Darknet signatures

                        Przemyslaaw Blaskiewicz, Anna lauks-Dutka 
                    Department of Fundamentals of Computer Science
                   Wroclaw University of Science and Technology, Poland

                                 Miroslaw Kutylowski
                      NASK - National Research Intitute,  Warsaw, Poland


Recently, solutions such as distributed ledgers have been promoted in order to achieve 
undeniability of past transactions. This creates the situation where signed transactions 
of a given person are recorded in a distributed data structure and are publicly available.

We show that behind such a fully legitimate and trustworthy ledger system one can create 
a perfect infrastructure for covert ledger for presumably illegal activities. 
Namely, a signer may hide signatures behind purely innocent signatures of legitimate transactions. 
Such signatures - called darknet signatures within this paper – can be used by the
signer to authenticate illegal operations, where signature delivery is by
means of the distributed ledger and thereby to some degree anonymous. 
The darknet signatures are undetectable apart from the designated recipient – the signatures posted
in the ledger look exactly the same as in case of an honest signer. In particular no extra data 
need to appear, thereby there are no witnesses of malicious activities in the form, 
e.g. of suspicious random components.

To demostrate reality of the threat, we present a simple example realization 
of darknet signatures hidden in Schnorr signatures. As the construction is quite elementary 
in this case, it should serve as a strong warning against using probabilistic signature schemes 
in a distributed ledger. 

accepted for DSPP'2023, Xi'an