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Object-Capability Model

Intro​

When thinking about security, it is good to start with a specific threat model. Our threat model is the following:

We assume that a thriving ecosystem of Cosmos SDK modules that are easy to compose into a blockchain application will contain faulty or malicious modules.

The Cosmos SDK is designed to address this threat by being the foundation of an object capability system.

The structural properties of object capability systems favor modularity in code design and ensure reliable encapsulation in code implementation.

These structural properties facilitate the analysis of some security properties of an object-capability program or operating system. Some of these — in particular, information flow properties — can be analyzed at the level of object references and connectivity, independent of any knowledge or analysis of the code that determines the behavior of the objects.

As a consequence, these security properties can be established and maintained in the presence of new objects that contain unknown and possibly malicious code.

These structural properties stem from the two rules governing access to existing objects:

  1. An object A can send a message to B only if object A holds a reference to B.
  2. An object A can obtain a reference to C only if object A receives a message containing a reference to C. As a consequence of these two rules, an object can obtain a reference to another object only through a preexisting chain of references. In short, "Only connectivity begets connectivity."

For an introduction to object-capabilities, see this Wikipedia article.

Ocaps in practice​

The idea is to only reveal what is necessary to get the work done.

For example, the following code snippet violates the object capabilities principle:

type AppAccount struct {...}
account := &AppAccount{
    Address: pub.Address(),
    Coins: sdk.Coins{sdk.NewInt64Coin("ATM", 100)},
}
sumValue := externalModule.ComputeSumValue(account)

The method ComputeSumValue implies a pure function, yet the implied capability of accepting a pointer value is the capability to modify that value. The preferred method signature should take a copy instead.

sumValue := externalModule.ComputeSumValue(*account)

In the Cosmos SDK, you can see the application of this principle in simapp.

https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/simapp/app.go

The following diagram shows the current dependencies between keepers.

Keeper dependencies