The width-w NAF method provides small memory and fast elliptic scalar multiplications secure against side channel attacks

The side channel attack (SCA) is a serious attack on wearable devices that have scarce computational resources. Cryptographic algorithms on them should be efficient using small memory - we have to make efforts to optimize the trade-off between efficiency and memory. In this paper we present efficient SCA-resistant scalar multiplications based on window method. Möller proposed an SPA-resistant window method based on 2^w-ary window method, which replaces w-consecutive zeros to 1 plus w-consecutive 1 and it requires 2^w points of table (or 2^w-1 + 1 points if the signed 2^w-ary is used). The most efficient window method with small memory is the width-w NAF, which requires 2^w-2 points of table. In this paper we convert the width-w NAF to an SPA-resistant addition chain. Indeed we generate a scalar sequence with the fixed pattern, e.g. |0..0cursive Greek chi|0..0cursive Greek chi|...|0..0cursive Greek chi|, where cursive Greek chi is positive odd points < 2^w. Thus the size of the table is 2^w-1, which is optimal in the construction of the SPA-resistant chain based on width-2 NAF. The table sizes of the proposed scheme are 6% to 50% smaller than those of Möller's scheme for w = 2,3,4,5, which are relevant choices in the sense of efficiency for 160-bit ECC.