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crtauth - a public key backed client/server authentication system

The latest version of this software can be fetched from GitHub.

crtauth is a system for authenticating a user to a centralized server. The initial use case is to create a convenient authentication for command line tools that interacts with a central server without resorting to authentication using a shared secret, such as a password.

The code available in this project is written in Python. There is also a Java version, implementing the same protocol available at

crtauth leverages the public key cryptography mechanisms that is commonly used by ssh(1) to authenticate users to remote systems. The goal of the system is to make the user experience as seamless as possible using the ssh-agent program to manage access to encrypted private keys without asking for a password each time the command is run

The name of the project is derived from the central concepts challenge, response, token and authentication, while at the same time reminding us old timers of the soon to be forgotten cathode ray tube screen technology.

Using the library

For the server side functionality there is a high level API available in the wsgi module. It provides wsgi middleware functionality that can be used to protect a service using the crtauth authentication mechanism. hello_world_server gives a minimal example on how this API is used. If crtauth is to be used in a non-WSGI environment, there is a lower level API available in the server module.

For clients an authentication plugin for Python Requests is available. An example use of the client module can be seen in the hello_world_client example.

Technical details

This section gives big picture overview of how crtauth operates. For the specifics of the protocol and it's messages, please see the specification.

Command line tools that connect to a central server to perform some action or fetch some information can be a very useful thing. crtauth is currently specified to work with HTTP as transport, but it is entirely possible to re-use that exposes information about servers using an HTTP-based API.

The basic operation of the protocol follows the following pattern

  • The client requests a challenge from the server, providing a username.
  • The server creates a challenge that gets sent back to the client.
  • The client signs the challenge and returns the response to the server.
  • The server verifies that the response is valid and if so it issues an access token to the client.
  • The access token is provided to when calling protected services.
  • The server validates that the token is valid and if so, provides access to the client.

The that implement this mechanism has two parts, one for the server and one for the client. A server that wants to authenticate clients instantiates an AuthServer instance (defined in the crtauth.server module) with a secret and a KeyProvider instance as constructor arguments. The very simple FileKeyProvider reads public keys from a filesystem directory using a filename pattern derived from the username of the connecting user.

Once there is an AuthServer instance, it can generate a challenge string for a specific user using the create_challenge() method.

The client part of the mechanism is also contained in the crtauth.server module, in the create_response() function. It takes a challenge string provided by the server and returns a response string suitable for sending back to the server.

The server in turn validates the response from the client and if it checks out it returns an access token that can be used by the client to make authenticated requests. This validation is done in the create_token() method of the AuthServer class.

For subsequent calls to protected services, the provided access token can be verified using the validate_token() method of the AuthServer instance.

SSH keys from LDAP

This library also provides functionality to extract public ssh keys for connecting users using an LDAP directory. To use this functionality, which is available in the module, the python-ldap module needs to be installed.


crtauth is free software, this code is released under the Apache Software License, version 2. The original code is written by Noa Resare with contributions from John-John Tedro, Erwan Lemmonier, Martin Parm and Gunnar Kreitz

All code is Copyright (c) 2011-2017 Spotify AB


a public key backed client/server authentication system




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