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= Keep ECDSA v2

The Keep Network offers threshold ECDSA protocol to generate ECDSA wallets
without any single signer having access to the corresponding private key. This
functionality is used by TBTC v2 to manage Bitcoin wallets used by the TBTC Bridge.

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== Overview

Keep ECDSA allows creating threshold ECDSA wallets where where `n` parties share
the power to issue digital signatures under a single public key. A threshold `t`
is specified such that any subset of `t + 1` players can jointly sign, but any
smaller subset cannot.

`WalletRegistry` smart contract is an on-chain registry of ECDSA wallets
controlled by an off-chain network of nodes. The distributed key generation
protocol used by the off-chain network of nodes should have three properties:

- The signing group as a whole should have an ECDSA public key, which will be
  shared on the host chain (Ethereum) and will correspond to the Bitcoin wallet
  owned by that signing group.
- Each member of the signing group should have a threshold ECDSA secret key
  share, which can be used to create a threshold ECDSA signature share for any
  transactions involving the signing group’s wallet.
- Each member of the signing group should be able to combine a threshold number
  of signature shares from itself and other members of the group to produce a
  signed version of a given transaction to be performed on behalf of the signing
  group.

== Prior Work

Smart contracts for the first version of Keep ECDSA are available in
link:https://github.com/keep-network/keep-ecdsa/tree/main/solidity[`keep-ecdsa` repository].
The new version is optimised for larger groups by implementing optimistic
selection of group members during DKG protocol. Staker rewards are redesigned
and allocated for all sortition pool members. Most parameters are now governable.

== The Mechanism

=== Wallet Creation

A new wallet is created on request from the `walletOwner` address. Signing group
creation starts with an owner's call to `WalletRegistry.requestNewWallet()`.
This transaction locks the sortition pool and sends a request to the Random
Beacon for a new relay entry. From this moment, no operator can enter
or leave the pool. Once a new relay entry appears on the chain and gets
delivered to `WalletRegistry` by the Random Beacon contract via
`WalletRegistry.__beaconCallback` callback function call, all off-chain
clients perform group selection calling `WalletRegistry.selectGroup()` view
function for free. Relay entry provided by the Random Beacon is used as a seed
for the group selection. After determining signing group members, clients should 
perform off-chain distributed key generation (DKG).
<<operator-only,One of the group members>> submits the result to the chain
calling `WalletRegistry.submitDkgResult(DKG.Result calldata dkgResult)`
function. Once the result is submitted, a challenge period starts.

During the challenge period, anyone can notify that the submitted DKG result is
malicious by calling `WalletRegistry.challengeDkgResult(DKG.Result calldata dkgResult)`
function. A malicious DKG result may contain corrupted data, group members not
selected by the pool, or incorrect supporting signatures. If such malicious
result is submitted and successfully challenged, the result submitter gets
slashed and the malicious result is immediately discarded. The address which
notified about malicious DKG result is <<punishment,rewarded>>. DKG timeout
timer is reset, and group members have another chance to submit a valid result.

Once the challenge period passes, and no valid challenge is reported, the DKG
result submitter should mark the DKG result as approved calling
`WalletRegistry.approveDkgResult(DKG.Result calldata dkgResult)`.
This transaction also unlocks the sortition pool.
The submitter receives an ETH reimbursement for both `submitDkgResult` and
`approveDkgResult` transactions as described in
<<transaction-incentives,Transaction Incentives>> section. In case the original
submitter does not call the `approveDkgResult` function within a specific number
of blocks, anyone can do that and receive the submitter's reimbursement.

In case the DKG result was not submitted before the timeout, anyone can 
notify about the timed out DKG by calling `WalletRegistry.notifyDkgTimeout()`
function and unlock the sortition pool as part of this transaction. 
In case the relay entry was not produced by the Random Beacon on time,
anyone can notify a seed timeout by calling `WalletRegistry.notifySeedTimeout()`
and unlock the sortition pool as a part of this transaction.

