neo-go/docs/notary.md

NeoGo P2P signature collection (notary) service

P2P signature (notary) service is a NeoGo node extension that allows several parties to sign one transaction independently of chain and without going beyond the chain environment. The on-chain P2P service is aimed to automate, accelerate and secure the process of signature collection. The service was initially designed as a solution for multisignature transaction forming and described in the proposal.

The original problem definition:

Several parties want to sign one transaction, it can either be a set of signatures for multisignature signer or multiple signers in one transaction. It's assumed that all parties can generate the same transaction (with the same hash) without any interaction, which is the case for oracle nodes or NeoFS inner ring nodes.

As some of the services using this mechanism can be quite sensitive to the latency of their requests processing, it should be possible to construct a complete transaction within the time frame between two consecutive blocks.

Components and functionality

The service consists of a native contract and a node module. Native contract is mostly concerned with verification, fees and payment guarantees, while module is doing the actual work. It uses generic Conflicts and NotValidBefore transaction attributes for its purposes as well as an additional special one (Notary assisted).

A new designated role is added, P2PNotary. It can have an arbitrary number of keys associated with it.

To use the service, one should pay some GAS, so below we operate with FEE as a unit of cost for this service. FEE is set to be 0.1 GAS.

We'll also use NKeys definition as the number of keys that participate in the process of signature collection. This is the number of keys that could potentially sign the transaction, for transactions lacking appropriate witnesses that would be the number of witnesses, for "M out of N" multisignature scripts that's N, for combination of K standard signature witnesses and L multisignature "M out of N" witnesses that's K+N*L.

Transaction attributes

Conflicts

This attribute makes the chain accept one transaction of the two conflicting only and adds an ability to give a priority to any of the two if needed. This attribute was originally proposed in neo-project/neo#1991.

The attribute has Uint256 data inside containing the hash of conflicting transaction. It is allowed to have multiple attributes of this type.

NotValidBefore

This attribute makes transaction invalid before certain height. This attribute was originally proposed in neo-project/neo#1992.

The attribute has uint32 data inside which is the block height starting from which the transaction is considered to be valid. It can be seen as the opposite of ValidUntilBlock. Using both allows to have a window of valid block numbers that this transaction could be accepted into. Transactions with this attribute are not accepted into mempool before specified block is persisted.

It can be used to create some transactions in advance with a guarantee that they won't be accepted until the specified block.

NotaryAssisted

This attribute holds one byte containing the number of transactions collected by the service. It could be 0 for fallback transaction or NKeys for a normal transaction that completed its P2P signature collection. Transactions using this attribute need to pay additional network fee of (NKeys+1)×FEE. This attribute could be only be used by transactions signed by the notary native contract.

Native Notary contract

It exposes several methods to the outside world:

Method	Parameters	Return value	Description
onNEP17Payment	from (uint160) - GAS sender account. amount (int) - amount of GAS to deposit. data represents array of two parameters: 1. to (uint160) - account of the deposit owner. 2. till (int) - deposit lock height.	bool	Automatically called after GAS transfer to Notary native contract address and records deposited amount as belonging to to address with a lock till till chain's height. Can only be invoked from native GAS contract. Must be witnessed by from. to can be left unspecified (null), with a meaning that to is the same address as from. amount can't be less than 2×FEE for the first deposit call for the to address. Each successive deposit call must have till value equal to or more than the previous successful call (allowing for renewal), if it has additional amount of GAS it adds up to the already deposited value.
lockDepositUntil	address (uint160) - account of the deposit owner. till (int) - new height deposit is valid until (can't be less than previous value).	void	Updates deposit expiration value. Must be witnessed by address.
withdraw	from (uint160) - account of the deposit owner. to (uint160) - account to transfer GAS to.	bool	Sends all deposited GAS for from address to to address. Must be witnessed by from. to can be left unspecified (null), with a meaning that to is the same address as from. It can only be successful if the lock has already expired, attempting to withdraw the deposit before that height fails. Partial withdrawal is not supported. Returns boolean result, true for successful calls and false for failed ones.
balanceOf	addr (uint160) - account of the deposit owner.	int	Returns deposited GAS amount for specified address (integer).
expirationOf	addr (uint160) - account of the deposit owner.	int	Returns deposit lock height for specified address (integer).
verify	signature (signature) - notary node signature bytes for verification.	bool	This is used to verify transactions with notary contract specified as a signer, it needs one signature in the invocation script and it checks for this signature to be made by one of designated keys, effectively implementing "1 out of N" multisignature contract.
getMaxNotValidBeforeDelta		int	Returns MaxNotValidBeforeDelta constraint. Default value is 140.
setMaxNotValidBeforeDelta	value (int)	void	Set MaxNotValidBeforeDelta constraint. Must be witnessed by committee.

