According to Using Composer documentation page:

*Optional: Save authentication information in auth.json to avoid repeatedly typing it*
To avoid having to always fill your credentials, add an auth.json file at your project root, next composer.json, or in your COMPOSER_HOME directory for machine-wide use.

I tried this procedure with a eZP 5.4 installation and the auth.json was ignored. After manually inserting the credentials, the auth.json file was created under */home/user/.composer/*

Steps to reproduce

1. Create an auth.json file in your ezroot folder, according to the documentaion
2. Execute **composer update --prefer-dist