Data processing device and method for processing secret data

A method of processing secret data by a computing device for protection against a side-channel attack during performance of a cryptographic operation including an encryption operation, a decryption operation, and/or an authentication operation, the method comprising:

• during the cryptographic operation and for each data word of a first data word and a second data word from the secret data,
• breaking down, by the computing device, the data word into multiple partial representatives according to an Exclusive-Or sharing of the data word;
• transforming, by the computing device, at least one of the partial representatives to produce a transformed version of the partial representative;
• storing, by the computing device, for the at least one of the partial representatives, the transformed version of the partial representative in a processor register;
• and
• combining, by the computing device, the first data word and the second data word, involving, if the combination of the first data word with the second data word requires a partial representative of the first data word to be combined with a partial representative of the second data word that is stored in a processor register after having been transformed, combining the partial representative of the first data word with the partial representative of the second data word by means of a processor operation that takes into consideration that the partial representative of the second data word has been transformed.