See the Notary deposit guide section on how to deposit funds to Notary native contract and manage the deposit.

P2PNotaryRequest payload

A new broadcasted payload type is introduced for notary requests. It's distributed via regular inv-getdata mechanism like transactions, blocks or consensus payloads. An ordinary P2P node verifies it, saves in a structure similar to mempool and relays. This payload has witness (standard single-signature contract) attached signing all of the payload.

This payload has two incomplete transactions inside:

• Fallback tx. This transaction has P2P Notary contract as a sender and service request sender as an additional signer. It can't have a witness for Notary contract, but it must have proper witness for request sender. It must have NotValidBefore attribute that is no more than MaxNotValidBeforeDelta higher than the current chain height and it must have Conflicts attribute with the hash of the main transaction. It at the same time must have Notary assisted attribute with a count of zero.

• Main tx. This is the one that actually needs to be completed; it:

  1. either doesn't have all witnesses attached
  2. or has a partial multisignature only
  3. or have not all witnesses attached and some of the rest are partial multisignature

  This transaction must have Notary assisted attribute with a count of NKeys (and Notary contract as one of the signers).

See the Notary request submission guide to learn how to construct and send the payload.

Notary node module

Node module with the designated key monitors the network for P2PNotaryRequest payloads. It maintains a list of current requests grouped by main transaction hash. When it receives enough requests to correctly construct all transaction witnesses, it does so, adds a witness of its own (for Notary contract witness) and sends the resulting transaction to the network.

If the main transaction with all witnesses attached still can't be validated due to any fee (or other) issues, the node waits for NotValidBefore block of the fallback transaction to be persisted.

If NotValidBefore block is persisted and there are still some signatures missing (or the resulting transaction is invalid), the module sends all the associated fallback transactions for the main transaction.

After processing, service request is deleted from the module.

See the [NeoGo P2P signature extensions](#NeoGo P2P signature extensions) on how to enable notary-related extensions on chain and [NeoGo Notary service node module](#NeoGo Notary service node module) on how to set up Notary service node.

Environment setup

To run P2P signature collection service on your network, you need to do:

• Set up [P2PSigExtensions](#NeoGo P2P signature extensions) for all nodes in the network.
• Set notary node keys in RoleManagement native contract.
• [Configure](#NeoGo Notary service node module) and run appropriate number of notary nodes with keys specified in RoleManagement native contract (at least one node is necessary to complete signature collection).

After service is running, you can [create and send](#Notary request lifecycle guide) notary requests to the network.

NeoGo P2P signature extensions

As far as Notary service is an extension of the standard NeoGo node, it should be enabled and properly configured before usage.

Configuration

To enable P2P signature extensions add P2PSigExtensions subsection set to true to ProtocolConfiguration section of your node config. This enables all notary-related logic in the network, i.e. allows your node to accept and validate NotValidBefore, Conflicts and NotaryAssisted transaction attribute, handle, verify and broadcast P2PNotaryRequest P2P payloads, properly initialize native Notary contract and designate P2PNotary node role in RoleManagement native contract.

If you use custom NativeActivations subsection of the ProtocolConfiguration section in your node config, specify the height of the Notary contract activation, e.g. 0.

Note, that even if P2PSigExtensions config subsection enables notary-related logic in the network, it still does not turn your node into notary service node. To enable notary service node functionality refer to the NeoGo Notary service documentation.

Example

  P2PSigExtensions: true
  NativeActivations:
    Notary: [0]
    ContractManagement: [0]
    StdLib: [0]
    CryptoLib: [0]
    LedgerContract: [0]
    NeoToken: [0]
    GasToken: [0]
    PolicyContract: [0]
    RoleManagement: [0]
    OracleContract: [0]

NeoGo Notary service node module

NeoGo node can act as notary service node (the node that accumulates notary requests, collects signatures and releases fully-signed transactions). It must have a wallet with a key belonging to one of network's designated notary nodes (stored in RoleManagement native contract). Also, the node must be connected to a network with enabled P2P signature extensions, otherwise problems with states and peer disconnections will occur.