Off-chain clients are expected to follow the <<operator-only,submission order>>
when submitting DKG result to avoid front-running and minimize the cost, but no
ordering is enforced on-chain.

The sortition pool weights operators by their authorized stake amount and allows
selecting the same operator multiple times. Inactive/disqualified members during
the off-chain DKG protocol are marked as ineligible for <<rewards,rewards>> for
a governable period of time when the DKG result is approved.

Each ECDSA wallet created in the system remains active until it is closed
by the `walletOwner` with a call to `WalletRegistry.closeWallet()`.

=== Signing

`WalletRegistry` does not expose functions for requesting and submitting ECDSA
signatures. Wallet signing group needs to monitor the `walletOwner` contract and
is responsible for handling requests from the `walletOwner` - this logic is not
a part of `WalletRegistry`. For TBTC v2, the `walletOwner` is the `Bridge` contract.
The wallet signing group reacts on the state changes in the `Bridge` by
producing appropriate ECDSA signatures, moving funds on Bitcoin, and proving it
to the `Bridge` contract.

=== Timeouts

==== DKG Timeout

There is a governable timeout for DKG to complete and for the result to be
submitted. DKG timeout includes the time it takes to execute off-chain protocol
to generate a key, and the time it takes to submit the result.
When DKG timeout is exceeded, anyone can call `RandomBeacon.notifyDkgTimeout()`.
This function unlocks the sortition pool and clears up DKG data, but no slashing
for DKG timeout is executed and no one is marked as ineligible for rewards.

==== DKG Seed Timeout

There is a governable timeout for a new signing group selection seed to be
provided.

For a signing group member selection to be executed by the sortition pool,
Random Beacon needs to provide a group selection seed. Request to the Random
Beacon is one of the first steps of the new wallet creation process in
`WalletRegistry.requestNewWallet()`

When Random Beacon did not provide a seed and a timeout for a seed is exceeded,
anyone can call `WalletRegistry.notifySeedTimeout()`. This function unlocks the
sortition pool and clears up DKG data, but no slashing for DKG timeout is
executed by `WalletRegistry` and no one is marked as ineligible for rewards.
Random Beacon has its own mechanism of slashing for not providing relay entry
on time.

[[inactivity]]
=== Inactivity notification and Heartbeat failures

Off-chain clients are free to execute any heartbeat protocol they want to ensure
signing group member key share is still available and nodes are operating properly.

[TIP]
One example of a heartbeat protocol is signing some piece of information every
n-th block. Wallet signing group members need to ensure the signed piece of
information can not be used in a fraudulent way and can not be used to accuse
them for committing a fraud in TBTC `Bridge`.

Group members can agree to punish members who are permanently inactive and issue
an operator inactivity claim. If the required threshold of group members signed
the operator inactivity claim, they can submit it to
`WalletRegistry.notifyOperatorInactivity(Inactivity.Claim calldata claim, uint256 nonce, int32[] calldata groupMembers)`
function and have the group members who are inactive excluded from the sortition
pool <<rewards,rewards>> for a governable time period.

This approach is theoretically susceptible to group members colluding together,
but because a reasonably high number of operators is needed to sign a claim and
operators signing the claim receive nothing in return,
we consider this approach safe and good enough. An important advantage of this
approach is that honest players can decide off-chain when it makes sense to
submit an operator inactivity claim and mark someone as ineligible for rewards.
For example, marking an operator ineligible for rewards for the next two weeks
has a higher impact than prolonging reward ineligibility for 10 minutes for an
operator that was already marked as ineligible for rewards. This approach does
not increase the gas cost of a happy path and leaves some freedom to group
members. They can mark as ineligible operators who turned off their nodes,
operators whose nodes never participate in signing because they are
misconfigured, or operators who notoriously miss their turn in submitting relay
entries.

