Reference (aragonOS 3)

Documentation for aragonOS v3.1.2.

This document provides a technical overview of the architecture and can be used as a specification and developer guide. For a less technical introduction to aragonOS 3 you can check the alpha release blog post.

General architecture and design philosophy

Using aragonOS to build a system allows for decoupling of specific business logic of a protocol or application from its authentication logic. It allows you to code your application without thinking about authentication or governance at all. By inheriting from the AragonApp base class and defining actions that require authentication with a special modifier, aragonOS can handle authentication for the protocol.

Basic concepts: Proxy and Forwarder

Before describing general ideas about the architecture, it is important to understand two concepts the entire framework builds upon:

Proxy

A Proxy is a very simple smart contract construct which consists of decoupling the instance of a particular smart contract with the location of its actual business logic. We call individual instances of contracts Proxy and the logic base contracts. A Proxy delegates all its logic to a base contract. Upgradeability is achieved because this link to the base contract can be modified, effectively updating the Proxy business logic. We created ERC897 to standardize Proxy interfaces for better interoperability in the ecosystem.

Forwarder

A Forwarder is a contract that, given some conditions, will pass along a certain action to other contract(s).

Thanks to the fact that proxies allow a certain instance of a contract to never have to change its address even if its underlying logic changes, it's easy to identify a certain instance of a contract and helps with the decoupling of authentication and logic. For example, just by checking that an action's sender address is an instance of a Voting app with a particular address, we can know that the action must have been approved by a vote.

Architecture: Kernel and apps

An organization or protocol that is built with aragonOS is composed of two types of smart contracts:

Kernel

The kernel is at the core of every organization and there is only one instance of it per organization. It manages a very important mapping of base contract addresses of each application registered in the kernel (such as the ACL) or the kernel’s own base contract.

Apps

Apps are contracts that rely on the kernel for their upgradeability and access control.

Design philosophy

The design philosophy we use when developing Aragon apps is very similar to the UNIX philosophy: we try to architect them to do one thing and one thing well, and to respect and implement the few aragonOS interfaces so that they play nicely with the rest of the ecosystem.

This results in purely technical benefits such as testability, but it is also very powerful when apps are combined and the output of one app becomes the input of an other one (forwarders resemble UNIX pipes in some way).

Lifecycle of an aragonOS call

Kernel

The app mapping

At the core of the kernel lives a critical mapping called the app mapping.

Modifying this mapping can have completely destructive consequences and can result in loss of funds. The permission to execute this action has to be well protected behind the ACL.

function setApp(bytes32 namespace, bytes appId, address app) public;

• Namespace: specifies what type of app record is being set.

• AppId: used to identify what app is being set. It is the ENS namehash of the aragonPM repo (e.g. namehash('voting.aragonpm.eth')).

• App: Address of a contract that can have different meaning depending on the namespace.

Namespaces

• Core namespace (keccak256('core')): in this namespace is where the core components of the kernel reside. The only thing in the core mapping is the reference to the kernel base contract.

• Base namespace (keccak256('base')): keeps track of the base contracts for appIds.

• App namespace (keccak256('app')): some apps use the app namespace as a way to reference other apps. For example this is used to store the reference to the ACL instance or the EVMScriptsRegistry.

Upgradeability

Upgradeability of apps and the kernel is done by setting a new address for a specific key in the apps mapping in the kernel.

Kernel upgradeability

Kernel instances for different organizations can share the same implementation. Every Kernel instance is a KernelProxy . The logic for upgrading to a new implementation is in the implementation itself. An upgrade to the Kernel could render it un-upgradeable.

Upgrading the kernel of an organization is done by changing the Kernel appId in the Core namespace

kernel.setApp(kernel.CORE_NAMESPACE(), kernel.KERNEL_APP_ID(), newKernelCodeAddr)

AppProxies and upgradeability

In a similar fashion to the Kernel, apps can share implementation code to save gas on deployment. AppProxies rely on the Kernel for their upgradeability.

Upgrading an app is done by setting a new app address for the appId for the Base namespace in the kernel.

kernel.setApp(kernel.APP_BASES_NAMESPACE(), votingAppId, newVotingAppCodeAddr)

aragonOS provides two different types of App Proxies:

• UpgradeableAppProxy: in every call to the proxy, it checks with the Kernel what the current code for that appId is and forwards the call.

• PinnedAppProxy: on contract creation it checks and saves the app code currently in the Kernel. This cannot be upgraded unless the app code has explicit logic to change that storage slot.

There is an extra function in the Kernel that allows for setting the app code and at the same time creating a new proxy. This function only sets the code the first time.

kernel.newAppInstance(votingAppId, votingApp)
kernel.newPinnedAppInstance(votingAppId, votingApp)

ACL

A Permission is defined as the ability to perform actions (grouped by roles) in a certain app instance (identified by its address).

