Codex Contracts

An experimental implementation of the smart contracts that underlay the Codex storage network. Its goal is to experiment with the rules around the bidding process, the storage contracts, the storage proofs and the host collateral. Neither completeness nor correctness are guaranteed at this moment in time.

Running

To run the tests, execute the following commands:

npm install
npm test

You can also run fuzzing tests (using Echidna) on the contracts:

npm run fuzz

To start a local Ethereum node with the contracts deployed, execute:

npm start

This will create a deployment-localhost.json file containing the addresses of the deployed contracts.

Overview

The Codex storage network depends on hosts offering storage to clients of the network. The smart contracts in this repository handle interactions between client and hosts as they negotiate and fulfill a contract to store data for a certain amount of time.

When all goes well, the client and hosts perform the following steps:

Client                 Host          Marketplace Contract
  |                     |                      |
  |                                            |
  | --------------- request (1) -------------> |
  |                                            |
  | ----- data (2) ---> |                      |
  |                     |                      |
                        | ----- fill (3) ----> |
                        |                      |
                        | ---- proof (4) ----> |
                        |                      |
                        | ---- proof (4) ----> |
                        |                      |
                        | ---- proof (4) ----> |
                        |                      |
                        | <-- payment (5) ---- |

1. Client submits a request for storage, containing the size of the data that it wants to store and the length of time it wants to store it
2. Client makes the data available to hosts
3. Hosts submit storage proofs to fill slots in the contract
4. While the storage contract is active, host prove that they are still storing the data by responding to frequent random challenges
5. At the end of the contract the hosts are paid

Contracts

A storage contract contains of a number of slots. Each of these slots represents an agreement with a storage host to store a part of the data. Hosts that want to offer storage can fill a slot in the contract.

A contract can be negotiated through requests. A request contains the size of the data, the length of time during which it needs to be stored, and a number of slots. It also contains the reward that a client is willing to pay and proof requirements such as how often a proof will need to be submitted by hosts. A random nonce is included to ensure uniqueness among similar requests.

When a new storage contract is created the client immediately pays the entire price of the contract. The payment is only released to the host upon successful completion of the contract.

Collateral

To motivate a host to remain honest, it must put up some collateral before it is allowed to participate in storage contracts. The collateral may not be withdrawn as long as a host is participating in an active storage contract.

Should a host be misbehaving, then its collateral may be reduced by a certain percentage (slashed).

Proofs

Hosts are required to submit frequent proofs while a contract is active. These proofs ensure with a high probability that hosts are still holding on to the data that they were entrusted with.

To ensure that hosts are not able to predict and precalculate proofs, these proofs are based on a random challenge. Currently we use ethereum block hashes to determine two things: 1) whether or not a proof is required at this point in time, and 2) the random challenge for the proof. Although hosts will not be able to predict the exact times at which proofs are required, the frequency of proofs averages out to a value that was set by the client in the request for storage.

Hosts have a small period of time in which they are expected to submit a proof. When that time has expired without seeing a proof, validators are able to point out the lack of proof. If a host misses too many proofs, it results into a slashing of its collateral.

References

• A marketplace for storage durability (design document)
• Timing of Storage Proofs (design document)

To Do

• Contract repair

  Allow another host to take over a slot in the contract when the original host missed too many proofs.

• Reward validators

  A validator that points out missed proofs should be compensated for its vigilance and for the gas costs of invoking the smart contract.

• Analysis and optimization of gas usage