  Cryptographic algorithms rely on the secrecy of their corresponding keys. On
embedded systems with standard CMOS chips, where secure permanent memory such
as flash is not available as a key storage, the secret key can be derived from
Physical Unclonable Functions (PUFs) that make use of minuscule manufacturing
variations of, for instance, SRAM cells. Since PUFs are affected by
environmental changes, the reliable reproduction of the PUF key requires error
correction. For silicon PUFs with binary output, errors occur in the form of
bitflips within the PUFs response. Modelling the channel as a Binary Symmetric
Channel (BSC) with fixed crossover probability $p$ is only a first-order
approximation of the real behavior of the PUF response. We propose a more
realistic channel model, refered to as the Varying Binary Symmetric Channel
(VBSC), which takes into account that the reliability of different PUF response
bits may not be equal. We investigate its channel capacity for various
scenarios which differ in the channel state information (CSI) present at
encoder and decoder. We compare the capacity results for the VBSC for the
different CSI cases with reference to the distribution of the bitflip
probability according a work by Maes et al.
