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FediDict UX Design, Part 2

Leonora Tindall 2018/09/02

This is the second of several posts on the design of my current ActivityPub-enabled Rust web application, FediDict. Each of these posts was be released to my Patreon patrons before being made publicly available. I recommend that you read the first UX design post first.

Any software that accepts input over the network will, eventually, be subject to attack, of some kind. Any federated software service must accept input over the network - that’s the whole point. In the case of FediDict, these attacks could come from either the user-facing side or the federation-facing, or server-to-server, side.

Either way, some of the technology choices I’ve made will help FediDict withstand attack, and in this post, I’ll explore how a solid access control model can help secure a system while still enabling rich control by administrators and a good user experience.

Inherent Sturdiness

FediDict is built in Rust, a language that promises memory safety for all programs written without the unsafe keyword. This eliminates (or, at least, mitigates to the point of insignificance) the majority of common memory-based attacks, like buffer overruns, use-after-free bugs, double-free bugs, and other such issues.

I’m also using a proven database system, though I’m not sure if PostgreSQL or MariaDB will be my final choice yet. Either one has a track record of high security, and FediDict will interface with them using Diesel, an object-relational mapping library which uses static analysis to ensure that certain types of database errors and bugs (like SQL injection) are not possible.

Access Control

Nonetheless, bugs exist in any piece of software, and application-level bugs are often far more impactful than are those in underlying code, precisely because they are so hard to predict and expose precisely what the attacker wants - the data held in the application.

One of the best ways to mitigate application-level bugs is to have a consistent (and consistently enforced) access control model. Rust’s type system helps me here.

A Pyramid Model

A dictionary produced by a small team of professionals is an easy thing to secure, but a crowdsourced (or, a semi-crowdsourced) one is not. In FediDict, I’ll have a few basic levels of access:

  1. The Guest. A guest has absolutely no write access, at all, but they have read access to most things. I’ll define exactly what later.
  2. The Contributor. A contributor has all the privelages a Guest has, as well as write access to the moderation queue (but, you’ll note, not to the site directly), and to the vote count of existing definitions, and to the report queue.
  3. The Moderator. A moderator has all the privelages a Contributor has, as well as being able to read the moderation queue and write from the moderation queue to the public database of definitions.

This is a tidy and convenient pyramid of access. Each level builds on the last, each group is smaller than the last, and all these privelages can be enforced with simple conditions, even at the database level.

Access becomes slightly more complex when I bring federated users into the mix. Because ActivityPub supports “liking” posts remotely, and I wish to support this as well, some people can write to the vote count of definitions without being Contributors.

Roles, not Tiers

This would appear to pose a problem, but in fact, it’s easy to solve with a concept known as Role Based Access Control, or RBAC. RBAC is a technique used by many complex systems, including Amazon Web Services and Google Cloud, but is applicable to simpler systems as well.

With RBAC, I’ll define a set of roles and assign those roles to users based on various factors. Then, when a user attempts to perform an action, FediDict will check for these roles. For instance, the roles could be:

I could use these roles in Rust through an enum. I’m not sure just how it will look, but for example:

impl Definition {
    // ... other code for definitions
    /// approve() sets a definition as approved, by the authority of the
    /// given user.
    fn approve(self, authority: &User) -> Result<Definition, Error> {
        if authority.has_role(Roles::QueueApprover) {
            let mut new = self;
            new.approved_by = Some(;
            return Ok(new);
        } else {
            return Err(Error::PermissionDenied(
                        "approve a definition from the moderation queue");

This could then be used in a pipeline. For instance:

// Assume I have user, the current user, and def, a definition, from elsewhere
// in the code.

In the case of an error, the information given is enough to render a message like:

Error: Permission denied. In order to approve a definition from the moderation queue, you need the QueueApprover role, which you do not have. Contact an administrator for assistance.

More Flexibility Without (Much) Complexity

I can still model tiers with these roles. A Guest has only the DefinitionReader role, while a Contributor has DefinitionReader, DefinitionSubmitter, and DefinitionEvaluator. A Moderator has all of those roles, in addition to QueueReader, QueueApprover, and QueueRejecter.

