Security Device

A security device, comprising:

• a modular reducer circuit configured to perform a modulo reduction by a modulus of each binary number of a sequence of binary numbers forming a data word, wherein each binary number consists of n bits and the modulus is smaller than 2n−1−1, by processing each binary number of the sequence by
• one or more first iterations comprising, in reaction to a first detector circuit of the security device detecting that the most significant bit of the binary number is set,
• changing the binary number by deleting its most significant bit and,
• further changing the binary number by adding the difference between 2n−1 and the modulus to the binary number
• followed by one or more second iterations comprising, in reaction to a second detector circuit of the security device detecting that the most significant bit of the sum of the binary number with the difference between 2n−1 and the modulus is set, setting the binary number to the sum of the binary number and the difference between 2n−1 and the modulus, wherein the most significant bit of the sum is deleted.