Integrated Circuit Fuse-Based Scan Chain Security
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Solution Overview
Problem
Integrated circuits with secret information face a conflict between testability and secrecy, as existing fuse-based solutions are vulnerable to repair, allowing unauthorized access to secret data through scan chains.
Innovation Solution
The method involves using multiple types of fuse elements, where consistent access to the scan chain is permitted only when specific combinations of fuse elements are blown or not blown, making it difficult to identify which ones need to be repaired, and incorporating authorization control circuits to ensure secure testing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fuse elements are used to prevent access to secret information via scan chain, then secrecy is improved, but the circuit becomes vulnerable to repair attacks
Solution Approach 1:
The protection mechanism is divided into multiple independent fuse elements (first fuse elements and second fuse elements) that control different aspects of scan chain access. The first fuse elements control initial access to the test control circuit, while the second fuse elements control subsequent testing operations. This segmentation ensures that repairing one set of fuses is insufficient to compromise security, as the other set remains intact.
Solution Approach 2:
The first fuse elements are blown in advance during manufacturing or initial setup to establish the first level of security. This preliminary action creates a permanent barrier that prevents unauthorized access to the test control circuit unless the specific first fuse elements are repaired. The second fuse elements remain intact until needed for legitimate testing operations.
2Ease of operation
If scan chain is used for testing circuit elements, then testability is improved, but secret information becomes accessible
Solution Approach 1:
The test control circuit acts as an intermediary between the scan chain and the functional circuits containing secret information. It monitors and controls access to the scan chain through fuse element verification, allowing legitimate testing operations while blocking unauthorized access to secret data. The intermediary ensures that scan chain operations occur only under authorized conditions.
Solution Approach 2:
Different portions of the circuit are given different security characteristics. The test control circuit and its associated fuse elements have high security requirements to protect secret information, while other parts of the circuit maintain normal testability through the scan chain. This local differentiation allows simultaneous achievement of secrecy and testability in appropriate circuit regions.
3Reliability
If fuses are blown to prevent scan chain access, then secrecy is improved, but testing capability is reduced
Solution Approach 1:
The security configuration is made dynamic through the selective blowing of different fuse elements. The first fuse elements are blown to establish baseline security, while the second fuse elements remain intact to preserve testing capability. When legitimate testing is required, the system can transition between secure and testable states by controlling which fuse elements are blown, providing flexibility without compromising overall security.
Data Source
AI summary
An integrated circuit (10) comprises a functional circuit (12a-c) that contain information that must be secured against unauthorized access. The integrated circuit comprises a test access circuit (14, 16) coupled to the functional circuit (12a-c), and a plurality of fuse elements (18) coupled to the test access circuit (14, 16). The fuse elements (18) are connected in a circuit configuration that makes the functional circuit (12a-c) consistently accessible via the test access circuit (14, 16) only when first fuse elements (18) of the plurality are in a blown state and second fuse elements (18) of the plurality are in a not-blown state. As a result the integrated circuit can be tested after selectively blowing all of the first fuse elements (18). After testing at least part of the second fuse elements (18) is blown. As a result, a person that does not know which fuse elements are first fuse elements and which are second fuse elements is presented with difficulties to restore the integrated circuit to a state where test access with the danger of access to the secured information is possible.