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cosmos-sdk/docs/architecture/adr-030-authz-module.md

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ADR 030: Authorization Module

Changelog

  • 2019-11-06: Initial Draft
  • 2020-10-12: Updated Draft
  • 2020-11-13: Accepted

Status

Accepted

Abstract

This ADR defines the x/authz module which allows accounts to grant authorizations to perform actions on behalf of that account to other accounts.

Context

The concrete use cases which motivated this module include:

  • the desire to delegate the ability to vote on proposals to other accounts besides the account which one has delegated stake
  • "sub-keys" functionality, as originally proposed in #4480 which is a term used to describe the functionality provided by this module together with the fee_grant module from ADR 029 and the group module.

The "sub-keys" functionality roughly refers to the ability for one account to grant some subset of its capabilities to other accounts with possibly less robust, but easier to use security measures. For instance, a master account representing an organization could grant the ability to spend small amounts of the organization's funds to individual employee accounts. Or an individual (or group) with a multisig wallet could grant the ability to vote on proposals to any one of the member keys.

The current implementation is based on work done by the Gaian's team at Hackatom Berlin 2019.

Decision

We will create a module named authz which provides functionality for granting arbitrary privileges from one account (the granter) to another account (the grantee). Authorizations must be granted for a particular Msg service methods one by one using an implementation of Authorization.

Types

Authorizations determine exactly what privileges are granted. They are extensible and can be defined for any Msg service method even outside of the module where the Msg method is defined. Authorizations use the new ServiceMsg type from ADR 031.

Authorization

type Authorization interface {
	// MethodName returns the fully-qualified Msg service method name as described in ADR 031.
	MethodName() string

	// Accept determines whether this grant permits the provided sdk.ServiceMsg to be performed, and if
	// so provides an upgraded authorization instance.
	// Returns:
	// + allow: true if msg is authorized
	// + updated: new Authorization instance which should overwrite the current one with new state
	// + delete: true if Authorization has been exhausted and can be deleted from state
	Accept(msg sdk.ServiceMsg, block abci.Header) (allow bool, updated Authorization, delete bool)
}

For example a SendAuthorization like this is defined for MsgSend that takes a SpendLimit and updates it down to zero:

type SendAuthorization struct {
	// SpendLimit specifies the maximum amount of tokens that can be spent
	// by this authorization and will be updated as tokens are spent. If it is
	// empty, there is no spend limit and any amount of coins can be spent.
	SpendLimit sdk.Coins
}

func (cap SendAuthorization) MethodName() string {
	return "/cosmos.bank.v1beta1.Msg/Send"
}

func (cap SendAuthorization) Accept(msg sdk.ServiceMsg, block abci.Header) (allow bool, updated Authorization, delete bool) {
	switch req := msg.Request.(type) {
	case bank.MsgSend:
		left, invalid := cap.SpendLimit.SafeSub(req.Amount)
		if invalid {
			return false, nil, false
		}
		if left.IsZero() {
			return true, nil, true
		}
		return true, SendAuthorization{SpendLimit: left}, false
	}
	return false, nil, false
}

A different type of capability for MsgSend could be implemented using the Authorization interface with no need to change the underlying bank module.

Msg Service

service Msg {
  // GrantAuthorization grants the provided authorization to the grantee on the granter's
  // account with the provided expiration time.
  rpc GrantAuthorization(MsgGrantAuthorization) returns (MsgGrantAuthorizationResponse);

  // ExecAuthorized attempts to execute the provided messages using
  // authorizations granted to the grantee. Each message should have only
  // one signer corresponding to the granter of the authorization.
  // The grantee signing this message must have an authorization from the granter.
  rpc ExecAuthorized(MsgExecAuthorized) returns (MsgExecAuthorizedResponse)


  // RevokeAuthorization revokes any authorization corresponding to the provided method name on the
  // granter's account that has been granted to the grantee.
  rpc RevokeAuthorization(MsgRevokeAuthorization) returns (MsgRevokeAuthorizationResponse);
}

message MsgGrantAuthorization{
  string granter = 1;
  string grantee = 2;
  google.protobuf.Any authorization = 3 [(cosmos_proto.accepts_interface) = "Authorization"];
  google.protobuf.Timestamp expiration = 4;
}

message MsgExecAuthorized {
    string grantee = 1;
    repeated google.protobuf.Any msgs = 2;
}

message MsgRevokeAuthorization{
  string granter = 1;
  string grantee = 2;
  string method_name = 3;
}

Router Middleware

The authz Keeper will expose a DispatchActions method which allows other modules to send ServiceMsgs to the router based on Authorization grants:

type Keeper interface {
	// DispatchActions routes the provided msgs to their respective handlers if the grantee was granted an authorization
	// to send those messages by the first (and only) signer of each msg.
    DispatchActions(ctx sdk.Context, grantee sdk.AccAddress, msgs []sdk.ServiceMsg) sdk.Result`
}

This allows the functionality provided by authz to be used for future inter-module object capabilities permissions as described in ADR 033

CLI

tx exec Method

When a CLI user wants to run a transaction on behalf of another account using MsgExecAuthorized, they can use the exec method. For instance gaiacli tx gov vote 1 yes --from <grantee> --generate-only | gaiacli tx authz exec --send-as <granter> --from <grantee> would send a transaction like this:

MsgExecAuthorized {
  Grantee: mykey,
  Msgs: []sdk.SericeMsg{
    ServiceMsg {
      MethodName:"/cosmos.gov.v1beta1.Msg/Vote"
      Request: MsgVote {
	    ProposalID: 1,
	    Voter: cosmos3thsdgh983egh823
	    Option: Yes
      }
    }
  }
}

tx grant <grantee> <authorization> --from <granter>

This CLI command will send a MsgGrantAuthorization transaction. authorization should be encoded as JSON on the CLI.

tx revoke <grantee> <method-name> --from <granter>

This CLI command will send a MsgRevokeAuthorization transaction.

Built-in Authorizations

SendAuthorization

// SendAuthorization allows the grantee to spend up to spend_limit coins from
// the granter's account.
message SendAuthorization {
  repeated cosmos.base.v1beta1.Coin spend_limit = 1;
}

GenericAuthorization

// GenericAuthorization gives the grantee unrestricted permissions to execute
// the provide method on behalf of the granter's account.
message GenericAuthorization {
  string method_name = 1;
}

Consequences

Positive

  • Users will be able to authorize arbitrary actions on behalf of their accounts to other users, improving key management for many use cases
  • The solution is more generic than previously considered approaches and the Authorization interface approach can be extended to cover other use cases by SDK users

Negative

Neutral

References