Data Recovery from "Scrubbed" NAND Flash Storage: Need for Analog Sanitization

Digital sanitization of flash based non-volatile memory system is a well-researched topic. Since flash memory cell holds information in the analog threshold voltage, flash cell may hold the imprints of previously written data even after digital sanitization. In this paper, we show that data is partially or completely recoverable from the flash media sanitized with "scrubbing" based technique, which is a popular technique for page deletion in NAND flash. We find that adversary may utilize the data retention property of the memory cells for recovering the deleted data using standard digital interfaces with the memory. We demonstrate data recovery from commercial flash memory chip, sanitized with scrubbing, by using partial erase operation on the chip. Our results show that analog scrubbing is needed to securely delete information in flash system. We propose and implement analog scrubbing using partial program operation based on the file creation time information.