Redmine

Hi,
I want to build a tool to automate some of my processes in company.
Therefore i want to build a "customer-dashboard" where my customers can log in.
I want to use my redmine as a leading user-management.

For the new application i want to import users from redmine, but in order to make it possible for customers to log in, i need the login credentials.
The REST API provides information about the username, but not the Password.

With the encrypted password (and of course infos about the algorythm used) i can rebuild the "login-function" of redmine.
The API does not provide the encrypted password.

How can i get it?
Is it possible to extend the API? or maybe there is a hidden function i can use?

Thanks in advance.