ed25519 base point coordinates

Asked Jul 21 '21 at 14:06

Active Jul 21 '21 at 14:15

Viewed 114 times

For the ed25519 standard Base Point is $B = (x, 4/5)$. According to Stack Question the y coordinate equals

46316835694926478169428394003475163141307993866256225615783033603165251855960

But how this value is received? As I understand $y = 4 \cdot 5^{-1} \mod l$, where $l = 2^{252} + 27742317777372353535851937790883648493$.

So $5^{-1} \mod l$ gives

1447401115466452442794637312608598848171423271875981521200390187657090850198

and then $4 \cdot 5^{-1} \mod l$ calculates to

5789604461865809771178549250434395392685693087503926084801560750628363400792

Where do I make the mistake?

edited Jul 21 '21 at 14:15

Maarten Bodewes

asked Jul 21 '21 at 14:06

2

It's 4/5 mod p (the prime), not mod l (the group order). – the default. Jul 21 '21 at 14:13
What is the value of p? – Кирилл Волков Jul 21 '21 at 14:13
2^255 - 19, according to Google (which probably explains the curve name). – the default. Jul 21 '21 at 14:14
Yes, that's the origin of the name. Similarly, Poly1305 has nothing to do with the value 1305 :) – Maarten Bodewes Jul 21 '21 at 14:15
Oh, yes. I am foolish. Thank you very much! – Кирилл Волков Jul 21 '21 at 14:16
Please report back with the findings, would be nice if @thedefault. writes a short answer based on it! – Maarten Bodewes Jul 21 '21 at 14:17

0 Answers0