We refer to a Permission Instance as an entity holding a certain permission.

Managing permissions

First of all, you need to define your base ACL instance for your kernel with:

acl = ACL(kernel.acl())

Then you can execute the following actions:

Create permission

acl.createPermission(address entity, address app, bytes32 role, address manager)

createPermission() will fail if that permission has pre-existing permission instances.

This action is identical to grantPermission() except it allows the creation of a new permission if it doesn’t exist yet.

A role in the ACL protects access to createPermission() as this important function could be used in malicious ways. When the Kernel is initialized, it also creates the permission that grants the initializing address the ability to create new permissions.

Note that creating permissions is made mandatory by the ACL: all actions requiring yet-to-be-created permissions are disallowed by default. Any permission checks on non-existent permissions are failed automatically.

Grant permission

acl.grantPermission(address entity, address app, bytes32 role)

Grants role in app for an entity. Only callable by the manager of a certain permission. This entity would then be allowed to call all actions that their role can perform on that particular app until the permission manager revokes their role with revokePermission().

The grantPermission() action doesn’t require protection with the ACL because an entity can only make changes to a permission if it is the permission's manager.

Revoke permission

acl.revokePermission(address entity, address app, bytes32 role)

Revokes role in app for an entity. Only callable by the manager of a certain permission.

The revokePermission() action doesn’t need to be protected by the ACL either as an entity can only make changes if it is the manager for a given permission.

Adding permissions

Apps have the choice of which actions to protect behind the ACL as some actions may make sense to be completely public. Protecting an action behind the ACL is done in the smart contract by simply adding the authentication modifier auth() or authP()(passing the role required as a parameter) to the action. On executing the action, the auth()/authP() modifiers check with the Kernel whether the entity performing the call holds the required role or not.

Basic ACL

As an example, the following steps show a complete flow for user "Root" to create a new DAO with the basic permissions set so that a Voting app can manage the funds stored in a Vault app:

1. Deploy the Kernel and the ACL

2. Executing kernel.initialize(acl, rootAddress) which in turns calls acl.initialize(rootAddress) creates the "permissions creator" permission under the hood: createPermission(rootAddress, aclAddress, CREATE_PERMISSIONS_ROLE, rootAddress)

3. Deploy the Voting app

4. Grant the Voting app the ability to call createPermission(): grantPermission(votingAppAddress, aclAddress, CREATE_PERMISSIONS_ROLE) (must be executed by rootAddress)

5. Deploy the Vault app, which has a action called transferTokens()

6. Create a new vote via the Voting app to create the TRANSFER_TOKENS_ROLE permission createPermission(votingAppAddress, vaultAppAddress, TRANSFER_TOKENS_ROLE, votingAppAddress)

7. If the vote passes, the Voting app then has access to all actions in the Vault protected by TRANSFER_TOKENS_ROLE, which in this case is just transferTokens()

8. Fund transfers from the Vault can now be controlled via votes from the Voting app. Each time a user wishes to transfer funds, they can create a new vote via the Voting app to propose an execution of the Vault's transferTokens() action. The transferTokens() action will be executed if and only if the vote passes.

Note that the Voting app is also able to revoke or regrant the TRANSFER_TOKENS_ROLE permission as it is that permission's manager on vaultAppAddress.

Permission managers

As we have seen, when a permission is created a Permission Manager is set for that specific permission. The permission manager is able to grant or revoke permission instances for that permission.

The Permission Manager can be changed with this command:

acl.setPermissionManager(address newManager, address app, bytes32 role)

Changes the permission manager to newManager. Only callable by the manager of a certain permission.

The new permission manager replaces the old permission manager resulting in the old manager losing any management power over that permission.

createPermission() executes a special case of this action to set the initial manager for the newly created permission. From that point forward, the manager can only be changed with setPermissionManager().

There's also a getter for the Permission Manager:

acl.getPermissionManager(address app, bytes32 role)

Parameter interpretation

When a permission is granted to an entity by the permission manager it can be assigned an array of parameters that will be evaluated every time the ACL is checked to see if the entity can perform the action.

Parameters allow the ACL to perform certain computations with the arguments of a permission in order to decide whether to allow the action or not. This moves the ACL from being a purely binary access list to a more sophisticated system that allows for fine-grained control.

An ACL parameter is comprised of a data structure with 3 values:

• Argument Value (uint240): the value to compare against, depending on the argument. It is a regular Ethereum memory word that loses its two most significant bytes of precision. The reason for this was to allow parameters to be saved in just one storage slot, saving significant gas. Even though uint240s are used, it can be used to store any integer up to 2^30 - 1, addresses, and bytes32. In the case of comparing hashes, losing 2 bytes of precision shouldn't be a dealbreaker if the hash algorithm is secure.

