International Association
for Cryptologic Research

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.

In response to standardization requests regarding password-authenticated key exchange (PAKE) protocols, the IRTF working group CFRG has setup a PAKE selection process in 2019, which led to the selection of the CPace protocol in the balanced setting, in which parties share a common password. In subsequent standardization efforts, the CPace protocol further developed, yielding a protocol family whose actual security guarantees in practical settings are not well understood. In this paper, we provide a comprehensive security analysis of CPace in the universal composability framework. Our analysis is realistic in the sense that it captures adaptive corruptions and refrains from modeling CPace's MapToPoint function that maps field elements to curve points as an idealized function. In order to extend our proofs to different CPace variants optimized for specific elliptic-curve ecosystems, we employ a new approach which represents the assumptions required by the proof as libraries accessed by a simulator. By allowing for the modular replacement of assumptions used in the proof, this new approach avoids a repeated analysis of unchanged protocol parts and lets us efficiently analyze the security guarantees of all the different CPace variants. As a result of our analysis, all of the investigated practical CPace variants enjoy adaptive UC security.

Increasingly connectivity becomes integrated in products and devices that previously operated in a stand-alone setting. This observation holds for many consumer applications in the so-called "Internet of Things" (IoT) as well as for corresponding industry applications (IIoT), such as industrial process sensors. Often the only practicable means for authentication of human users is a password. The security of password-based authentication schemes frequently forms the weakest point of the security infrastructure. In this paper we first explain why a tailored protocol designed for the IIoT use case is considered necessary. The differences between IIoT and the conventional Internet use-cases result in largely modified threats and require special procedures for allowing both, convenient and secure use in the highly constrained industrial setting. Specifically the use of a verifier-based password-authenticated key-exchange (V-PAKE) protocol as a hedge against public-key-infrastructure (PKI) failures is considered important. Availability concerns for the case of failures of (part of) the communication infrastructure makes local storage of access credentials mandatory. The larger threat of physical attacks makes it important to use memory-hard password hashing. This paper presents a corresponding tailored protocol, AuCPace, together with a security proof within the Universal Composability (UC) framework considering fully adaptive adversaries. We also introduce a new security notion of partially augmented PAKE that provides specific performance advantages and makes them suitable for a larger set of IIoT applications. We also present an actual instantiation of our protocol, AuCPace25519, and present performance results on ARM Cortex-M0 and Cortex-M4 microcontrollers. Our implementation realizes new speed-records for PAKE and X25519 Diffie-Hellman for the ARM Cortex M4 architecture.

Connectivity becomes increasingly important also for small embedded systems such as typically found in industrial control installations. More and more use-cases require secure remote user access increasingly incorporating handheld based human machine interfaces, using wireless links such as Bluetooth. Correspondingly secure operator authentication becomes of utmost importance. Unfortunately, often passwords with all their well-known pitfalls remain the only practical mechanism.We present an assessment of the security requirements for the industrial setting, illustrating that offline attacks on passwords-based authentication protocols should be considered a significant threat. Correspondingly use of a Password Authenticated Key Exchange protocol becomes desirable. We review the significant challenges faced for implementations on resource-constrained devices.We explore the design space and shown how we succeeded in tailoring a particular variant of the Password Authenticated Connection Establishment (PACE) protocol, such that acceptable user interface responsiveness was reached even for the constrained setting of an ARM Cortex-M0+ based Bluetooth low-energy transceiver running from a power budget of 1.5 mW without notable energy buffers for covering power peak transients.