Cryptographic Device Tamper-Resistant Shift Function
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Solution Overview
Problem
White-box implementations of block ciphers, such as DES, are vulnerable to differential fault attacks, where attackers can modify variables to emulate physical faults and recover secret keys, even if direct key extraction is prevented.
Innovation Solution
Incorporating tamper-resistance shift functions before the round function and output units to restrict modifications and errors, thereby handicapping differential fault attacks by ensuring that only authorized changes to encoded values are allowed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a white-box implementation of block cipher is used, then direct key extraction is prevented, but the implementation becomes vulnerable to differential fault attacks
Solution Approach 1:
The patent applies preliminary encoding transformations to the block cipher implementation before it is exposed to potential attacks. The cipher is transformed into an equivalent form with encoded variables and table look-up operations, which prevents direct key extraction while maintaining cryptographic functionality. This preliminary transformation is the foundation that enables subsequent fault attack resistance mechanisms.
Solution Approach 2:
The patent introduces intermediate encoding layers and verification mechanisms as mediators between the cryptographic operations and the external environment. These intermediaries include encoded variable representations, verification variables, and controlled modification mechanisms that prevent direct access to sensitive internal states while allowing controlled interactions. The mediators ensure that even if an attacker can observe or modify variables, they cannot directly extract keys or easily mount differential fault attacks.
2Difficulty of detecting and measuring
If variables are made accessible for observation, then implementation transparency is achieved, but secret key information may be exposed through fault attacks
Solution Approach 1:
The patent creates encoded copies of the cryptographic variables and operations rather than exposing the original plaintext representations. The block cipher is implemented using encoded variables that are mathematical equivalents but visually and structurally different from the original form. Table look-up operations work on these encoded copies, providing implementation transparency through observable operations while the encoded nature prevents direct inference of secret keys even when variables are observed.
Data Source
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AI summary
A cryptographic device (100) for calculating a block cipher (500) on a block cipher input (105) and producing a block cipher output (106), the block cipher calculation operating on encoded values (210), the cryptographic device comprising a round function unit (140; 300)for applying the final round (118) of the multiple rounds of cryptographic processing implementing the block cipher, a first output unit (160) and second output unit (180) for decoding encoded output data (132, 152).