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How Ubuntu does encrypt user (session) password?

What algorithms/mechanisms are involved in that?

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See man crypt, which says, in part:

NOTES
Glibc notes
   The  glibc2  version  of  this  function supports additional encryption
   algorithms.

   If salt is a character  string  starting  with  the  characters  "$id$"
   followed by a string terminated by "$":

          $id$salt$encrypted

   then  instead  of  using  the DES machine, id identifies the encryption
   method used and this then determines  how  the  rest  of  the  password
   string is interpreted.  The following values of id are supported:

          ID  | Method
          ─────────────────────────────────────────────────────────
          1   | MD5
          2a  | Blowfish (not in mainline glibc; added in some
              | Linux distributions)
          5   | SHA-256 (since glibc 2.7)
          6   | SHA-512 (since glibc 2.7)

   So    $5$salt$encrypted    is   an   SHA-256   encoded   password   and
   $6$salt$encrypted is an SHA-512 encoded one.

   "salt" stands for the up to 16 characters following "$id$" in the salt.
   The  encrypted  part  of  the  password  string  is the actual computed
   password.  The size of this string is fixed:

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