Magento 2 Token based authentication:
Official docs: http://devdocs.magento.com/guides/v2.2/get-started/authentication/gs-authentication-token.html
Discussion about Magento 2 API authentication: https://community.magento.com/t5/Just-Ask-Alan/Magento-2-OAuth-authentication-and-REST-API-access/td-p/22528