Abstract
Edwards curves have attracted great interest for several reasons. When curve parameters are chosen properly, the addition formulas use only 10M + 1S. The formulas are strongly unified, i.e., work without change for doublings; even better, they are complete, i.e., work without change for all inputs. Dedicated doubling formulas use only 3M + 4S, and dedicated tripling formulas use only 9M + 4S.
This paper introduces inverted Edwards coordinates. Inverted Edwards coordinates (X 1:Y 1:Z 1) represent the affine point (Z 1/X 1,Z 1/Y 1) on an Edwards curve; for comparison, standard Edwards coordinates (X 1:Y 1:Z 1) represent the affine point (X 1/Z 1,Y 1/Z 1).
This paper presents addition formulas for inverted Edwards coordinates using only 9M + 1S. The formulas are not complete but still are strongly unified. Dedicated doubling formulas use only 3M + 4S, and dedicated tripling formulas use only 9M + 4S. Inverted Edwards coordinates thus save 1M for each addition, without slowing down doubling or tripling.
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Bernstein, D.J., Lange, T. (2007). Inverted Edwards Coordinates. In: Boztaş, S., Lu, HF.(. (eds) Applied Algebra, Algebraic Algorithms and Error-Correcting Codes. AAECC 2007. Lecture Notes in Computer Science, vol 4851. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-77224-8_4
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DOI: https://doi.org/10.1007/978-3-540-77224-8_4
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