Notary service node doesn't need RPC service to be enabled because it receives notary requests and broadcasts completed transactions via P2P protocol. However, enabling RPC service allows to send notary requests directly to the notary service node and avoid P2P communication delays.

Configuration

To enable notary service node check firstly that [P2PSignatureExtensions](#NeoGo P2P signature extensions) are properly set up. Then add P2PNotary subsection to ApplicationConfiguration section of your node config.

Parameters:

• Enabled: boolean value, enables/disables the service node, true for service node to be enabled
• UnlockWallet: notary node wallet configuration:
  • Path: path to NEP-6 wallet.
  • Password: password for the account to be used by notary node.

Example

P2PNotary:
  Enabled: true
  UnlockWallet:
    Path: "/notary_node_wallet.json"
    Password: "pass"

Notary request lifecycle guide

Below are presented all stages each P2P signature collection request goes through. Use stages 1 and 2 to create, sign and submit P2P notary request. Stage 3 is performed by the notary service; it does not require user's intervention and is given for informational purposes. Stage 4 contains advice to check for notary request results.

1. Notary deposit

To guarantee that payment to the notary node will still be done if things go wrong, sender's deposit to the Notary native contract is used. Before the notary request will be submitted, you need to deposit enough GAS to the contract, otherwise, request won't pass verification.

Notary native contract supports onNEP17Payment method. Thus, to deposit funds to the Notary native contract, transfer the desired amount of GAS to the contract address. Use func (*Client) TransferNEP17 with the data parameter matching the following requirements:

• data should be an array of two elements: to and till.
• to denotes the receiver of the deposit. It can be nil in case to equals the GAS sender.
• till denotes chain's height before which deposit is locked and can't be withdrawn. till can't be set if you're not the deposit owner. Default till value is the current chain height + 5760. till can't be less than the current chain height. till can't be less than the currently set till value for that deposit if the deposit already exists.

Note, that the first deposit call for the to address can't transfer less than 2×FEE GAS. Deposit is allowed for renewal, i.e. consequent deposit calls for the same to address add up a specified amount to the already deposited value.

After GAS transfer is successfully submitted to the chain, use [Notary native contract API](#Native Notary contract) to manage your deposit.

Note, that regular operation flow requires the deposited amount of GAS to be sufficient to pay for all fallback transactions that are currently submitted (all in-flight notary requests). The default deposit sum for one fallback transaction should be enough to pay the fallback transaction fees which are system fee and network fee. Fallback network fee includes (NKeys+1)×FEE = (0+1)×FEE = FEE GAS for NotaryAssisted attribute usage and regular fee for the fallback size. If you need to submit several notary requests, ensure that the deposited amount is enough to pay for all fallbacks. If the deposited amount is not enough to pay the fallback fees, Insufficiend funds error will be returned from the RPC node after notary request submission.

2. Request submission

Once several parties want to sign one transaction, each of them should generate the transaction, wrap it into P2PNotaryRequest payload and send it to the known RPC server via submitnotaryrequest RPC call. Note, that all parties must generate the same main transaction while fallbacks can differ.

To create a notary request, you can use NeoGo RPC client. Follow the steps to create a signature request:

1. Prepare a list of signers with scopes for the main transaction (i.e. the transaction that signatures are being collected for, that will be Signers transaction field). Use the following rules to construct the list:

   • First signer is the one who pays the transaction fees.
   • Each signer is either a multisignature or a standard signature or a contract signer.
   • Multisignature and signature signers can be combined.
   • Contract signer can be combined with any other signer.

   Include Notary native contract in the list of signers with the following constraints:

   • Notary signer hash is the hash of a native Notary contract that can be fetched from func (*Client) GetNativeContractHash.
   • A notary signer must have None scope.
   • A notary signer shouldn't be placed at the beginning of the signer list because Notary contract does not pay main transaction fees. Other positions in the signer list are available for a Notary signer.

2. Construct a script for the main transaction (that will be Script transaction field) and calculate system fee using regular rules (that will be SystemFee transaction field). Probably, you'll perform one of these actions:

   1. If the script is a contract method call, use invokefunction RPC API func (*Client) InvokeFunction and fetch the script and the gas consumed from the result.
   2. If the script is more complicated than just a contract method call, construct the script manually and use invokescript RPC API func (*Client) InvokeScript to fetch the gas consumed from the result.
   3. Or just construct the script and set system fee manually.

