opkssh is a tool which enables ssh to be used with OpenID Connect allowing SSH access management via identities like alice@example.com instead of long-lived SSH keys. It does not replace ssh, but rather generates ssh public keys that contain PK Tokens and configures sshd to verify the PK Token in the ssh public key. These PK Tokens contain standard OpenID Connect ID Tokens. This protocol builds on the OpenPubkey which adds user public keys to OpenID Connect without breaking compatibility with existing OpenID Provider.

Currently opkssh is compatible with Google, Microsoft/Azure and Gitlab OpenID Providers (OP). If you have a gmail, microsoft or a gitlab account you can ssh with that account.

To ssh with opkssh you first need to download the opkssh binary and then run:

opkssh login

This opens a browser window where you can authenticate to your OpenID Provider. This will generate an SSH key in ~/.ssh/id_ecdsas which contains your OpenID Connect identity. Then you can ssh under this identity to any ssh server which is configured to use opkssh to authenticate users using their OpenID Connect identities.

ssh user@example.com

To ssh with opkssh, Alice first needs to install opkssh using homebrew or manually downloading the binary.

To install with homebrew run:

brew tap openpubkey/opkssh
brew install opkssh

To install manually, download the opkssh binary and run it:

To install on Windows run:

curl https://github.com/openpubkey/opkssh/releases/latest/download/opkssh-windows-amd64.exe -o opkssh.exe

To install on OSX run:

curl -L https://github.com/openpubkey/opkssh/releases/latest/download/opkssh-osx-amd64 -o opkssh; chmod +x opkssh

To install on linux run:

curl -L https://github.com/openpubkey/opkssh/releases/latest/download/opkssh-linux-amd64 -o opkssh; chmod +x opkssh

After downloading opkssh, on OSX or Linux run:

opkssh login

on Windows run:

.\opkssh.exe login

This opens a browser window to select which OpenID Provider you want to authenticate against. After successfully authenticating opkssh generates an SSH public key in ~/.ssh/id_ecdsas which contains your PK Token. By default this ssh key expires after 24 hours and you must run opkssh login to generate a new ssh key.

Since your PK Token has been saved as an SSH key you can SSH as normal:

ssh root@example.com

This works because SSH sends the SSH public key opkssh wrote in ~/.ssh/id_ecdsas to the server and sshd running on the server will send the public key to the opkssh command to verify.

To configure a linux server to use opkssh simply run (with root level privileges):

wget -qO- "https://raw.githubusercontent.com/openpubkey/opkssh/main/scripts/install-linux.sh" | sudo bash

This downloads the opkssh binary, installs it as /usr/local/bin/opkssh, and then configures ssh to use opkssh as an additional authentication mechanism.

To allow a user, alice@gmail.com, to ssh to your server as root, run:

sudo opkssh add root alice@gmail.com google

We use two features of SSH to make this work. First we leverage the fact that SSH public keys can be SSH certificates and SSH Certificates support arbitrary extensions. This allows us to smuggle your PK Token, which includes your ID Token, into the SSH authentication protocol via an extension field of the SSH certificate. Second, we use the AuthorizedKeysCommand configuration option in sshd_config (see sshd_config manpage) so that the SSH server will send the SSH certificate to an installed program that knows how to verify PK Tokens.

All opkssh configuration files are space delimited and live on the server. We currently have no configuration files on the client.

/etc/opk/providers contains a list of allowed OPs (OpenID Providers), a.k.a. IDPs. This file functions as an access control list that enables admins to determine the OpenID Providers and Client IDs they wish to rely on.

• Column 1: Issuer URI of the OP
• Column 2: Client-ID, the audience claim in the ID Token
• Column 3: Expiration policy, options are:
  • 24h - user's ssh public key expires after 24 hours,
  • 48h - user's ssh public key expires after 48 hours,
  • 1week - user's ssh public key expires after 1 week,
  • oidc - user's ssh public key expires when the ID Token expires
  • oidc-refreshed - user's ssh public key expires when their refreshed ID Token expires.

By default we use 24h as it requires that the user authenticate to their OP once a day. Most OPs expire ID Tokens every one to two hours, so if oidc the user will have to sign multiple times a day. oidc-refreshed is supported but complex and not currently recommended unless you know what you are doing.

The default values for /etc/opk/providers are:

# Issuer Client-ID expiration-policy 
https://accounts.google.com 206584157355-7cbe4s640tvm7naoludob4ut1emii7sf.apps.googleusercontent.com 24h
https://login.microsoftonline.com/9188040d-6c67-4c5b-b112-36a304b66dad/v2.0 096ce0a3-5e72-4da8-9c86-12924b294a01 24h

/etc/opk/providers requires the following permissions (by default we create all configuration files with the correct permissions):

sudo chown root:opksshuser /etc/opk/providers
sudo chmod 640 /etc/opk/providers

/etc/opk/auth_id is the global authorized identities file. This is a server wide file where policies can be configured to determine which identities can assume what linux user accounts. Linux user accounts are typically referred to in SSH as principals and we continue the use of this terminology.

• Column 1: The principal, i.e., the account the user wants to assume
• Column 2: Email address or subject ID of the user (choose one)
  • Email - the email of the identity
  • Subject ID - an unique ID for the user set by the OP. This is the sub claim in the ID Token.
• Column 3: Issuer URI

# email/sub principal issuer 
alice alice@example.com https://accounts.google.com
guest alice@example.com https://accounts.google.com 
root alice@example.com https://accounts.google.com 
dev bob@microsoft.com https://login.microsoftonline.com/9188040d-6c67-4c5b-b112-36a304b66dad/v2.0

To add new rule run:

sudo opkssh add {USER} {EMAIL} {ISSUER}

These auth_id files can be edited by hand or you can use the add command to add new policies. For convenience you can use the shorthand google or azure rather than specifying the entire issuer. This is especially useful in the case of azure where the issuer contains a long and hard to remember random string. For instance:

sudo opkssh add dev bob@microsoft.com azure

/etc/opk/auth_id requires the following permissions (by default we create all configuration files with the correct permissions):

sudo chown root:opksshuser /etc/opk/auth_id
sudo chmod 640 /etc/opk/auth_id

This is a local version of the auth_id file. It lives in the user's home directory (/home/{USER}/.opk/auth_id) and allows users to add or remove authorized identities without requiring root level permissions.

It can only be used for user/principal whose home directory it lives in. That is, if it is in /home/alice/.opk/auth_id it can only specify who can assume the principal alice on the server.

# email/sub principal issuer 
alice alice@example.com https://accounts.google.com

It requires the following permissions:

chown {USER}:{USER} /home/{USER}/.opk/auth_id
chmod 600 /home/{USER}/.opk/auth_id

We use a low privilege user for the SSH AuthorizedKeysCommandUser. Our install script creates this user and group automatically by running:

sudo groupadd --system opksshuser
sudo useradd -r -M -s /sbin/nologin -g opksshuser opksshuser

We then add the following lines to /etc/ssh/sshd_config

AuthorizedKeysCommand /usr/local/bin/opkssh verify %u %k %t
AuthorizedKeysCommandUser opksshuser

To log in using a custom OIDC provider, run:

opkssh login -provider https://authentik.local/application/o/opkssh/ -client-id SuperClientID

In the /etc/opk/providers file, add the following line to recognize the custom provider:

https://authentik.local/application/o/opkssh/,SuperClientID 24h

Then, add a user with:

opkssh add root alice@example.com https://authentik.local/application/o/opkssh/

We document how to manually install opkssh on a server here.