Algorithm of elliptic curve point addition in mixed Lopez Dahab-affine coordinates

Asked May 22 '19 at 07:32

Active May 22 '19 at 09:18

Viewed 176 times

In Guide to Elliptic Curve Cryptography, it provides the formulas to calculate the point addition in mixed Lopez Dahab-affine coordinates, as shown follows.

In the actual algorithm,

The code involves additional conditional statements to handle some special cases.

I am not sure what is the purpose of the condition block in line 9.

Could anyone explain the special case that this block trying to handle? Is this block always necessary?

edited May 22 '19 at 09:18

asked May 22 '19 at 07:32

mksit

9.1 is for the case P=Q. 9.2 is for the case P=-Q. So unless you can guarantee these two conditions will never occur then yes, this block is always necessary. – President James K. Polk May 22 '19 at 12:36
@JamesKPolk Why P = -Q when X3 = 0 and Y3 != 0 ? – mksit May 22 '19 at 17:13

Algorithm of elliptic curve point addition in mixed Lopez Dahab-affine coordinates

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