3. Calculate the height main transaction is valid until (that will be ValidUntilBlock transaction field). Consider the following rules for VUB value estimation:

   • VUB value must not be lower than the current chain height.
   • The whole notary request (including fallback transaction) is valid until the same VUB height.
   • VUB value must be lower than notary deposit expiration height. This condition guarantees that the deposit won't be withdrawn before notary service payment.
   • All parties must provide the same VUB for the main transaction.

4. Construct the list of main transaction attributes (that will be Attributes transaction field). The list must include NotaryAssisted attribute with NKeys equals the overall number of the keys to be collected excluding notary and other contract-based witnesses. For m out of n multisignature request NKeys = n. For multiple standard signature request, signers NKeys equals the standard signature signers count.

5. Construct a list of accounts (wallet.Account structure from the wallet package) to calculate network fee for the transaction using the following rules. This list will be used in the next step.

   • The number and the order of the accounts should match the transaction signers constructed at step 1.
   • An account for a contract signer should have Contract field with Deployed set to true if the corresponding contract is deployed on chain.
   • An account for a signature or a multisignature signer should have Contract field with Deployed set to false and Script set to the signer's verification script.
   • An account for a notary signer is just a placeholder and should have Contract field with Deployed set to false, i.e. the default value for Contract field. That's needed to skip notary verification during regular network fee calculation at the next step.

6. Fill in the main transaction Nonce field.

7. Construct a list of main transactions witnesses (that will be Scripts transaction field). Uses standard rules for witnesses of not yet signed transaction (it can't be signed at this stage because network fee is missing):

   • A contract-based witness should have Invocation script that pushes arguments on stack (it may be empty) and empty Verification script. If multiple notary requests provide different Invocation scripts, the first one will be used to construct contract-based witness.
   • A Notary contract witness (which is also a contract-based witness) should have empty Verification script. Invocation script should be of the form [opcode.PUSHDATA1, 64, make([]byte, 64)...], i.e. to be a placeholder for a notary contract signature.
   • A standard signature witness must have regular Verification script filled even if the Invocation script is to be collected from other notary requests. Invocation script should be empty.
   • A multisignature witness must have regular Verification script filled even if Invocation script is to be collected from other notary requests. Invocation script either should be empty.

8. Calculate network fee for the transaction (that will be NetworkFee transaction field). Use func (*Client) CalculateNetworkFee method with the main transaction given to it.

9. Fill in all signatures that can be provded by the client creating request, that includes simple-signature accounts and multisignature accounts where the client has one of the keys (in which case an invocation script is created that pushes just one signature onto the stack).

10. Define lifetime for the fallback transaction. Let the fallbackValidFor be the lifetime. Let N be the current chain's height and VUB be ValidUntilBlock value estimated at step 3. Then, the notary node is trying to collect signatures for the main transaction from N up to VUB-fallbackValidFor. In case of failure after VUB-fallbackValidFor-th block is accepted, the notary node abandons attempts to complete the main transaction and tries to push all associated fallbacks. Use the following rules to define fallbackValidFor:

    • fallbackValidFor shouldn't be more than MaxNotValidBeforeDelta value.
    • Use func (*Client) GetMaxNotValidBeforeDelta to check MaxNotValidBefore value.

11. Construct a script for the fallback transaction. The script may do something useful, i.g. invoke method of a contract. However, if you don't need to perform anything special on fallback invocation, you can use simple opcode.RET script.

12. Sign and submit P2P notary request. Use func (*Client) SignAndPushP2PNotaryRequest for it.

    • Use the signed main transaction from step 9 as mainTx argument.
    • Use the fallback script from step 10 as fallbackScript argument.
    • Use -1 as fallbackSysFee argument to define system fee by test invocation or provide any custom value.
    • Use 0 as fallbackNetFee argument not to add extra network fee to the fallback.
    • Use the fallbackValidFor estimated at step 9 as fallbackValidFor argument.
    • Use your account you'd like to send request (and fallback transaction) from to sign the request (and fallback transaction).

    SignAndPushP2PNotaryRequest will construct and sign a fallback transaction, construct and sign a P2PNotaryRequest and submit it to the RPC node. The resulting notary request and an error are returned.

After P2PNotaryRequests are sent, participants should wait for one of their transactions (main or fallback) to get accepted into one of subsequent blocks.

