Patent ID: 7787620

Claim:
An improved method for performing a symmetric block cipher operation on a message block using a tamper resistant device having a microprocessor, the improved method comprising: (a) receiving an input message; (b) obtaining unpredictable information; (c) using said microprocessor, producing at least one masked intermediate parameter and a corresponding blinding parameter from at least said input message and said unpredictable information, such that: (i) the value of said masked intermediate parameter is unpredictable to an attacker; (ii) the value of said blinding parameter is unpredictable to an attacker; (iii) the value of an intermediate value for said block cipher operation is represented by a mathematical relationship between said masked intermediate parameter and said blinding parameter; (d) using said microprocessor, performing a plurality of rounds of said block cipher operation, where each said round includes changing said value for at least one of said parameters, and at least one round includes (x) obtaining additional unpredictable information relative to a prior round, and (y) using said additional unpredictable information, processing said masked intermediate parameter and said blinding parameter to obtain a new value for said masked intermediate parameter and a new value for said blinding parameter relative to a prior round; and (e) producing an output message from at least said masked intermediate parameter and said blinding parameter after said round operations in step (d).