Modular RESTful authentication API for Web Apps (Part 4)

This is the forth part of my Authentication and security for REST API in the context of Web Apps series of posts. In the previous post, we designed a RESTful authentication API. In this post, we will integrate several user authentication schemes to it (criteria 5).

As for the previous post, is used as the base IRI for new Linked-Data vocabulary terms and rest-auth is used as prefix for such terms.

It is important to note that some authentication schemes that have been traditionally implemented as single step processes (such as user/password authentication) have been switched to two or multi-steps processes in recent years for security reasons.

While a multi-steps process is not, a priori, RESTful, we offer an API behavior that would in fact remove any hidden state.

However, having API operations that are designed to be called in a pre-determined sequence could confuse user-agents and users, and generate increased error rates. To solve this problem, implementations SHOULD annotate their API Documentations with explicit relationships between the calls. For example, the Service Ontology could be used to achieve that.

Password-based authentication

Currently, the simplest and most common authentication mechanism is password based authentication: The user provides the server with a username and a password, if they match what the server has in its database, the credentials are accepted and the user is authenticated.

The way it is implemented on most websites however is rather lacking. Many websites do not perform CSRF protection on the login form, and the vast majority do not offer the user a chance to identify the platform before filling in their password, thus making the user vulnerable to phishing attempts (criteria 8).

Login CSRF allows an attacker to login the victim in the attacker’s own account. Any information the victim would then fill in, will be saved in the attacker’s account. If the attacked platform is linking accounts with sensitive platforms or data (e.g., credit card information) this might be a very serious flaw.

Since our API already request authentication operations to be protected from CSRF this situation will be averted.

Platform identification is usually the responsibility of an Extended-Validation TLS certificate. However, such certificates are expensive, can be hacked, and most users do not check them. Other platforms require the user to choose a “secret” image and a “personal” phrase (thereafter, anti-phishing proof). These should be changed on a regular basis and are shown back to the user after they entered their login but before they enter their password.

This is an excellent solution and extremely easy to implement. In particular in an SPA this should be easy to do without overwhelming the user with multiple login steps. For example, the anti-phishing proof could be fetched and shown automatically to the user when they remove the focus from the login field.

To support this authentication scheme, the API MUST provide two POST operations:

  1. The first one MUST require only the user’s public credentials (login name) as input (which is usually a nick name or email address).
  2. The second one MUST require a JWS JWT and the user’s secret credentials (password) as input.

The API MUST respond to the first operation with a very short term, signed JWS JWT. This token MUST NOT be used to replace the current credential token. This token MUST NOT be stored in any permanent storage. This token exp claim SHOULD grant the token a lifetime of a few minutes at most. This token sub claim MUST have the value the user provided as a login. This token MUST include appropriate private claims to present the user with their anti-phishing proof. This token MUST include private claims copying the value of the rest-auth:use-cookie and rest-auth:remember-me inputs.

This token is not intended to be used as API credentials, but only to provide the equivalent of a nonce, thus making replay attacks impossible.

Additionally, this token SHOULD be signed, and not simply granted a HMAC code. This additional restriction allows an automated agent to fetch the API’s public key and double check the signature. This will ensure the server is truly the one who provided the token. This automated check MUST NOT replace the user’s own verification of the anti-phishing proof without further check on the server signature (e.g., ensuring the certificate used to sign the token is actually well known to be associated with the intended service).

Other data than secret images and personal phrase MAY be used as anti-phishing proof, as long as they enable the user to double check (manually or automatically) that they are actually in communication with the service they initially opened an account with.

The client is then expected to invoke the second operation with both the token provided to them as a result of the first operation and the user’s secret credentials. The API MUST then process the user’s credentials and accept or reject these credentials.

If the API accepts the credentials, it should return an appropriate JWE JWT credentials token.

The rest-auth:use-cookie and rest-auth:remember-me inputs provided to the second operation MUST match the ones provided to the first operation. The API SHOULD ignore the values provided with the second operation if they don’t match.

These passwords SHOULD be required to be appropriately strong.

One-time passwords

These time or counter dependent passwords are getting more and more common. They are typically used for Multi-Factor Authentication.

In this case, we actually propose to offer them as a primary secret credential for authentication. Once users are getting more used to them, they are usually harder to loose than a password. Most users will more easily forget their password than, say, loose their smartphones. Moreover, we remove the need to create and remember a password, which is often the most problematic (both UX- and security-wise) step in the account creation process.

The API requirements are exactly the same as for password-based authentication, except instead of expecting a password as the second operation’s input, the API should expect the one-time password.

The API MAY provide the two authentications schemes:

If the API chooses to use a unified set of operations, it MAY use pattern-based restrictions to differentiate one time password from traditional password. For example, most one time passwords are a sequence of six-to-eight digits. Any traditional password that is such a sequence SHOULD therefore be forbidden.

Other two-parties single or multi-factor authentication schemes

Many other authentication schemes can be used with similar same operation sets. Adding secret credential inputs to the second operation allows easy Multi-Factor Authentication.

Many other authentication schemes requiring a single or two round-trips to the server (e.g., SRP) could be implemented (more or less seamlessly) in addition or replacement of the traditional or one time password schemes.

External trustee authentication schemes

External trustee authentication schemes such as OpenID, OAuth2, or SAML can be easily integrated with this API.

The simplest way would be to provide a POST operation for each such scheme. The OAuth provider or OpenID’s URL/XRI could be provided as the operation’s input.

As these authentication schemes rely on an external trustee, this means the Web App will be unloaded and reloaded with the trustee’s information.

It is very important to protect the part of the app loading the trustee’s information against Login CSRF.

To this end, the initial POST operation, providing the API with the user selection of the trustee MUST provide a return URL containing the anonymous JWT’s jti as a CSRF token. When returning from the trustee’s platform, the Web App MUST send:

If the jti of the anonymous JWT and the CSRF token do not match, the API MUST NOT issue an authenticated token. It SHOULD instead return a “400 Bad Request” Response. This along with the usual CSRF protection already granted by our JWT token ensures the API and Web App will not accept unrequested credentials tokens.

Our API is now able to integrate with widely different, including the most common, user authentication scheme. However, there are some security flaws that need to be further discussed in order to be more thoroughly mitigated against.