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 1000-999999999 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>
  </listitem>
</varlistentry>