• Argument ID (uint8): Determines how the comparison value is fetched. From 0 to 200 it refers to the argument index number passed to the role. After 200, there are some special Argument IDs:

  • BLOCK_NUMBER_PARAM_ID (id = 200): Sets comparison value to the block number at the time of execution. This allows for setting up timelocks depending on blocks.

  • TIMESTAMP_PARAM_ID (id = 201): Sets comparison value to the timestamp of the current block at the time of execution. This allows for setting up timelocks on time.

  • SENDER_PARAM_ID (id = 202): Sets comparison value to the sender of the call. (Currently useless)

  • ORACLE_PARAM_ID (id = 203): Checks with an oracle at the address in the argument value and returns whether it returned true or false (no comparison with the argument value).

  • LOGIC_OP_PARAM_ID (id = 204): Evaluates a logical operation and returns true or false depending on its result (no comparison with the argument value).

  • PARAM_VALUE_PARAM_ID (id = 205): Uses value as return. Commonly used with the RET operation to just return a value. If the value in the param is greater than 0, it will evaluate to true, otherwise it will return false.

• Operation type (uint8): Determines what operation is made to compare the value fetched using the argument ID or the argument value. For all comparisons, both values are compared in the following order args[param.id] <param.op> param.value. Therefore for a greater than operation, with a param = {id: 0, op: Op.GT, value: 10}, it will interpret whether the argument 0 is greater than 10. The implemented operation types are:

  • None (Op.NONE): Always evaluates to false regardless of parameter or arguments.

  • Equals (Op.EQ): Evaluates to true if every byte matches between args[param.id] and param.value.

  • Not equals (Op.NEQ): Evaluates to true if any byte doesn't match.

  • Greater than (Op.GT): Evaluates to true if args[param.id] > param.value.

  • Less than (Op.LT): Evaluates to true if args[param.id] < param.value.

  • Greater than or equal (Op.GTE): Evaluates to true if args[param.id] >= param.value.

  • Less than or equal (Op.LTE): Evaluates to true if args[param.id] <= param.value.

  • Return (Op.RET): Evaluates to true if args[param.id] is greater than one. Used with PARAM_VALUE_PARAM_ID, it makes args[param.id] = param.value, so it returns the parameter associated value.

While also representing an operation, when the argument ID is LOGIC_OP_PARAM_ID only the Ops below are valid. These operations use the parameter's value to point to other parameter indices in the parameter array. Any values are encoded as uint32 numbers, each left-shifted 32 bits (for example, an Op that takes two inputs with a value of 0x00....0000000200000001 would have input 1, 1, and input 2, 2, refering to params at index 1 and 2). Here are the available logic Ops:

• Not (Op.NOT): Takes 1 parameter index and evaluates to the opposite of what the linked parameter evaluates to.

• And (Op.AND): Takes 2 parameter indices and evaluates to true if both evaluate to true.

• Or (Op.OR): Takes 2 parameter indices and evaluates to true if any of them evaluate to true.

• Exclusive or (Op.XOR): Takes 2 parameter indices and evaluates to true if only one of the parameters evaluate to true.

• If else (Op.IF_ELSE): takes 3 parameters, evaluates the first parameter and if true, evalutes as the second parameter's evaluation, or as the third parameter's evaluation if false.

Parameter execution

When evaluating a rule the ACL will always evaluate the result of the first parameter. This first parameter can be an operation that links to other parameters and its evaluation depends on those parameters' evaluation. Execution is recursive and the result evaluated is always the result of the evaluation of the first parameter.

Parameter encoding

To encode some logic operations (AND, OR, IF-ELSE) which link to other parameters, the following helpers are provided, where the function arguments always refer to parameter indexes in the Param array they belong to:

If-Else (ternary) operation

encodeIfElse(uint condition, uint success, uint failure)

Binary operations (And, Or)

encodeOperator(uint param1, uint param2)

