Scan Chain Security via Open-Circuit Deadlock
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Solution Overview
Problem
Existing JTAG scan chain security methods are either permanent, rendering normal use impossible after securing, or they leave vulnerabilities that can be exploited, such as the JTAG infrastructure remaining accessible to attackers.
Innovation Solution
A scan chain security capability that enables controlled interruption and re-establishment of the scan chain using a scan chain security component, configured to create and remove open-circuit conditions in response to validation signals, allowing secure and reversible locking and unlocking of the scan chain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the TAP is burned or removed to prevent exploitation of the scan chain, then security against malicious access is improved, but the JTAG infrastructure becomes inaccessible for legitimate testing purposes
Solution Approach 1:
The patent introduces a deadlock mechanism that is activated in advance before the scan chain can be exploited. The OCD cell is configured to detect specific scan register values and automatically create an open-circuit condition, preventing malicious access before it can occur. This preliminary protective action ensures that even if attackers identify the JTAG wires, they cannot exploit the scan chain without first triggering the security mechanism.
Solution Approach 2:
The OCD cell acts as an intermediary component between the scan chain and potential attackers. It monitors the scan register values and mediates access by either allowing normal operation or creating a deadlock condition. This intermediary layer provides security without completely removing the JTAG infrastructure, enabling legitimate testing while blocking malicious exploitation.
2Reliability
If a fuse is burnt to disable JTAG access to the configuration area, then security is improved, but no JTAG access is possible until the burnt part is replaced
Solution Approach 1:
The patent implements a dynamic security mechanism where the OCD cell can switch between locked and unlocked states based on the scan register values. Unlike a burnt fuse that provides permanent disablement, the deadlock mechanism can be dynamically activated or deactivated by changing the scan register contents, allowing the system to adapt its security state as needed while maintaining restorability.
Solution Approach 2:
The security mechanism allows for temporary discarding of access (by creating a deadlock) while preserving the ability to recover it. The open-circuit condition created by the OCD cell can be removed by changing the scan register values, enabling the system to temporarily block access for security purposes while maintaining the capability to restore full functionality when needed.
3Object-affected harmful factors
If the scan chain is permanently secured by burning or removing components, then malicious access is prevented, but the DfT infrastructure becomes dead weight and cannot be used for in-field testing
Solution Approach 1:
The OCD cell is configured in advance to monitor specific scan register values and automatically activate the deadlock mechanism when threats are detected. This preliminary protective action prevents malicious exploitation while leaving the JTAG infrastructure intact and functional for legitimate testing purposes, maintaining adaptability without compromising security.
Solution Approach 2:
The OCD cell serves as an intermediary that selectively blocks malicious access while allowing legitimate testing operations. By monitoring scan register values and creating deadlocks only under specific conditions, it preserves the versatility of the DfT infrastructure for authorized purposes while preventing harmful exploitation.
4Ease of operation
If battery backup is used to support secure storage for JTAG access control, then subsequent JTAG access is enabled, but the real estate on the device becomes problematic
Solution Approach 1:
The patent extracts the security functionality from bulky external components (like battery-backed storage) and integrates it directly into the existing scan chain logic through the OCD cell. This eliminates the need for additional hardware real estate while maintaining secure access control capabilities, as the security mechanism is embedded within the existing JTAG infrastructure.
Solution Approach 2:
The OCD cell performs multiple functions: it monitors scan register values, detects potential exploitation attempts, creates deadlock conditions, and can be reset through normal scan operations. This multi-functional approach replaces what would otherwise require separate security hardware, battery backup, and storage components, significantly reducing the device real estate requirements.
Data Source
Figure 1A~1B
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AI summary
A scan chain security capability is provided herein. The scan chain security capability enables secure control over normal use of a scan chain of a system, e.g., for purposes such as testing prior to deployment or sale of the system, in-field testing after deployment or sale of the system, in-field modification of the system, and the like. The scan chain security capability enables secure control over normal use of a scan chain by enabling control over interruption of a scan chain and re-establishment of an interrupted scan chain. A scan chain security component is configured for removing an open- circuit condition from the scan chain in response to a control signal. The control signal may be generated in response to validation of a security key, in response to successful completion of a challenge-based authentication process, or in response to any other suitable validation or authentication. The scan chain security component also may be configured for creating an open- circuit condition in the scan chain in response to a second control signal. The second control signal may be a scan register value received via the scan chain.