A Broadband Subliminal Channel in Signatures Without Sharing the Signing Key

The utilization of the broadband subliminal channel allows a sender to covertly transmit a message to a receiver through digital signatures. This method requires the sender to relinquish the signing key to the receiver. As a result, the receiver has the ability to employ the signing key to sign any data on behalf of the sender without the sender’s knowledge or consent. Meanwhile, difficulties may arise if the sender is unwilling to disclose the signing key to the receiver. In this paper, we propose a broadband subliminal channel that can be used in digital signature schemes (e.g., DSA, ECDSA, ElGamal, and Schnorr) without disclosing the signing key to the receiver. As it writes the message on a digital signature, we call it WMoS. We first implement WMoS in the Elliptic Curve Digital Signature Algorithm (ECDSA). We then provide the security proof to show that signatures generated in WMoS have the same security level as standard ECDSA signatures. Moreover, we discuss the variants of WMoS in ECDSA and use them to construct applications. Furthermore, we use the implementation of WMoS to generate a signature for an Ethereum transaction to demonstrate its feasibility. We also evaluate the efficiency of WMoS in ECDSA, and the results show that WMoS in ECDSA can generate a signature as efficiently as the standard ECDSA.