DEUCE: Write-Efficient Encryption for Non-Volatile Memories

Phase Change Memory (PCM) is an emerging Non Volatile Memory (NVM) technology that has the potential to pro-vide scalable high-density memory systems. While the non-volatility of PCM is a desirable property in order to save leakage power, it also has the undesirable effect of making PCMmain memories susceptible to newer modes of security vulnerabilities, for example, accessibility to sensitive data if a PCM DIMM gets stolen. PCM memories can be made secure by encrypting the data. Unfortunately, such encryp-tion comes with a significant overhead in terms of bits writ-ten to PCM memory, causing half of the bits in the line to change on every write, even if the actual number of bits be-ing written to memory is small. Our studies show that a typ-ical writeback modifies, on average, only 12 % of the bits in the cacheline. Thus, encryption causes almost a 4x increase in the number of bits written to PCM memories. Such ex-traneous bit writes cause significant increase in write power, reduction in write endurance, and reduction in write band-width. To provide the benefit of secure memory in a write efficient manner this paper proposes Dual Counter Encryp-tion (DEUCE). DEUCE is based on the observation that a typical writeback only changes a few words, so DEUCE re-encrypts only the words that have changed. We show that DEUCE reduces the number of modified bits per writeback for a secure memory from 50 % to 24%, which improves per-formance by 27 % and increases lifetime by 2x