how to verify a user password with Pdf Encryption V 5

Hi, everyone

I am trying to verify a pasword with an encrypted pdf .

For my test, i have used a pdf wwith a blank user password.

I have join a hexadecimal dump of the crypt dictionary

Because my password is empty, i compute a sha256 hash of the user validation salt (the selected part of the U entry)

But the result is not equal to the 32 first bytes of the U entry.

What i have missed ?

Thanks in advance