`Inactivity.Claim` has an additional boolean field of `heartbeatFailed`. If too
many members are inactive during the heartbeat failing, it means that the wallet
is at risk of losing the possibility to sign transactions. `walletOwner`
(TBTC `Bridge`) is informed about a failed heartbeat by
`IWalletOwner.__ecdsaWalletHeartbeatFailedCallback` callback function call and starts the process of moving funds out
of the problematic wallet.

[[rewards]]
=== Rewards

T rewards are allocated to all operators registered in the ECDSA sortition
pool, excluding operators who were marked as ineligible for rewards as a result
of being reported by other group members as <<inactivity,inactive>> or as
a result of being inactive or disqualified during the DKG. Rewards are allocated
proportionally to the operator's weight in the pool. 

[[transaction-incentives]]
=== Transaction Incentives

There are three types of transactions: <<operator-only,Operator-Only>>,
<<public-knowledge,Public-Knowledge>>, and <<punishment,Punishment>>.

[[operator-only]]
==== Operator-Only
Operator-Only transactions are where only the operators have access to the
information required to assemble the transaction with the right input
parameters.

In order to avoid all operators racing to submit the transaction at the same
time, we have an off-chain informal agreement to submit based on the operator's
position in the group (can use the hash of the group's pubkey).

If the designated operator does not submit their transaction before a timeout
expires, the duty moves to the next operator and the group can sign a
transaction to mark that operator as <<inactivity,inactive>>. Since there is no
slashing reward, and since this transaction can only be submitted by an operator,
this transaction is also Operator-Only.

In order to compensate the operator for posting the transaction, the gas spent
will be reimbursed by a DAO-funded ETH pool in the same transaction. It is
important to note, that the system has a governable cap for the gas price to
protect against malicious operators trying to drain the pool (see `Reimbursable`
and `ReimbursementPool` smart contracts).

Operator-only transactions are `submitDkgResult`,
`notifyOperatorInactivity`, and `approveDkgResult` for a certain number of
blocks, before a timeout for the original DKG result submitter to call this
function elapses.

[[public-knowledge]]
==== Public-Knowledge
Public-Knowledge transactions are where anyone has access to the information
required to assemble the transaction and the transaction does not lead to
punishment.

In order to prevent wasting gas on racing to submit, such transactions need to
be executed rarely, and off-chain clients should follow the informal agreement
about the submission order.

To compensate these transactions, whoever posts them will have the gas spent
reimbursed by a DAO-funded ETH pool in the same transaction.

The only two public knowledge transactions are `notifyDkgTimeout` and
`notifySeedTimeout`.

`approveDkgResult` turns into a public knowledge transaction in case the
original submitter has not approved the result before the timeout.

[[punishment]]
==== Punishment
Punishment transactions are where anyone has access to the information required
to assemble the transaction (like <<public-knowledge,Public-Knowledge>>) and
the transaction leads to slashing.

In these transactions, maintaining system health is more important than
optimizing gas via preventing racing, so we offer up bounties in the form of
a notifier reward from slashed tokens to whichever submitter submits first. We
do not compensate gas. Notification rewards are distributed by Threshold Network
`TokenStaking` contract.

The only punishment transaction in `WalletRegistry` itself if `challengeDkgResult`.
Additionally, `walletOwner` can implement its own punishment transactions, and
slash the signing group members with a call to `WalletRegistry.seize` function.

== Parameters

[%header,cols="3m,4,^1,^2m"]
|=== 
^|Property Name
^|Description
|Governable
|Default Value

4+s|DKG

|groupSize
|Size of a signing group for a wallet.
|No
|`100`

|groupThreshold
|The minimum number of group members needed to interact according to the protocol
to produce a signature
|No
|`51`

|activeThreshold
|The minimum number of active and properly behaving group members during the DKG
needed to accept the result.
|No
d|`90` +
_90% of groupSize_

|singnatureByteSize
|Size in bytes of a single signature produced by operator supporting DKG result.
|No
|`65`

|seedTimeout
|Time in blocks for Random Beacon to provide group selection seed.
|Yes
d|`11_520 blocks` +
_~48h assuming 15s block time_

|resultChallengePeriodLength
|Time in blocks during which the submitted DKG result can be challenged.
|Yes
d|`11_520 blocks` +
_~48h assuming 15s block time_

|resultSubmissionTimeout
|Time in blocks during which a DKG result is expected to be submitted.
|Yes
d|`2000 blocks` +
_100 members * 20 blocks = 2000 blocks_

|submitterPrecedencePeriodLength
|Time in blocks during which only the DKG result submitter is allowed to approve it.
|Yes
|`20 blocks`


