let users change own pwd with htpasswd

[lleemon]

I have created a password file using htpasswd utility.
It contails 2 users:
Johnencryptedpwd)
Jackencryptedpwd)

I have a cgi-page that allows only admins to add users to this file. I want to create a page for all users to be able to enter their username, current pwd, new password, new password again.

A script will change the old password with the new password depending on the following:
1. User exists
2. old pwd matches what currently is stored.

If 1 and 2 are true a script can call htpasswd and change the password.

My main question is: How do I decrypt my password file or compare the incoming old pwd to see if they match?

Thanks for any advice or directions.