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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?

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    $\begingroup$ It's 4/5 mod p (the prime), not mod l (the group order). $\endgroup$ Jul 21, 2021 at 14:13
  • $\begingroup$ What is the value of p? $\endgroup$ Jul 21, 2021 at 14:13
  • $\begingroup$ 2^255 - 19, according to Google (which probably explains the curve name). $\endgroup$ Jul 21, 2021 at 14:14
  • $\begingroup$ Yes, that's the origin of the name. Similarly, Poly1305 has nothing to do with the value 1305 :) $\endgroup$
    – Maarten Bodewes
    Jul 21, 2021 at 14:15
  • $\begingroup$ Oh, yes. I am foolish. Thank you very much! $\endgroup$ Jul 21, 2021 at 14:16

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