4+s|Slashing

|maliciousDkgResultSlashingAmount
|Slashing amount for submitting malicious DKG result.
|Yes
d|`400e18` +
_400 T_

|dkgMaliciousResultNotificationRewardMultiplier
|Percentage of the staking contract malicious behavior notification reward which
will be transferred to the notifier reporting about a malicious DKG result.
|Yes
|`100`

|sortitionPoolRewardsBanDuration
|Duration of the sortition pool rewards ban imposed on operators who were
inactive/disqualified during off-chain DKG or were voted by the group as
inactive for other reasons.
|Yes
|`2 weeks`

4+s|Gas offsets

|dkgResultSubmissionGas	
|Calculated gas cost for submitting a DKG result. This will be refunded as part
of the DKG approval process.
|Yes
|`290_000`

|dkgResultApprovalGasOffset
|Gas that is meant to balance the DKG result approval's overall cost.
|Yes
|`72_000`

|notifyOperatorInactivityGasOffset
|Gas that is meant to balance the operator inactivity notification cost.
|Yes
|`93_000`

|notifySeedTimeoutGasOffset
|Gas that is meant to balance the DKG seed delivery timeout notification cost.
|Yes
|`7_250`

|notifyDkgTimeoutNegativeGasOffset
|Gas that is meant to balance the DKG timeout notification cost.
|Yes
|`2_300`

4+s|Authorization

|minimumAuthorization
|The minimum authorization amount required so that operator can participate in
the Random Beacon.
|Yes
d|`40_000e18` +
_40 000 T_

|authorizationDecreaseDelay
|Delay in seconds that needs to pass between the time authorization decrease is
requested and the time that request gets approved.
|Yes
d|`3_888_000 seconds` +
_45 days_

|authorizationDecreaseChangePeriod
|Time period in seconds before the authorization decrease delay end, during
which the authorization decrease request can be overwritten.
|Yes
d|`3_888_000 seconds` +
_45 days_

4+s|Wallet Registry

|walletOwner	
|Wallet owner address capable of requesting new wallets, closing and slashing
existing ones.
|Yes
d|TBTC `Bridge` contract address

|randomBeacon	
|Random Beacon contract address, needed to produce seed for wallet signing group
member selection.
|Yes
d|`RandomBeacon` contract address

|===

== Build

The contracts use https://hardhat.org/[*Hardhat*] development
environment. To build and deploy contracts, please follow the instructions
presented below.

=== Prerequisites

Please make sure you have the following prerequisites installed on your machine:

- https://nodejs.org[Node.js] >=14
- https://yarnpkg.com[Yarn] >=1.22

=== Build contracts

To build the smart contracts, install node packages first:

```sh
yarn install
```

Once packages are installed, you can build the smart contracts using:

```sh
yarn build
```

Compiled contracts will land in the `build/` directory.

==== TypeScript Typings

Typings are generated for the contracts in `typechain/` directory.

=== Test contracts

There are multiple test scenarios living in the `test` directory.
You can run them by doing:

```sh
yarn test
```

=== Deploy contracts

To deploy contract execute:

```
yarn deploy --network <NETWORK>
```

After the Bridge contract from tbtc-v2 is deployed it has to be set as the
`walletOwner` in the `WalletRegistry`:

```
npx hardhat --network <NETWORK> initialize-wallet-owner --wallet-owner-address <BRIDGE_ADDRESS>
```
