Hiding Information in Flash Memory Yinglei Wang Wingkei

Hiding Information in Flash Memory • Yinglei Wang, Wing-kei Yu, Sarah Q. Xu, Edwin Kan, and G. Edward Suh • Cornell University • Tuan Tran

Introduction Steganography? Yes

Presentation Outline �Overview �Flash Memory Background �Information Hiding Algorithm �Evaluation �Conclusion 3

Overview �The goal of the hiding technique is to make the detection, retrieval, and removal of hidden information sufficiently time consuming for an attacker. 4

Overview Flash Interface Requirements for the technique �Work with flash and float-gate non-volatile memory which can read, program and erase to specific memory location. �Can be implemented as a software update. 5

Flash Memory Background Float Gate Transistors � The floating gate is an insulated conductor surrounded by oxide. � Information is stored as the presence or absence of trapped charge on the floating gate. 6

Flash Memory Background Float Gate Transistors �Flash cells without charge are read as 1. �Flash cells have charge are read as 0. �Single-Level Cells: one bit is stored per cell. �Multi-Level Cells: multiple bits are stored per cell. 7

Flash Memory Background Flash Organization and Operation �Read: transistor is turned on and the amount of current is detected. �Erase: pushes charge off the floating-gate by applying a large negative voltage on the control gate. �Write: stores charge on the floating-gate �Page: the smallest unit in which data is read or written �Block: the smallest unit for an erase operation �Flash does not provide bit-level write or erase. 8

Flash Memory Background Aging �The voltages involved place great stress on the device oxide, wearing out the device. �The bit is rendered non-operational, leaving it in a stuck-at state. �The program time that is required to flip a state from ‘ 1’ to ‘ 0’ for a cell tends to reduce. 9

Flash Memory Background Partial Programming �Program time: the time it takes to program a Flash cell. �Flash memory interface requires all bits in a page to be programmed together. �The program time only reveals how long programming the entire page takes. 10

Flash Memory Background Partial Programming �Partial program: aborting a program operation before completion. �Partial programs: will accumulate charge on the floating gate and eventually result in the cell entering a stable programmed state. �The number of partial program operations to flip a bit from 1 to 0 represents the program time for the bit. 11

Information Hiding Algorithm Overview �The program time is the time it takes for a bit to change from the erased state (1) to the programmed state (0). �Need to be able to intentionally change and control each bit’s program time. �Stress some bits within a page more than others by controlling the value written to it. 12

Information Hiding Algorithm Overview �The program times of individual bits vary significantly due to manufacturing variations. �Encode one bit of hidden information using many bits in Flash memory. �Use a key (hiding key) to select which Flash bits will be grouped together. 13

Information Hiding Algorithm: � Choose set of page/block. � Divide the bits into fixed size group. � The algorithm determines which value ( 0 or 1) need to be written. 14

Information Hiding Algorithm: � Decide on a N to exert on Flash. � N is chosen to ensure good BER. � Each page is programmed N time to imprint the payload into Flash. 15

Information Hiding Algorithm Recovery Algorithm: � Use partial programming to measure the program time. � Choose M such that at the end of M partial programs, more than half of the bits, are programmed. � If bits do not flip, its program time is set to constant. 16

Information Hiding Algorithm Recovery Algorithm: � Compute the median program time. � If bit’s program time is above the median, set it to 1. � If bit’s program time is below the median, set it to 0. � X is chosen empirically. 17

Information Hiding Algorithm Recovery Algorithm: � Divide bits into group. � Compute average program time for each group. � Bit’s payload is set to 1 if the average program time of the group is below Th, 0 otherwise. � Th: the average program times of the more-stressed and less-stressed groups 18

Evaluation Setup �Use custom Flash test board. �Use multiple types of memory flash chip. �Used the first 4, 096 bits of 16, 896 -bit pages. 19

Evaluation Robustness – Bit Error Rate �Bit Error Rate : metric for measuring robustness. �Hide a randomly generated message into Flash memory and compared the retrieved message with the original. �Select 5120 groups and 5000 PE cycles: BER = 0. 0029 20

Evaluation Robustness � BER decreases as the hiding stress increases. � More stress increases the program time difference between bits hiding 1 s and 0 s. 21

Evaluation Robustness � BER decreases with an increasing group size. � The capacity decreases as more physical bits are included. � the statistical variations among groups will decrease as the group size increases. 22

Evaluation Robustness � Neighboring pages have a strong influence on each other. � Subset of pages with specific interval K. � There is not much benefit to using a group size beyond 128 and a page interval beyond 4. 23

Evaluation Effectiveness � Aim to simulate the normal usage of the Flash chip. � In each program operation for the initial stress, random data are programmed. � As initial stress level increases, the BER also increases 24

Evaluation Performance �For hiding : �Throughput : 16. 6 bits/second. �Higher with smaller number of PE cycles or group. �For reading: �Throughput: 564 bit/second. �Higher if hiding scheme uses a smaller number of Flash bits to encode each hidden bit. 25

Evaluation Detectability �Information hiding scheme uses per-bit program time. �The hiding operation does not change normal Flash functions. �An attacker needs to rely on checking the analog properties of the Flash memory. 26

Evaluation Detectability � There is no visible pattern in per-page program time. � The program time of a page shows distinct values. � The program time values for each chip stay the same. 27

Evaluation Retrieval without the Hiding Key � 10% of Correct Group Members � Group size is a security parameter 28

Evaluation Erase Tolerance � Stress the chip after hiding info. � Program every bit of the page to 0. � BER is quite reasonable. 29

Evaluation Different Flash Models �Tested several different Flash memory models. �Chips from the same manufacturer perform similarly. �In MLC chip: �Bits split into a fast group and a slow group. �Only the faster programming bits work for info hiding. 30

Conclusion �Demonstrate a technique to hide information using the program time of individual bits in Flash memory. �Using groups of bits to store one bit of payload allows the technique to effectively hide information robustly with low bit error rates. �Without the key, measuring analog characteristics of the Flash chip can not reveals whether the chip contains hidden information. 31

Q&A 32