SSH Agent Background

OpenSSH includes ssh-agent, a program which holds private keys used for public key authentication. When a ssh client authenticates with a remote server and SSH_AUTH_SOCK is set, the client will connect to the agent's authentication socket and list the identities loaded into the agent. It then offers these identities to the remote host and if there is a matching identity on the remote host (in ~/.ssh/authorized_keys) the remote host will reply with a challenge for that identity. The client then sends that challenge to the agent and the agent returns the challenge signed with the private key of the identity. The client then sends this signed challenge to the remote host, thereby proving it has access to the identity. In the case of RSA key authentication, the remote host verifies the signed challenge by decrypting it the public key thereby proving the client had use of the private key. Other identity types work similarly.

The ssh client enables forwarding the agent connection to remote hosts, allowing a user to login to a third host using the identities stored on their local machine. This forwarding process can be repeated as necessary.

We can use this framework to allow groups of local users to use identities stored in one restricted user account. This allows a teams to share a common set of authentication credentials without giving each member a copy of these credentials, thus providing a single point to grant and revoke access.

Here is a diagram of a normal authentication sequence:

The authorization proxy and ssh-agent are running as a separate user which has the shared authentication credentials.

Agent Multiplexer

The ssh agent multiplexer is invoked by each user at login. It examines their environment for an existing forwarded SSH authentication socket and creates a new listening authentication socket. The multiplexer acts as a proxy for each ssh authentication socket connection. For enumeration requests, it connects to both the existing authentication socket and the authorization proxy, requests the list of identities from each socket, and returns the combined results. For signing requests, it forwards the request to the authentication socket which listed that identity. All other requests are forwarded the existing authentication socket. This allows the user to add and remove identities from their local agent.

Authorization Proxy

The authorization proxy runs as the restricted user which has access to the SSH private keys. The key pairs for each identity are loaded into the user's SSH agent (ssh-agent(1)) and the authorization proxy controls access to this agent. This is necessary because ssh-agent uses both file permissions on the agent socket and getsockopt(SOL_SOCKET, SO_PEERCRED) to restrict access the same user as the ssh-agent is running as. By inserting this additional proxy in to the agent path we can provide limited access to these identities to other users with appropriate access.

The authorization proxy can share much of its code with the agent multiplexer since it also needs to accept multiple concurrent ssh agent connections, modify list identities responses and govern sign requests.

Authorization will be stored in a SQLite database:

    CREATE TABLE IF NOT EXISTS identities (
        key_digest TEXT UNIQUE NOT NULL,
        name TEXT NOT NULL
    );

    CREATE TABLE IF NOT EXISTS authorizations (
        username TEXT NOT NULL,
        identity_id INTEGER         -- REFERENCES identities(rowid)
    );

Setup

    $ python setup.py install

Testing

Start a test ssh-agent

    $ ssh-agent
    SSH_AUTH_SOCK=/tmp/ssh-2Lp0DynTskn4/agent.11829; export SSH_AUTH_SOCK;
    SSH_AGENT_PID=11830; export SSH_AGENT_PID;
    echo Agent pid 11830;


    $ sshagentmux --socket /tmp/ssh-2Lp0DynTskn4/agent.11829 -- ssh-add -l