Examples of rules

The interpreter supports encoding complex rules in what would look almost like a programming language. For example let’s look at the following test case:

    function testComplexCombination() {
        // if (oracle and block number > block number - 1) then arg 0 < 10 or oracle else false
        Param[] memory params = new Param[](7);
        params[0] = Param(LOGIC_OP_PARAM_ID, uint8(Op.IF_ELSE), encodeIfElse(1, 4, 6));
        params[1] = Param(LOGIC_OP_PARAM_ID, uint8(Op.AND), encodeOperator(2, 3));
        params[2] = Param(ORACLE_PARAM_ID, uint8(Op.EQ), uint240(new AcceptOracle()));
        params[3] = Param(BLOCK_NUMBER_PARAM_ID, uint8(Op.GT), uint240(block.number - 1));
        params[4] = Param(LOGIC_OP_PARAM_ID, uint8(Op.OR), encodeOperator(5, 2));
        params[5] = Param(0, uint8(Op.LT), uint240(10));
        params[6] = Param(PARAM_VALUE_PARAM_ID, uint8(Op.RET), 0);

        assertEval(params, arr(uint256(10)), true);

        params[4] = Param(LOGIC_OP_PARAM_ID, uint8(Op.AND), encodeOperator(5, 2));
        assertEval(params, arr(uint256(10)), false);
    }

When assigned to a permission, this rule will evaluate to true (and therefore allow the action) only on the following conditions:

• If an oracle accepts it, and

• The block number is greater than the previous block number, and

• Either the oracle allows it (again! testing redundancy too) or the first parameter of the rule is lower than 10.

The possibilities for customizing an organization or protocol's governance model are truly endless and there is no need to write any actual Solidity.

Events

createPermission(), grantPermission(), and revokePermission() all fire the same SetPermission event that Aragon clients are expected to cache and process into a locally stored version of the ACL:

SetPermission(address indexed from, address indexed to, bytes32 indexed role, bool allowed)

setPermissionManager() fires the following event:

ChangePermissionManager(address indexed app, bytes32 indexed role, address indexed manager)

Forwarders and EVMScript

Forwarders are one of the most important concepts of aragonOS. Rather than hardcoding the notion of a vote into each separate app’s functionality and ACL one can instead use a generic Voting App, which implements the forwarding interface, to pass actions forward to other apps after successful votes. If the Voting App is set up to only allow a token’s holders to vote, that means any actions/calls being passed from it must have also been approved by the token’s holders.

Forwarding and transaction pathing

The forwarding interface also allows a frontend interface, like the Aragon client, to calculate "forwarding paths". If you wanted to perform an action but you don't have the required permissions, a client can think of alternative paths for execution. For example, you might be in the Vault app's interface wishing to perform a token transfer. If you only had the permission to create votes, the client would directly prompt you to create a vote rather than let you complete the transfer. The flow is illustrated in the following animation:

Vote forwarding scenario. (Please note that the governance model and characters are fictional). 👇

EVMScripts

We have designed our own scripting format, known as EVMScripts, to encode complex actions into a bytes representation that can be stored and later executed by another entity. EVMScripts can be installed on a per-organization basis through a EVMScriptRegistry and aragonOS comes complete with the ability to install multiple script executors in an organization.

EVMScript executors are contracts that take a script and an input and return an output after execution.

Script executors and EVMScriptRegistry

EVMScript executors must implement the following interface:

interface IEVMScriptExecutor {
    function execScript(bytes script, bytes input, address[] blacklist) external returns (bytes);
}

Because script executors get are called with a delegatecall, in order to prevent self-destructs, IEVMScriptExecutor.execScript(...) MUST return at least 32 bytes so in case an executor selfdestructs it could be detected.

CallsScript

aragonOS provides the CallsScript executor as a simple way to concatenate multiple calls. It cancels the operation if any of the calls fail.

• Script body: See CallsScript source code for spec of the payload.

• Input: None

• Output: None.

• Blacklist: Entire script reverts if a call to one of the addresses in the blacklist is performed.

DelegateScript delegatecalls into a given contract which basically allows for any arbitrary computation within the EVM in the caller’s context.

• Script body: Address of the contract to make the call to.

• Input: calldata for the delegatecall that will be performed.

• Output: raw return data of the call.

• Blacklist: impossible to enforce. If there are any addresses in the blacklist the script will revert as it is not possible to check whether a particular address will be called.

DeployDelegateScript

Is a superset of the DelegateScript but it takes a contract’s initcode bytecode as its script body instead of just an address. On execution, it deploys the contract to the blockchain and executes it with a delegatecall.

• Script body:: initcode for contract being created.

• Input: calldata for the delegatecall that will be performed after contract creation.

• Output: raw return data of the call.

• Blacklist: impossible to enforce. If there are any addresses in the blacklist the script will revert as it is not possible to check whether a particular address will be called.

Making an app a Forwarder

Examples of forwarders can be found in the aragon-apps repo. Both the Voting and the Token Manager are forwarders.

Warning

EVMScripts are very powerful and risk causing security breaches! For example, the Token Manager, which allows any token holder to forward actions, needs to have the token address in its blacklist as otherwise any token holder would effectively have control over the token in the same way that the Token Manager does!\