embedding degree
3 and cryptographic size. This clearly showed that curves with small
embedding degrees are indeed weak. The article
[14] was presented by A. Guillevic during the
SAC 2016 conference in New Foundland.