Migrate user passwords from drupal 6 to already existing drupal 8 site I have a drupal 6 site and a drupal 8 site. The drupal 8 site already exists and I do not want to do this on an empty drupal 8 install. Also I only want to migrate passwords. The users with their corresponding UIDs have already been brought over, and they do appear in the list of users, however their old passwords do not work on the new site. I'm assuming because drupal 8 uses a different encryption system. 

I looked into using this method https://www.drupal.org/docs/8/upgrade/upgrading-from-drupal-6-or-7-to-drupal-8 based on the answers provided for this question How do I migrate user passwords?
But that seems to only apply to situations when the site does not exist and I want to perform a brand new install of Drupal 8. And it supposes that I want to migrate all data when I only want to migrate the users. 

Upgrading and moving over all data (nodes, block layouts, etc.) is not an option. I do not want that data in the existing drupal 8 site. I need only the users (preferably just the user passwords). 

I'm willing to make a custom module to perform this import. Is there a way to migrate only the user passwords from a drupal 6 site into a drupal 8 site?
If you only want to migrate users and their passwords then you can do the same by using drush commands.
Firstly you have to install certain modules:
1.Migrate Upgrade
2.Migrate Plus
3.Migrate Tools

Then you have to create a manifest.yml file at the root of your d8 site.
The manifest.yml must look like this:

# user 
- d6_user 
- d6_user_profile_field 
- d6_user_profile_field_instance 
- d6_user_profile_entity_display 
- d6_user_profile_entity_form_display 
- d6_profile_values:user 
- d6_filter_format 
- d6_user_role 
- d6_user_picture_entity_display 
- d6_user_picture_entity_form_display 
- d6_user_picture_file 
- d6_user_picture_field 
- d6_user_picture_field_instance 


Then run the following drush command:

drush migrate-manifest --legacy-db url=mysql://d6user:d6pass@localhost/drupal_6 manifest.yml 


For further information follow the following page:https://www.drupal.org/docs/8/upgrade/upgrade-using-drush