Soft Decision Error Correction for Compact Memory-Based PUFs Using a Single Enrollment

Secure storage of cryptographic keys in hardware is an essential building block for high security applications. It has been demonstrated that Physically Unclonable Functions (PUFs) based on uninitialized SRAM are an effective way to securely store a key based on the unique physical characteristics of an Integrated Circuit (IC). The start-up state of an SRAM memory is unpredictable but not truly random as well as noisy, hence privacy amplification techniques and a Helper Data Algorithm (HDA) are required in order to recover the correct value of a full entropy secret key. At the core of an HDA are error correcting techniques. The best known method to recover a full entropy 128-bit key requires 4700 SRAM cells. Earlier work by Maes et al. has reduced the number of SRAM cells to 1536 by using soft decision decoding; however, this method requires multiple measurements (and thus also power resets) during the storage of a key, which will be shown to be an unacceptable overhead for many applications. This article demonstrates how soft decision decoding with only a single measurement during storage can reduce the required number of SRAM cells to 3900 (a 17% reduction) without increasing the size of en-/decoder. The number of SRAM cells can even be reduced to 2900 (a 38% reduction). This does increase cost of the decoder, but depending on design requirements it can be shown to be worthwhile. Therefore, it is possible to securely store a 128-bit key at a very low overhead in an IC or FPGA.