Parties and Authority¶
Daml is designed for distributed applications involving mutually distrusting parties. In a well-constructed contract model, all parties have strong guarantees that nobody cheats or circumvents the rules laid out by templates and choices.
In this section you will learn about Daml’s authorization rules and how to develop contract models that give all parties the required guarantees. In particular, you’ll learn how to:
- Pass authority from one contract to another
- Write advanced choices
- Reason through Daml’s Authorization model
Remember that you can load all the code for this section into a folder called
intro6 by running
daml new intro6 --template daml-intro-6
Preventing IOU Revocation¶
SimpleIou contract from Transform Data Using Choices and Add Constraints to a Contract has one major problem: The contract is only signed by the
issuer. The signatories are the parties with the power to create and archive contracts. If Alice gave Bob a
SimpleIou for $100 in exchange for some goods, she could just archive it after receiving the goods. Bob would have a record of such actions, but would have to resort to off-ledger means to get his money back.
template SimpleIou with issuer : Party owner : Party cash : Cash where signatory issuer
simple_iou_test = do alice <- allocateParty "Alice" bob <- allocateParty "Bob" -- Alice and Bob enter into a trade. -- Alice transfers the payment as a SimpleIou. iou <- submit alice do createCmd SimpleIou with issuer = alice owner = bob cash = Cash with amount = 100.0 currency = "USD" passTime (days 1) -- Bob delivers the goods. passTime (minutes 10) -- Alice just deletes the payment. submit alice do archiveCmd iou
For a party to have any guarantees that only those transformations specified in the choices are actually followed, they either need to be a signatory themselves, or trust one of the signatories to not agree to transactions that archive and re-create contracts in unexpected ways. To make the
SimpleIou safe for Bob, you need to add him as a signatory.
template Iou with issuer : Party owner : Party cash : Cash where signatory issuer, owner choice Transfer : ContractId Iou with newOwner : Party controller owner do assertMsg "newOwner cannot be equal to owner." (owner /= newOwner) create this with owner = newOwner
There’s a new problem here: There is no way for Alice to issue or transfer this
Iou to Bob. To get an
Iou with Bob’s signature as
owner onto the ledger, his authority is needed.
iou_test = do alice <- allocateParty "Alice" bob <- allocateParty "Bob" -- Alice and Bob enter into a trade. -- Alice wants to give Bob an Iou, but she can't without Bob's authority. submitMustFail alice do createCmd Iou with issuer = alice owner = bob cash = Cash with amount = 100.0 currency = "USD" -- She can issue herself an Iou. iou <- submit alice do createCmd Iou with issuer = alice owner = alice cash = Cash with amount = 100.0 currency = "USD" -- However, she can't transfer it to Bob. submitMustFail alice do exerciseCmd iou Transfer with newOwner = bob
This may seem awkward, but notice that the
ensure clause is gone from the
Iou again. The above
Iou can contain negative values so Bob should be glad that
Alice cannot put his signature on any
You’ll now learn a couple of common ways of building issuance and transfer workflows for the above
Iou, before diving into the authorization model in full.
Use Propose-accept Workflows for One-off Authorization¶
If there is no standing relationship between Alice and Bob, Alice can propose the issuance of an Iou to Bob, giving him the choice to accept. You can do so by introducing a proposal contract
template IouProposal with iou : Iou where signatory iou.issuer observer iou.owner choice IouProposal_Accept : ContractId Iou controller iou.owner do create iou
Note how we have used the fact that templates are records here to store the
Iou in a single field.
iouProposal <- submit alice do createCmd IouProposal with iou = Iou with issuer = alice owner = bob cash = Cash with amount = 100.0 currency = "USD" submit bob do exerciseCmd iouProposal IouProposal_Accept
IouProposal contract carries the authority of
iou.issuer by virtue of them being a signatory. By exercising the
IouProposal_Accept choice, Bob adds his authority to that of Alice, which is why an
Iou with both signatories can be created in the context of that choice.
The choice is called
Accept, because propose-accept patterns are very common. In fact, you’ll see another one just below. As each choice defines a record type, you cannot have two choices of the same name in scope. It’s a good idea to qualify choice names to ensure uniqueness.
