1. A data processing method comprising: applying a transformation to randomly masked input data comprising input data that is masked with a random additive mask, wherein said transformation replaces said additive mask with a multiplicative-additive mask comprising an additive component, wherein said additive component comprises k bits and said input data comprises n bits; using a first subset of said masked input data and said additive component of said multiplicative-additive mask in combination to select a lookup table (LUT) from a plurality of LUTs, wherein there are up to 2 k LUTs in said plurality of LUTs and up to 2 n-k entries per LUT, wherein said LUT is selected using a swap operation in which indices of said LUTs are exchanged according to bit values of said additive mask, wherein k bits of said masked input data are used with said additive component to select said LUT; and using a second subset of said masked input data to select an entry in said LUT corresponding to a transformed version of said input data, wherein the remaining n-k bits of said masked input data are used to select said entry.