3. Signatures collection and transaction release

A valid P2PNotaryRequest payload is distributed via P2P network using standard broadcasting mechanisms until it reaches the designated notary nodes that have the respective node module active. They collect all payloads for the same main transaction until enough signatures are collected to create proper witnesses for it. Then, they attach all witnesses required and send this transaction as usual and monitor subsequent blocks for its inclusion.

All the operations leading to successful transaction creation are independent of the chain and could easily be done within one block interval. So, if the first service request is sent at the current height H, the main transaction is highly likely to be a part of H+1 block.

4. Results monitoring

Once the P2PNotaryRequest reaches RPC node, it is added to the notary request pool. Completed or outdated requests are removed from the pool. Use NeoGo notification subsystem to track request addition and removal:

• Use RPC subscribe method with notary_request_event stream name parameter to subscribe to P2PNotaryRequest payloads that are added or removed from the notary request pool.
• Use sender or signer filters to filter out a notary request with the desired request senders or main tx signers.

Use the notification subsystem to track that the main or the fallback transaction is accepted to the chain:

• Use RPC subscribe method with transaction_added stream name parameter to subscribe to transactions that are accepted to the chain.
• Use sender filter with the Notary native contract hash to filter out fallback transactions sent by the Notary node. Use signer filter with the notary request sender address to filter out the fallback transactions sent by the specified sender.
• Use sender or signer filters to filter out the main transaction with the desired sender or signers. You can also filter out the main transaction using Notary contract signer filter.
• Don't rely on sender and signer filters only, also check that the received transaction has NotaryAssisted attribute with the expected NKeys value.

Use the notification subsystem to track main or fallback transaction execution results.

Moreover, you can use all regular RPC calls to track main or fallback transaction invocation: getrawtransaction, getapplicationlog etc.

Notary service use-cases

Several use-cases where Notary subsystem can be applied are described below.

Committee-signed transactions

The signature collection problem occurs every time committee participants need to submit a transaction with m out of n multisignature, i.g.:

• transfer initial supply of NEO and GAS from a committee multisignature account to other addresses on new chain start
• tune valuable chain parameters like gas per block, candidate register price, minimum contract deployment fee, Oracle request price, native Policy values etc
• invoke non-native contract methods that require committee multisignature witness

Current solution offers off-chain non-P2P signature collection (either manual or using some additional network connectivity). It has an obvious downside of reliance on something external to the network. If it's manual, it's slow and error-prone; if it's automated, it requires additional protocol for all the parties involved. For the protocol used by oracle nodes, it also means nodes explicitly exposing to each other.

With the Notary service all signature collection logic is unified and is on chain already. The only thing that committee participants should perform is to create and submit a P2P notary request (can be done independently). Once the sufficient number of signatures is collected by the service, the desired transaction will be applied and pass committee witness verification.

NeoFS Inner Ring nodes

Alphabet nodes of the Inner Ring signature collection is a particular case of committee-signed transactions. Alphabet nodes multisignature is used for various cases, such as:

• main chain and side chain funds synchronization and withdrawal
• bootstrapping new storage nodes to the network
• network map management and epoch update
• containers and extended ACL management
• side chain governance update

Non-notary on-chain solution for Alphabet nodes multisignature forming is imitated via contracts collecting invocations of their methods signed by standard signature of each Alphabet node. Once the sufficient number of invocations is collected, the invocation is performed.

The described solution has several drawbacks:

• it can only be app-specific (meaning that for every use case this logic would be duplicated) because we can't create transactions from transactions (thus using proper multisignature account is not possible)
• for m out of n multisignature we need at least m transactions instead of one we really wanted to have; but actually we'll create and process n of them, so this adds substantial overhead to the chain
• some GAS is inevitably wasted because any invocation could either go the easy path (just adding a signature to the list) or really invoke the function we wanted to (when all signatures are in place), so test invocations don't really help and the user needs to add some GAS to all of these transactions

Notary on-chain Alphabet multisignature collection solution uses Notary subsystem to successfully solve these problems, e.g. to calculate precisely the amount of GAS to pay for contract invocation witnessed by Alphabet nodes (see nspcc-dev/neofs-node#47), to reduce container creation delay (see nspcc-dev/neofs-node#519) etc.

Contract-sponsored (free) transactions

The original problem and solution are described in neo-project/neo#2577 discussion.