Invisible Bits: Hiding Secret Messages in SRAM's Analog Domain

Electronic devices are increasingly the subject of inspection by authorities. While encryption hides secret messages, it does not hide the transmission of those secret messages - in fact, it calls attention to them. Thus, an adversary, seeing encrypted data, turns to coercion to extract the credentials required to reveal the secret message. Steganographic techniques hide secret messages in plain sight, providing the user with plausible deniability, removing the threat of coercion.

This paper unveils Invisible Bits a new steganographic technique that hides secret messages in the analog domain of Static Random Access Memory (SRAM) embedded within a computing device. Unlike other memory technologies, the power-on state of SRAM reveals the analog-domain properties of its individual cells. We show how to quickly and systematically change the analog-domain properties of SRAM cells to encode data in the analog domain and how to reveal those changes by capturing SRAM’s power-on state. Experiments with commercial devices show that Invisible Bits provides over 90% capacity - two orders-of-magnitude more than previous on-chip steganographic approaches, while retaining device functionality - even when the device undergoes subsequent normal operation or is shelved for months. Experiments also show that adversaries cannot differentiate between devices with encoded messages and those without. Lastly, we show how to layer encryption and error correction on top of our message encoding scheme in an end-to-end demonstration.