The above solves issuance, but not transfers. You can solve transfers exactly the same way, though, by creating a
template IouTransferProposal with iou : Iou newOwner : Party where signatory (signatory iou) observer (observer iou), newOwner choice IouTransferProposal_Cancel : ContractId Iou controller iou.owner do create iou choice IouTransferProposal_Reject : ContractId Iou controller newOwner do create iou choice IouTransferProposal_Accept : ContractId Iou controller newOwner do create iou with owner = newOwner
In addition to defining the signatories of a contract,
signatory can also be used to extract the signatories from another contract. Instead of writing
signatory (signatory iou), you could write
signatory iou.issuer, iou.owner.
IouProposal had a single signatory so it could be cancelled easily by archiving it. Without a
Cancel choice, the
newOwner could abuse an open TransferProposal as an option. The triple
Cancel is common to most proposal templates.
To allow an
iou.owner to create such a proposal, you need to give them the choice to propose a transfer on the
Iou contract. The choice looks just like the above
Transfer choice, except that a
IouTransferProposal is created instead of an
choice ProposeTransfer : ContractId IouTransferProposal with newOwner : Party controller owner do assertMsg "newOwner cannot be equal to owner." (owner /= newOwner) create IouTransferProposal with iou = this newOwner
Bob can now transfer his
Iou. The transfer workflow can even be used for issuance:
charlie <- allocateParty "Charlie" -- Alice issues an Iou using a transfer proposal. tpab <- submit alice do createCmd IouTransferProposal with newOwner = bob iou = Iou with issuer = alice owner = alice cash = Cash with amount = 100.0 currency = "USD" -- Bob accepts the transfer from Alice. iou2 <- submit bob do exerciseCmd tpab IouTransferProposal_Accept -- Bob offers Charlie a transfer. tpbc <- submit bob do exerciseCmd iou2 ProposeTransfer with newOwner = charlie -- Charlie accepts the transfer from Bob. submit charlie do exerciseCmd tpbc IouTransferProposal_Accept
Use Role Contracts for Ongoing Authorization¶
Many actions, like the issuance of assets or their transfer, can be pre-agreed. You can represent this succinctly in Daml through relationship or role contracts.
newOwner can transfer an asset, as demonstrated in the script above. In Composing Choices, you will see how to compose the
IouTransferProposal_Accept choices into a single new choice, but for now, here is a different way. You can give them the joint right to transfer an IOU:
choice Mutual_Transfer : ContractId Iou with newOwner : Party controller owner, newOwner do create this with owner = newOwner
Up to now, the controllers of choices were known from the current contract. Here, the
newOwner variable is part of the choice arguments, not the
This is also the first time we have shown a choice with more than one controller. If multiple controllers are specified, the authority of all the controllers is needed. Here, neither
newOwner can execute a transfer unilaterally, hence the name
template IouSender with sender : Party receiver : Party where signatory receiver observer sender nonconsuming choice Send_Iou : ContractId Iou with iouCid : ContractId Iou controller sender do iou <- fetch iouCid assert (iou.cash.amount > 0.0) assert (sender == iou.owner) exercise iouCid Mutual_Transfer with newOwner = receiver
IouSender contract now gives one party, the
sender the right to send
Iou contracts with positive amounts to a
nonconsuming keyword on the choice
Send_Iou changes the behaviour of the choice so that the contract it’s exercised on does not get archived when the choice is exercised. That way the
sender can use the contract to send multiple Ious.
Here it is in action:
-- Bob allows Alice to send him Ious. sab <- submit bob do createCmd IouSender with sender = alice receiver = bob -- Charlie allows Bob to send him Ious. sbc <- submit charlie do createCmd IouSender with sender = bob receiver = charlie -- Alice can now send the Iou she issued herself earlier. iou4 <- submit alice do exerciseCmd sab Send_Iou with iouCid = iou -- Bob sends it on to Charlie. submit bob do exerciseCmd sbc Send_Iou with iouCid = iou4
Daml’s Authorization Model¶
Hopefully, the above will have given you a good intuition for how authority is passed around in Daml. In this section you’ll learn about the formal authorization model to allow you to reason through your contract models. This will allow you to construct them in such a way that you don’t run into authorization errors at runtime, or, worse still, allow malicious transactions.
