S-box Selection in White-Box Cryptography
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Solution Overview
Problem
In whitebox cryptography, protecting inputs to the S-box layer of a symmetric cipher is challenging due to known attacks that target fixed encodings, and existing methods have weaknesses such as constant points that can be exploited by attackers.
Innovation Solution
The implementation of a cryptographic system using a substitution box with self-equivalent affine functions, where the minimum diversification number is maximized by constructing a set of self-equivalences to prevent fixed encodings and ensure that each input value can be mapped to multiple output values, thereby enhancing security against attacks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If fixed encodings are used in the S-box layer, then the implementation is simpler, but security is weakened due to exploitable constant points
Solution Approach 1:
The patent applies dynamics by making the S-box encoding variable rather than fixed. The system dynamically selects from multiple self-equivalent affine encodings based on the input value, ensuring that the encoding changes with each input. This prevents attackers from exploiting constant points while maintaining implementation feasibility through pre-computed encoding tables.
Solution Approach 2:
The patent changes the parameter of encoding from fixed to variable by introducing self-equivalent affine transformations. Different affine encodings are applied depending on the input value, changing the mapping parameters dynamically. This ensures that no single encoding is repeatedly used, thereby eliminating constant points that attackers could exploit.
2Reliability
If self-equivalent affine functions with high diversification number are used, then security is improved, but the complexity of selecting and implementing the appropriate S-box increases
Solution Approach 1:
The patent applies preliminary action by pre-computing and storing multiple self-equivalent affine encodings in lookup tables before execution. The diversification analysis and S-box selection criteria are predetermined and stored. During actual cryptographic operations, the system simply retrieves the appropriate pre-computed encoding based on the input, avoiding complex real-time calculations while maintaining high security.
3Object-affected harmful factors
If the minimum diversification number is maximized, then the number of exploitable constant points is reduced, but the computational overhead for determining and implementing the optimal S-box increases
Solution Approach 1:
The patent performs diversification analysis and S-box optimization in advance, storing the results in pre-computed tables. The minimum diversification number for each possible S-box configuration is calculated beforehand, and the optimal configurations are stored for quick retrieval. This eliminates the need for time-consuming computations during actual cryptographic operations, reducing time loss while maintaining maximum security.
Data Source
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Figure 4A~4D
AI summary
A method of implementing a cryptographic operation using a substitution box, comprising: specifying a set of self-equivalent functions for the substitution box; determining the minimum diversification number of the substitution box over the set of self-equivalent functions; comparing the minimum diversification number to a threshold value; including and implementing a cryptographic operation with selected substitution box when the minimum diversification number is greater or equal to a threshold value.