Generating PHPass for WordPress Passwords

I am working on an application wnere I need to check for authenticity of user according to his wordpress userid/password. Trouble is, I don’t know how to check whether a password is correct or not.

I read at WordPress MD5 Password that after wordpress 2.5 phpass http://www.openwall.com/phpass/ was being used for wordpress passwords instead of MD5.

Question is, If I have

1. Username Password in plain text
2. wordpress Authentication Unique Keys and Salts from wp-config.php
3. Database access

then How can:
1. I check whether user-name/password combination is correct.

1. Change password for user

2. Implement forgot password implementation

WordPress version 3.0.4, Networked blogs being used with around 20 sites.

I know I should be doing it with native wordpress functions, but in this case it is not an option due to nature of the WordPress settings.

I’ve spent over a week on this and still clueless! help would be appreciated.