  In this paper we describe our flexible ECDSA design for elliptic curve over
binary extended fields GF(2l). We investigated its resistance against
Horizontal Collision Correlation Attacks (HCCA). Due to the fact that our
design is based on the Montgomery kP algorithm using Lopez-Dahab projective
coordinates the scalar k cannot be successful revealed using HCCA, but this
kind of attacks can be helpful to divide the measured traces into parts that
correspond to processing of a single bit of the scalar k. The most important
contribution of this paper is that our flexible field multiplier is resistant
against horizontal attacks. This inherent resistance makes it a valuable
building block for designing unified field multipliers.
