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I have tried to calculate $trace$ of a coordinate $X$ of EC in binary representation. Before that I tried to pre-calculate traces of the various bits of $X$ using formula: $$Tr(X) = Tr(\sum_{i = 0}^{m-1} c_ix_i)= \sum_{i = 0}^{m-1}(c_i Tr(x_i) )$$

For the field $F_{2^{163}}$ that i use we have only 2 cases $x_i$ that yield $Tr(x_i) = 1$ to common $trace$. This cases when $i$ - 0 and 157. With 0 is ok but for others cases squaring the polynomials and reduction by modulus lead to the performance issues.

I would be appreciated if anyone proposes a more effective approach to such calculations. Or any references to ready implementations or services which can be used to calculate traces

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  • $\begingroup$ Needs a better title.. $\endgroup$
    – kelalaka
    Nov 25, 2018 at 18:02

1 Answer 1

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What do you mean by trace of "bits" of $X$? How do you define $x_i$?

Since $263$ is prime, the field $\mathbb{F}_{2^{163}}$ has no subfields other than the prime field $\mathbb{F}_2.$ This means that the trace function $$Tr:\mathbb{F}_{2^{163}}\rightarrow \mathbb{F}_2$$ is the only trace which exists for this field. However, this means that the function $Tr(u)$ is equidistributed on $\mathbb{F}_{2}$, which means it is $0$ half the time and $1$ half the time as $u$ ranges over $\mathbb{F}_{2^{163}}.$

This property would also apply to any "bit" of $X$, reasonably defined as the coordinate of $X$ with respect to some basis element.

Edit: If as in my comment $u=\alpha^i$, squaring gives $u^2=\alpha^{2i}.$ So square of $e_i$ is $e_{2i}$ but if $2i\geq n=163$ it needs to be reduced mod the field defining polynomial.

In $\mathbb{F}_{2^4}$. defined by $\alpha^4+\alpha+1=0,$ $\alpha=e_1=0010,$ $\alpha^2=e_2=0100,$ but $\alpha^4=\alpha+1=e_1+e_0=0011.$

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  • $\begingroup$ I mean that $trace$ is linear function and any coordinates can be separated per bits. $x_i$ are jusr $.....00000100 , ...0000001000 $ and so on $\endgroup$ Nov 25, 2018 at 19:41
  • $\begingroup$ OK but what $i=0,157$ represent is still unclear since the bits are basis dependent. What does your sentence "yield 0 to common trace" really mean? Write it mathematically. $\endgroup$
    – kodlu
    Nov 25, 2018 at 21:10
  • $\begingroup$ X coordinate for EC $F_{2^{163}} $ is bit row of 163 bits. So $x_0$ here all bits are 0 but the rightmost bit is 1, $x_{157}$ here all bits are 0 except 157-th bit that is 1 $\endgroup$ Nov 26, 2018 at 13:16
  • $\begingroup$ Oh, it seems like $x_i$ are what's usually called $e_i$, unit vectors with only one bit nonzero. $\endgroup$
    – kodlu
    Nov 26, 2018 at 19:43
  • $\begingroup$ Yes, @kodlu, but say me please what is the result of squaring such unit vector $e_i ^2$? I mean $e_i^2 = e_i$, right? $\endgroup$ Nov 27, 2018 at 17:05

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