Scan Chain Modifying Circuit for Secure IC Testing
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Solution Overview
Problem
Integrated circuits face a conflict between testability and secrecy, where testability requirements can compromise the protection of secret information, making it difficult to identify and access scan chain inputs and outputs, and existing solutions do not systematically address this issue.
Innovation Solution
A scan chain modifying circuit that varies the routing of test data through the scan chain, potentially using fuses or anti-fuses to enable or disable variable routing, creates a feedback loop, bypasses, or changes connections to obscure the identification of scan chain terminals, and employs a running key comparison to maintain a predetermined path or enable/disable transfer, making it harder to determine the scan chain inputs and outputs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If scan chain is used for testing, then testability is improved, but secrecy of circuit elements is compromised
Solution Approach 1:
The scan chain is segmented into multiple groups, with each group containing a subset of scan cells. By dividing the scan chain into segments and controlling access to each segment independently, the patent enables selective testing of circuit portions while protecting sensitive areas. This segmentation allows testers to access non-critical scan cells without exposing secret information stored in protected scan cells.
Solution Approach 2:
Secret information is extracted from the main scan chain by placing it in dedicated protected scan cells that are separated from the general testable circuit elements. These extracted secret elements are controlled by separate authorization mechanisms, allowing the main scan chain to function for testing while the extracted secret portions remain protected from unauthorized access.
2Object-affected harmful factors
If scan chain output is encrypted to protect secrecy, then secrecy is improved, but testability deteriorates
Solution Approach 1:
An authorization control circuit acts as an intermediary between the test equipment and the scan chain. This intermediary circuit selectively enables or disables access to different scan cell groups based on authorization status. When authorized, the intermediary allows normal scan operations; when unauthorized, it blocks access to protected scan cells while permitting testing of non-protected portions, thus maintaining testability without compromising secrecy.
3Object-affected harmful factors
If variable routing is used to obscure scan chain terminals, then secrecy is improved, but device complexity increases
Solution Approach 1:
The scan chain modifying circuit periodically changes the routing configuration of scan cells during the shifting process. By implementing time-varying permutations of scan cell connections, the patent makes it difficult to identify which external terminals correspond to scan chain inputs and outputs. The periodic reconfiguration occurs automatically during normal scan operations without requiring additional control circuits, thus limiting the increase in device complexity.
Data Source
AI summary
An integrated circuit comprises a scan chain with parallel inputs and outputs coupled to a functional circuit. A scan chain modifying circuit is provided coupled to the scan chain. When testing is authorized the scan chain modifying circuit operates in a mode wherein a normal shift path is provided through the scan chain. When testing is not authorized the scan chain modifying circuit operates to effect spontaneous dynamic changes in the shift path, which dynamically vary the length of the shift path between external terminals of the integrated circuit while shifting takes place. In an embodiment the dynamical variations are controlled by a running key comparison. In other embodiments running key comparison is used to disable transfer through the scan chain or operation of functional circuits.