In Choices In the Ledger Model you learned that a transaction is, equivalently, a tree of transactions, or a forest of actions, where each transaction is a list of actions, and each action has a child-transaction called its consequences.
Each action has a set of required authorizers – the parties that must authorize that action – and each transaction has a set of authorizers – the parties that did actually authorize the transaction.
The authorization rule is that the required authorizers of every action are a subset of the authorizers of the parent transaction.
The required authorizers of actions are:
- The required authorizers of an exercise action are the controllers on the corresponding choice. Remember that
archiveare just an implicit choice with the signatories as controllers.
- The required authorizers of a create action are the signatories of the contract.
- The required authorizers of a fetch action (which also includes
fetchByKey) are somewhat dynamic and covered later.
The authorizers of transactions are:
- The root transaction of a commit is authorized by the submitting party.
- The consequences of an exercise action are authorized by the actors of that action plus the signatories of the contract on which the action was taken.
An Authorization Example¶
Consider the transaction from the script above where Bob sends an
Iou to Charlie using a
It is authorized as follows, ignoring fetches:
- Bob submits the transaction so he’s the authorizer on the root transaction.
- The root transaction has a single action, which is to exercise
IouSendercontract with Bob as
senderand Charlie as
receiver. Since the controller of that choice is the
sender, Bob is the required authorizer.
- The consequences of the
Send_Iouaction are authorized by its actors, Bob, as well as signatories of the contract on which the action was taken. That’s Charlie in this case, so the consequences are authorized by both Bob and Charlie.
- The consequences contain a single action, which is a
Mutual_Transferwith Charlie as
ownerBob. The required authorizers of the action are the
owner, Bob, and the
newOwner, Charlie, which matches the parent’s authorizers.
- The consequences of
Mutual_Transferare authorized by the actors (Bob and Charlie), as well as the signatories on the Iou (Alice and Bob).
- The single action on the consequences, the creation of an Iou with
ownerCharlie has required authorizers Alice and Charlie, which is a proper subset of the parent’s authorizers.
You can see the graph of this transaction in the transaction view of the IDE:
TX 12 1970-01-01T00:00:00Z (Parties:276:3) #12:0 │ disclosed to (since): 'Bob' (12), 'Charlie' (12) └─> 'Bob' exercises Send_Iou on #10:0 (Parties:IouSender) with iouCid = #11:3 children: #12:1 │ disclosed to (since): 'Bob' (12), 'Charlie' (12), 'Alice' (12) └─> fetch #11:3 (Parties:Iou) #12:2 │ disclosed to (since): 'Bob' (12), 'Charlie' (12), 'Alice' (12) └─> 'Bob', 'Charlie' exercises Mutual_Transfer on #11:3 (Parties:Iou) with newOwner = 'Charlie' children: #12:3 │ disclosed to (since): 'Bob' (12), 'Charlie' (12), 'Alice' (12) └─> create Parties:Iou with issuer = 'Alice'; owner = 'Charlie'; cash = (Parties:Cash with currency = "USD"; amount = 100.0000000000)
Note that authority is not automatically transferred transitively.
template NonTransitive with partyA : Party partyB : Party where signatory partyA observer partyB choice TryA : ContractId NonTransitive controller partyA do create NonTransitive with partyA = partyB partyB = partyA choice TryB : ContractId NonTransitive with other : ContractId NonTransitive controller partyB do exercise other TryA
nt1 <- submit alice do createCmd NonTransitive with partyA = alice partyB = bob nt2 <- submit alice do createCmd NonTransitive with partyA = alice partyB = bob submitMustFail bob do exerciseCmd nt1 TryB with other = nt2
The consequences of
TryB are authorized by both Alice and Bob, but the action
TryA only has Alice as an actor and Alice is the only signatory on the contract.
Therefore, the consequences of
TryA are only authorized by Alice. Bob’s authority is now missing to create the flipped
NonTransitive so the transaction fails.
In Composing Choices you will put everything you have learned together to build a simple asset holding and trading model akin to that in the Daml IOU Quickstart Tutorial. In that context you’ll learn a bit more about the
Update action and how to use it to compose transactions, as well as about privacy on Daml ledgers.