# Authentication The SIGNIUS portal supports two authentication methods – API Key Authentication and mutual TLS Authentication ## API Key authentication The API key string is a random string of considerable length, for example: > `X-Api-Key: 3H06Ydk2Al4aSBuT6zXAuXEqOI6VdSxZyADZshuvmvcTqr60WQBAob78Lpkjh7Le`. When you use an API key to authenticate, you always pass the key’s string as a **X-Api-Key** header sent to the SIGNIUS API, which is later used to authorize the user executing action. In regards of security the key string should be considered a password. ## Mutual Certificate authentication To ensure maximum communication security **SIGNIUS API** allows the use of certificate authentication process. In this solution the certificate public keys have to be exchanged first, before the communication process occurs (see **Getting Started**). Afterwards during standard communication each time a request is sent to the Signius API first a mTLS connection has to be estabilished. In the process each of the sides (Client and Server) trust each other based on previously exchanged certificates. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.signius.eu/api/authentication.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.