LoMoS: Less-Online/More-Offline Signatures for Extremely Time-Critical Systems

The state-of-the-art digital signatures incur undesirable delays, hence are impractical for time-stringent Industrial Control Systems (ICSs). The recent revision to IEC 62351-6 standard stepped back from digital signatures in favor of symmetric key based solutions, thereby sacrificing key properties, e.g., scaling well for multiple destinations, easy key distribution and management, public verifiability, and non-repudiation. Inspired by the Online/Offline signatures, this paper presents a new digital signature model to provide the key properties of digital signatures within the delay requirements, hinting that this step back can be avoided. The Online/Offline signatures concept divides the signature generation into two phases; offline (before the message is given), online (using the outputs of the former for faster signing after the message is given). The conventional solutions following this concept potentially reduce the delay, yet do not meet IEC 61850 delay requirements as they still involve expensive operations in the online phase, and their offline phase hinders throughput. This paper introduces Less-online/More-offline Signatures (LoMoS) to enable minimal end-to-end delay and high message throughput. LoMoS entails avoiding expensive operations entirely during the online phase. We present a construction that converts any digital signature scheme into LoMoS, retains its properties, and unlike existing solutions, benefits from shorter messages.