US Patent 9363073 Cryptographic countermeasure method by deriving a secret data

A method of protecting a circuit from attacks aiming to discover secret data used during the execution of a cryptographic calculation by the circuit, by, executing a transformation calculation implementing a bijective transformation function, receiving as input a secret data, and supplying a transformed data, executing a cryptographic calculation receiving as input a data to process and the transformed data, and executing an inverse transformation calculation receiving as input the result of the cryptographic calculation, and supplying a result that the cryptographic calculation would have supplied if it had been applied to the data to process and directly to the secret data, the data to process belong to a stream of a multiplicity of data, the transformed data being supplied as input to the cryptographic calculation for all the data of the stream.