I can also construct other types of user from these roles. For instance, a site admin needs AccountCreator, AccountRemover, and AccountRoleAssigner, but could delegate account creation to others by giving them only AccountCreator.

Someone the organization trusts to remove spam but not to fully evaluate the validity of new definitions could have QueueReader and QueueRejecter but not QueueApprover.

A user interacting over ActivityPub can like and report as well as viewing the database, so they would get DefinitionReader and DefinitionEvaluator, but not any other roles.

With roles, I get the security provided by a solid tiered model with a flexibility that a tiered model can never provide.

Implementing Roles

This might seem like a difficult technique to implement, but actually, it’s very natural, especially using the combination of Rust’s expressive type system and a powerful RBDMS like PostgreSQL.

Each user will have a database record in the Users table with an associated ID. That record will specify their user name, e-mail address, (hashed) password, and any other information, including the ID of a row in the Roles table.

The Roles table will associate user IDs with a set of roles. Each role can be on, off, or none. This maps naturally to a nullable Boolean value in SQL, or to an Option<bool> in Rust.

Say, for instance, ther are three users, an admin, a moderator, and a contributor. The users table might hold three records, like so:

ID Username email password roles
12 ltindall a@b.c d7239dhs 02358
87 chughes c@x.y a2342nb0 83950
34 djanes q@m.z passw0rd 22741

And, the Roles table would have some associated records:

ID definition-reader definition-submitter queue-reader queue-approver account-creator
02358 null yes yes yes yes
83950 null yes yes yes no
22741 null null null null null

Here, some roles have explicit values and some do not. For instance, entry 83950 (associated with UID 87, user chuges) has no explicit value (null) for definition-reader and explicit values for all the other fields, while entry 22741 (associated with UID 34, user djanes) has default values everywhere.

This is valuable because, if a problem with the default settings is discovered, it would be quite difficult to set the roles for every user created before that point. With this system, values are only non-null if they have been explicitly set; null values resolve to defaults set elsewhere.

This approach also provides the flexibility I was looking for. If I decide to trust djanes with creating accounts but not, say, approving new definitions, I need only change the account-creator column of the Roles table to yes (or true, or however the database stores Boolean values) and that account can now create new accounts.

There is a major issue with this model, however. It does not have a good way to handle access control for users with no identity, or with an identity I don’t know about until they first interact with us over a federation protocol.

Anonymous Identity

Anonymous users - those interacting through an unauthenticated web interface and without identity provided by a federation partner - are actually pretty easy to handle. I already laid out the need for default permissions for logged-in users, so I can just add another set of defaults for not-logged-in users (probably just DefinitionReader).

New Introductions

Like other federated systems (such as e-mail), an ActivityPub identity is determined by both a username (like ltindall) and a domain (for instance, Assuming I hosted a FediDict instance on with the user accounts shown above, their full federated usernames would be,, and These are identities ComputerJargon.Online already knows about, but the system could get a “like” from a user on another ActivityPub-supporting instance at any time.

Let’s say Eugen, creator of Mastodon, follows the definitions created by users of ComputerJargon.Online and sees a definition he really likes. He could click the “like” button in the Mastodon UI, which would eventually notify FediDict that a like had been sent by Assuming ComputerJargon.Online had never interacted with Eugen before, he would not have an account in the ComputerJargon.Online database. So, how does FediDict determine his access rights?

Defaults come to the rescue again. With a set of defined defaults for accounts coming in from federation partners, FediDict simply has to create a new record for Eugen’s account in the accounts table (noting its remote origin) with a null access control row, giving it default values for remote accounts.

A Holistic View

These considerations alone are not enough to make a system secure, but with good underlying technologies and a solid access control model, which I’ll refine in future posts and as I begin to write more code, it should be easy to keep egregious bugs out and fix and issues that do make it into the codebase.

Security comes in layers, and this post has laid out a few of them: one at the very bottom, the language and underlying technology used to build the system, and one near the top, the access control model. I can fill in the rest as time goes on.