QUIDDIKEY

SRAM Physical Unclonable Functions or PUF use the behavior of standard SRAM, available in any digital chip, to differentiate chips from each other. Due to deep sub-micron process variations in the production process, every transistor in an IC and hence in an SRAM cell has slightly random electric properties. This randomness is expressed in the startup values of ‘uninitialized’ SRAM memory.

An SRAM memory consists of a number of SRAM cells. Each SRAM cell consists of two cross-coupled inverters that each are built up by a p- and n-MOS transistor. When power is applied to an SRAM cell, its logical state is determined by the relation between the threshold voltages of the p-MOS transistors in the invertors. The transistor that starts conducting first determines the outcome, a logical ‘0’ or ‘1’.

It turns out that an SRAM cell has its own preferred state every time the SRAM is powered resulting from the random differences in the threshold voltages. This preference is independent from the preference of the neighboring cells and independent of the location of the cell on the chip or on the wafer.

Hence the startup values of unitialized SRAM form a unique and random pattern of 0’s and 1’s. This pattern is like a silicon fingerprint since it is unique per SRAM and hence per chip. It can be used as a Physical Unclonable Function and is called the SRAM PUF response.

QUIDDIKEY Hardware IP Family

The SRAM PUF response is a noisy fingerprint, and turning it into a high-quality and secure key vault requires further processing. This is done with the QUIDDIKEY IP. QUIDDIKEY bundles the following techniques to reconstruct exactly the same cryptographic key every time and under all (environmental) circumstances in a very secure manner:

• error correction
• randomness extraction
• security countermeasures
• anti-aging techniques

More information on these techniques can be found here. QUIDDIKEY generates an Activation Code which, in combination with the SRAM startup behavior, is used to reconstruct on demand, in real time, an intrinsic PUF key which is never stored. When it is needed later it can be reconstructed. The intrinsic PUF key can be used as a root key for key derivation and wrapping. A key protected by QUIDDIKEY is integrity protected and can be retrieved only on the same device while it will be meaningless on other devices.