Score:1

ed25519 base point coordinates

ph flag

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?

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