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Access control in multi-party computation, by its nature, is set-centric. Unlike in traditional settings where only one entity, group, or role is involved in an action, in multi-party computation tasks an access is approved or denied by combining the decisions of all stakeholders. Ownership is also set-like, because the result of a joint computation task very likely belongs to all parties that have provided data.

We found that access control models in conventional systems like RBAC and ABAC are not powerful enough to govern data usages in Teaclave. Therefore, we invented our own access control model and mechanism.

Model

The access control model of Teaclave is configured through the file model.conf. The file has three sections:

• requests
• terms
• matchers

Before diving into the details about how our access control model works, we recommend readers learn about logic programming first, because our model configuration language is actually a home-baked tiny logic programming language.

request

A request is a query that should be answered by the access control service. The response is either "approved" or "denied." A request is defined as a tuple of any arity. For example,

[requests]
task_access_data = task, data

defines a request called task_access_data which contains two fields named task and data. This request can mean a task with the id task wants to access a piece of data hosted by Teaclave whose id is data.

term

Terms are relations over certain domains. Each term can be viewed as a table storing facts about the entities revelant to the access control logic. For example,

[terms]
data_owner = data, usr
task_participant = task, usr

For the task_access_data request, there are three relevant domains: data, usr, and task. Furthermore, two relations are required by Teaclave to make a decision, which are

• data_owner relation over (data X usr), denoting which user owns a piece of data.
• task_participant relation over (task X usr), denoting which users are the participants of a joint computation task.

An instance of the database describing the two terms could be

data_owner data_1, usr_1
data_owner data_2, usr_1
data_owner data_2, usr_2

task_participant task_1 usr_1
task_participant task_1 usr_2

The facts stored in this database instance indicate that data_1 is owned exclusively by usr_1, while data_2 is owned by usr_1 and usr_2 together. The facts also indicates that task_1 has two participants, i.e., usr_1 and usr_2.

matcher

The core logic used by Teaclave to resolve a request is defined as a matcher. We define the matcher for task_access_data request as the following

[matcher]
task_access_data = data_owner(task_access_data.data, _) <= task_participant(task_access_data.task, _)

data_owner(task_access_data.data, _) and task_participant(task_access_data.task, _) are term queries. The return value of data_owner(task_access_data.data, _) is a subset of the usr domain, where each value u in the fact meets the condition that

data_owner task_access_data.data u

is in the terms database. _ is called the query wild card and <= is the subset operator. Therefore, the matcher basically means that, the request for task to access data is approved only if all owners of data are have articipated in task.

Implementation

The access control module of Teaclave is implemented as a standalone service. Other components should send RPC requests to the service and get access control decisions as RPC responses.

The model configuration parser and request resolution engine are written in Python, powered by MesaPy. The access control service of Teaclave is a nice showcase of what MesaPy is capable of.

The implementation is purely experimental at this point. The performance is not optimized and the engine is likely not robust enough to avoid crashes while dealing with badly shaped requests. Contributions are welcome!