The goal of a steganalyst is to distinguish stego objects sent by a steganographer with a secret message hidden inside from cover objects with no meaningful message. The steganalyst is only as good as her knowledge of the sender’s choices during the embedding. To create a security margin for the real world, security definitions often borrow Kerckhoffs, principle, i.e., they assume that the attacker knows the system. However, a strict interpretation of the principle, “only the key is secret,” does not apply to steganography because a public cover would compromise security. In this paper, we study two interpretations of Kerckhoffs’ principle for steganography using a conceptual framework of the steganalyst’s knowledge. The framework connects both interpretations and encompasses other existing notions of knowledge in the literature. Viewing access to the cover as a side-channel attack allows us to adapt Kerckhoffs’ principle to Simmons’ prisoners’ problem.