“Act natural!”: Exchanging Private Messages on Public Blockchains

Messengers have become an essential means of interpersonal interaction. Yet untraceable private communication remains an elusive goal, as most messengers hide content, but not communication patterns. The knowledge of communication patterns can by itself reveal too much, as happened, e. g., in the context of the Arab Spring. Subliminal channels in cryptographic systems enable untraceable private communication in plain sight. In this context, bulletin boards in the form of blockchains are a natural object for subliminal communication: accessing them is innocuous, as they rely on distributed access for verification and extension. At the same time, blockchain users generate hundreds of thousands of transactions per day that are individually signed and placed on the blockchain. Thus blockchains may serve as innocuous repository for publicly accessible cryptographic transactions where subliminal channels can be placed. In this paper, we propose a public-key subliminal channel using secret-recoverable splittable signature schemes on blockchains and prove that our construction is undetectable in the random oracle model under common cryptographic assumptions. Our approach is applicable to any secret-recoverable splittable signature scheme and introduces a constant overhead of a single signature per message. Such schemes are used by 98 of the top 100 cryptocurrencies. We also analyze the applicability of our approach to the Bitcoin, Monero, and RippleNet networks and present proof of concept implementations for Bitcoin and RippleNet.