nats-iam-broker*

*for the Decentralised Auth Callout flow

A configurable NATS micro-service that exchanges Identity Provider (IdP) tokens¹ for NATS user authorization JWTs. This component helps implement the decentralised auth callout flow:

1. A prospective NATS user authenticates with their IdP, and passes the obtained token to NATS.
2. This micro-service (re-)validates the IdP token.
3. This micro-service mints a new NATS user access token with specific authorizations.

¹https://www.iana.org/assignments/jwt/jwt.xhtml

What does it do?

The assumption is that implementations of auth callout microservices are almost identical, but for the process of determining which authorizations should be assigned to the minted NATS user access tokens.

This component implements the boilerplate code and using YAML configuration and golang templating, allows construction of JWTs with claims of the form:

claims(NATS_JWT) = f(claims(idpJwt))

where:

• f is a function that extracts and transforms claims from the IdP's Jwt.
• idpClaims is the IdP JWT supplied from the user.

Examples

The mock OIDC/OAuth2 service https://oidctest.wsweet.org/ is used to obtain an authenticated JWT id_token.

The example scripts in the scripts/ and examples/ folders demonstrate how to configure, deploy and test this micro-service. These are structred as:

• scripts
  • *-toolkit.sh, helper bash functions.
  • /templates/*, templates to use with vault/openbao and vault-plugin-secret-nats.
• examples
  • /<example_name>
    • /<name>_initial_setup.sh, one-time script to setup decentralised accounts and users
    • /<name>_start_service.sh, configures and starts the auth callout micro-service
    • /<name>_simulate_login.sh, contacts IdP, authenticates, and uses the obtained token to establish a client connection with NATS
    • /run.sh, run all steps in the example

Assuming docker is installed, the examples can be run using: make example-<example_name>.

Example 1: RGB.org

The RGB.org organisation has three departments that share a single NATS deployment. A department consists of several teams, with each team developing a set of applications.

The blue department has this setup.

• there are three teams: App Team 1, App Team 2, App Team 3

  • they have each developed apps that require access to NATS.
  • users of App Teami's apps have NATS credentials minted by NATS account APPi

• the department has configured and deployed a shared instance of the nats-iam-broker microservice.

  • the microservice connects to NATS using user minter of NATS account MINT_11.
  • minting accounts have an additional user nobody, that has no permissions.

Bob is a member of App Team 3, and wishes to use their in-house app demo-app. This section describes the authentication and authorization flow.

1. Bob launches demo-app.

   • demo-app directs him to OIDC Provider2 for authentication, which Bob completes.
   • demo-app receives back a signed JWT token jwt.provider2.bob.

2. demo-app obtains credentials¹ for MINT_11(nobody) and packages this into a NATS authorization_request (perhaps by calling nats.connect())².

3. NATS creates a connection_id for the Bob's instance of demo-app, and directs the MINT_11(nobody) connection to the blue department's nats-iam-broker microservice. This is because the microservice connects to NATS using MINT_11(minter), and the two have a common NATS account. The connection between NATS and nats-iam-broker is private³.

4. nats-iam-broker microservice receives the request.

   • it unpacks jwt.provider2.bob and validates the token against OIDC Provider2.
   • it performs additional validations, like checking JWT expiry/clock skew, etc.
   • an unsuccessful validation reports an Authorization Violation back to the user.

5. nats-iam-broker microservice mints a new JWT.

   • it inspects Bob's user/profile information in jwt.provider2.bob.
   • it determines that the account to issue+sign the minted token is APP3.
   • it creates a set of unique authorizations for Bob's demo-app usage.
   • it sets a suitable token expiry etc.

6. nats-iam-broker signs the minted JWT, encrypts it for transport and sends it to NATS server.

7. NATS server decrypts³ and validates the nats-jwt, and binds the authorizations to the client's connection_id. Finally, it notifies Bob's instance of demo-app of the successful connection.

---

¹ although nobody user credentials have no NATS permissions, storing them externally can facilitate key rotation of signing_key(MINT_11).

² a (somewhat) arbitrary decision has been made to pass the third-party JWT in the password field, i.e., nats.Connect(UserCredentials(MINT_11(nobody)), password=jwt.provider2.bob)

³ this uses the XKey field. It is optional but recommended.

Example 2: Basic

This example demonstates a simpler setup consisting of:

• one auth callout microservice MINT, with standard users minter and nobody
• one appliction account APP1

As in the previous example, authorised NATS users are created and signed on-the-fly, following successful validation of the IdP token.

Setup

The application is intended to be deployed as either:

• a single golang binary plus a (mergable set of) YAML configuration file(s).
• a helm-chart, see charts/nats-iam-broker/README.md.

Standard go templating is used to construct roles dynamically. Templating functions are listed in the filters module.

The configuration structure is outlined here.

Key	Type	Description
nats.url	string	NATS server URL
service.name	string	name of deployed micro-service instance
service.version	string	version of deployed micro-service instance
service.description	string	description of deployed micro-service instance
service.creds_file	string	user credentials used to connect to NATS
service.account.name	string	(metadata) human-readable reminder of account used to sign and encrypt communications with NATS server
service.account.signing_nkey	string	key used to sign new user-jwt returned to NATS
service.account.encryption.enabled	bool	toggle for communication-encryption with NATS server
service.account.encryption.xkey_secret	string	trusted secret-key for encryption with NATS server
nats_jwt.exp_max	duration	maximum duration of minted nats user-jwt
idp.client_id	string	the client identifier registered with the IdP
idp.issuer_url	string	the url of the IdP issuer
idp.validation.claims	[]string	set of required claims on idp token
idp.validation.aud	[]string	set of allowed values for audience claim
idp.validation.exp.min	duration	minimum time to expiry for idp token from now
idp.validation.exp.max	duration	maximum duration of idp token from now
rbac.user_accounts	-	set of accounts configured to issue and sign nats user-jwts
rbac.user_accounts[i].name	string	name of user-jwt signing account
rbac.user_accounts[i].public_key	string	public key of user-jwt signing account
rbac.user_accounts[i].signing_nkey	string	signing key of user-jwt signing account in nkey format
rbac.roles	-	set of referenceable nats jwt permission groupings
rbac.roles[i].name	string	role name
rbac.roles[i].permissions	jwt.Permissions	nats-io/jwt permissions structure (see link)
rbac.roles[i].limits	jwt.Limits	nats-io/jwt limits structure (see link)
rbac.role_binding[i].user_account	string	user account to bind
rbac.role_binding[i].roles	[]string	set of roles to bind
rbac.role_binding[i].match.claim	string	name of IdP JWT claim to match on
rbac.role_binding[i].match.value	string	corresponding value of IdP JWT claim to match on

References

1. youtube, synadia auth-callout: https://www.youtube.com/watch?v=VvGxrT-jv64
2. youtube: synaida decentralised auth callout: https://www.youtube.com/watch?v=5pQVjN0ym5w
3. natsbyexample: https://natsbyexample.com/examples/auth/callout-decentralized/cli
4. rethinking connectivity 19: https://github.com/synadia-io/rethink_connectivity/tree/main/19-auth-callout
5. mock oidc service: https://lemonldap-ng.org/documentation/latest/testopenidconnect.html