ELECTRONIC CALCULATING DEVICE FOR PERFORMING OBFUSCATED ARITHMETIC

An electronic calculating device (100) for performing arithmetic in a commutative ring (Zn; Zn [x]/f(x)) is presented. The calculating device comprising a storage (110) arranged to store an increment table (T) defined for an increment ring element (1; ut), the increment table mapping an input ring element (k=uk1−uk2) to an output integer-list (T((k1 k2))=(I1, I2)) encoding an output ring element (I=uI1−uI2), such that the output ring element equals the increment ring element ring-added to the input ring element (I=k−1). Using the increment table, a ring addition unit (130) adds a first addition-input integer-list ((a1, a2)) encoding a first addition-input ring element and a second addition-input integer list ((b1, b2)) encoding a second addition-input ring element. The device may comprise a ring multiplication unit (140) also using the increment table.