SCA-secure ECC in software - mission impossible?

This paper describes an ECC implementation computing the X25519 key-exchange protocol on the ARM-Cortex M4 microcontroller. This software comes with extensive mitigations against various side-channel and fault attacks and is, to our best knowledge, the first to claim affordable protection against multiple classes of attacks that are motivated by distinct real-world application scenarios. We also present results of a comprehensive side-channel evaluation. We distinguish between X25519 with ephemeral keys and X25519 with static keys and show that the overhead to protect the two is about 36% and 239% 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 more efficient than widely-deployed ECC cryptographic libraries, which offer much less protections.