shadow/man/login.defs.d/SHA_CRYPT_MIN_ROUNDS.xml

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<varlistentry condition="sha_crypt">
  <term><option>SHA_CRYPT_MIN_ROUNDS</option> (number)</term>
  <term><option>SHA_CRYPT_MAX_ROUNDS</option> (number)</term>
  <listitem>
    <para>
      When <option>ENCRYPT_METHOD</option> is set to
      <replaceable>SHA256</replaceable> or
      <replaceable>SHA512</replaceable>, this defines the number of SHA
      rounds used by the encryption algorithm by default (when the number
      of rounds is not specified on the command line).
    </para>
    <para>
      With a lot of rounds, it is more difficult to brute forcing the
      password. But note also that more CPU resources will be needed to
      authenticate users.
    </para>
    <para>
      If not specified, the libc will choose the default number of rounds
      (5000).
    </para>
    <para>
      The values must be inside the 1000-999,999,999 range.
    </para>
    <para>
      If only one of the <option>SHA_CRYPT_MIN_ROUNDS</option> or
      <option>SHA_CRYPT_MAX_ROUNDS</option> values is set, then this value
      will be used.
    </para>
    <para>
      If <option>SHA_CRYPT_MIN_ROUNDS</option> &gt;
      <option>SHA_CRYPT_MAX_ROUNDS</option>, the highest value will be
      used.
    </para>
    <para condition="pam">
      Note: This only affect the generation of group passwords.
      The generation of user passwords is done by PAM and subject to the
      PAM configuration. It is recommended to set this variable
      consistently with the PAM configuration.
    </para>
  </listitem>
</varlistentry>