My problem is I need to do users authentication through API. So I need to replace

if (!www_authenticate("$td", "subscriber")) {
	www_challenge("$td", "1");
}

With

if (!my_auth_script()) {
	www_challenge("$td", "1");
}

The main problem is - how can I grab or compare users password? I know nonce, which I understand is MD5 salt. Can I, for example, grab users password from API, then grab the MD5 string and the nonce user sent me, calculate MD5 on base of API password and nonce - and then compare MD5 strings sent by user and calculated?