OPKSSH makes it easy to SSH with single sign-on technologies

OPKSSH makes it easy to SSH with single sign-on technologies like OpenID Connect, thereby removing the need to manually manage and configure SSH keys.

OPKSSH is a project that makes it easy to SSH with single sign-on (SSO) technologies like OpenID Connect (OIDC), removing the need to manually manage and configure SSH keys. It does this without adding a trusted party other than your identity provider (IdP). We are excited to announce that OPKSSH has been open-sourced under the umbrella of the OpenPubkey project. While the underlying protocol OpenPubkey became an open source Linux foundation project in 2023, OPKSSH was closed source and owned by BastionZero (now Cloudflare). Cloudflare has gifted this code to the OpenPubkey project, making it open source.

In this post, we describe what OPKSSH is, how it simplifies SSH management, and what OPKSSH being open source means for you.

Background

A cornerstone of modern access control is single sign-on (SSO), where a user authenticates to an identity provider (IdP), and in response the IdP issues the user a token. The user can present this token to prove their identity, such as "Google says I am Alice". SSO is the rare security technology that both increases convenience - users only need to sign in once to get access to many different systems - and increases security.

OpenID Connect

OpenID Connect (OIDC) is the main protocol used for SSO. As shown below, in OIDC the IdP, called an OpenID Provider (OP), issues the user an ID Token which contains identity claims about the user, such as "email is alice@example.com". These claims are digitally signed by the OP, so anyone who receives the ID Token can check that it really was issued by the OP.

Unfortunately, while ID Tokens do include identity claims like name, organization, and email address, they do not include the user's public key. This prevents them from being used to directly secure protocols like SSH or End-to-End Encrypted messaging.

Note that throughout this post we use the term OpenID Provider (OP) rather than IdP, as OP specifies the exact type of IdP we are using, i.e., an OpenID IdP. We use Google as an example OP, but OpenID Connect works with Google, Azure, Okta, etc.

Figure 1: Shows a user Alice signing in to Google using OpenID Connect and receiving an ID Token

OpenPubkey

OpenPubkey, shown below, adds public keys to ID Tokens. This enables ID Tokens to be used like certificates, e.g. "Google says alice@example.com is using public key 0x123." We call an ID token that contains a public key a PK Token. The beauty of OpenPubkey is that, unlike other approaches, OpenPubkey does not require any changes to existing SSO protocols and supports any OpenID Connect compliant OP.

Figure 2: Shows a user Alice signing in to Google using OpenID Connect/OpenPubkey and then producing a PK Token

While OpenPubkey enables ID Tokens to be used as certificates, OPKSSH extends this functionality so that these ID Tokens can be used as SSH keys in the SSH protocol. This adds SSO authentication to SSH without requiring changes to the SSH protocol.

Why this matters

OPKSSH frees users and administrators from the need to manage long-lived SSH keys, making SSH more secure and more convenient.

"In many organizations - even very security-conscious organizations - there are many times more obsolete authorized keys than they have employees. Worse, authorized keys generally grant command-line shell access, which in itself is often considered privileged. We have found that in many organizations about 10% of the authorized keys grant root or administrator access. SSH keys never expire." - Challenges in Managing SSH Keys - and a Call for Solutions by Tatu Ylonen (Inventor of SSH)

In SSH, users generate a long-lived SSH public key and SSH private key. To enable a user to access a server, the user or the administrator of that server configures that server to trust that user's public key. Users must protect the file containing their SSH private key. If the user loses this file, they are locked out. If they copy their SSH private key to multiple computers or backup the key, they increase the risk that the key will be compromised. When a private key is compromised or a user no longer needs access, the user or administrator must remove that public key from any servers it currently trusts. All of these problems create headaches for users and administrators.

OPKSSH overcomes these issues:

Improved security: OPKSSH replaces long-lived SSH keys with ephemeral SSH keys that are created on-demand by OPKSSH and expire when they are no longer needed. This reduces the risk a private key is compromised, and limits the time period where an attacker can use a compromised private key. By default, these OPKSSH public keys expire every 24 hours, but the expiration