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Though the mass manufacture of integrated circuits is known to produce identical logical functionality, variability inherent to the manufacturing processes creates delay characteristics unique to each integrated circuit produced. To identify a unique integrated circuit, a sequence of input stimuli exercising a large number of paths is applied to prompt an output response which is dependent on the delays of a large number of gates and wires. This input sequence and response are the secret signature of the integrated circuit to be documented for identification and authentication. The signature or any key that can be extracted from this signature is known only to the organization recording the input and response, and is nearly impossible to guess given the exponential number of input stimuli sequences and responses possible. The secret signature of the integrated circuit is interpretable as the silicon realization of a Physical Unclonable Function (PUF) resultant from the variation in manufacturing and like a thumbprint for each silicon chip. 

By designing the paths chosen and input stimuli creating the signature, the computational barrier can be raised to render the unique delay virtually impossible to recreate. Additionally, package barriers and control barriers can be used to further bar counterfeit fabrication. A potential package barrier could be achieved by inserting a scattering of conducting particles into the packaging so that delays measured after the packaging is removed are completely different. A control barrier would prevent a counterfeiter from controlling applied inputs by using a hash function on the stimulus presented at the package inputs. 

Beyond its irreproducibility, these integrated circuits signatures offer durability and environmental variability resistance. The most significant issues affecting chip performance are variable ambient temperature and circuit aging. In an authentication application, environmental variations can result in change of signatures, but small changes can be “forgiven” during the verification step. If the signature is to be used as a secret cryptographic key, error correction techniques can ensure that the same secret key is (re)generated by the chip repeatedly even under extreme environmental variation.