A data encryption method performed with ring arithmetic operations wherein a modulus C is be chosen of the form 2w−L, wherein C is a w-bit number and L is a low Hamming weight odd integer less than 2(w−1)/2. And in some of those embodiments, the residue mod C is calculated via several steps. P is split into 2 w-bit words H1 and L1. S1 is calculated as equal to L1+(H12x1)+(H12x2)+ . . . +(H12xk)+H1. S1 is split into two w-bit words H2 and L2. S2 is computed as being equal to L2+(H22x1)+(H22x2)+ . . . +(H22xk)+H2. S3 is computed as being equal to S2+(2x1+ . . . +2xk+1). And the residue is determined by comparing S3 to 2w. If S3