Template attacks are a special kind of side-channel attacks that work in two stages. In a first stage, the attacker builds up a database of template traces collected from a device which is identical to the attacked device, but under the attacker’s control. In the second stage, traces from the target device are compared to the template traces to recover the secret key. In the context of attacking elliptic curve scalar multiplication with template attacks, one can interleave template generation and template matching and reduce the amount of template traces. This paper enhances the power of this technique by defining and applying the concept of online template attacks, a general attack technique with minimal assumptions for an attacker, who has very very limited control over the template device. We show that online template attacks need only one power consumption trace of a scalar multiplication on the target device; they are thus suitable not only against ECDSA and static elliptic curve Diffie–Hellman (ECDH), but also against elliptic curve scalar multiplication in ephemeral ECDH. In addition, online template attacks need only one template trace per scalar bit and they can be applied to a broad variety of scalar multiplication algorithms. To demonstrate the power of online template attacks, we recover scalar bits of a scalar multiplication using the double-and-add-always algorithm on a twisted Edwards curve running on a smartcard with an ATmega163 CPU.

This paper describes an ECC implementation computing the X25519 keyexchange protocol on the Arm Cortex-M4 microcontroller. For providing protections against various side-channel and fault attacks we first review known attacks and countermeasures, then we provide software implementations that come with extensive mitigations, and finally we present a preliminary side-channel evaluation. To our best knowledge, this is the first public software claiming affordable protection against multiple classes of attacks that are motivated by distinct real-world application scenarios. We distinguish between X25519 with ephemeral keys and X25519 with static keys and show that the overhead to our baseline unprotected implementation is about 37% and 243%, respectively. While this might seem to be a high price to pay for security, we also show that even our (most protected) static implementation is at least as efficient as widely-deployed ECC cryptographic libraries, which offer much less protection.

This work implements and evaluates the recent complete addition formulae for the prime order elliptic curves of Renes, Costello and Batina on an FPGA platform. We implement three different versions:(1) an unprotected architecture; (2) an architecture protected through coordinate randomization; and (3) an architecture with both coordinate randomization and scalar splitting in place. The evaluation is done through timing analysis and test vector leakage assessment (TVLA). The results show that applying an increasing level of countermeasures leads to an increasing resistance against side-channel attacks. This is the first work looking into side-channel security issues of hardware implementations of the complete formulae.