This article introduces a novel, low-cost technique for hiding data in commercially available resistive-RAM (ReRAM) chips. The data is kept hidden in ReRAM cells by manipulating its analog physical properties through switching ($\textit{set/reset}$) operations. This hidden data, later, is retrieved by sensing the changes in cells' physical properties (i.e., $\textit{set/reset}$ time of the memory cells). The proposed system-level hiding technique does not affect the normal memory operations and does not require any hardware modifications. Furthermore, the proposed hiding approach is robust against temperature variations and the aging of the devices through normal read/write operation. The silicon results show that our proposed data hiding technique is acceptably fast with ${\sim}0.4bit/min$ of encoding and ${\sim}15.625bits/s$ of retrieval rates, and the hidden message is unrecoverable without the knowledge of the secret key, which is used to enhance the security of hidden information.