Trustable oracles towards trustable blockchains

Pedro Duarte da Costa

Supervisor:

Filipe Figueiredo Correia

Second Supervisor:

Hugo Sereno Ferreira

pedro.duartecosta@fe.up.pt

16 July 2019

Master in Informatics and Computing Engineering

Blockchain

 

Smart Contracts

 

Shared distributed ledger

Strong consistency guarantees

Deterministic

Blockchain applications

Automated and decentralized contracts

The terms are written in code

2

The smart contract connectivity problem

 

The blockchain is deterministic. Once a transaction is validated we can be sure that it will remain unchanged.

 

But the internet is non-deterministic, thus contracts cannot directly query the internet.

3

Oracles

 

4

Problem

 

Trusting oracles to be reliable and honest.

5

Systematic Literature Review

RQ1: What kind of blockchain oracles have been proposed?

RQ2: What are the research trends on blockchain oracles?

6

(("blockchain" OR "block chain" OR "block-chain")  

AND  

("oracles" OR "oracle" OR "middle-ware" OR "middleware" OR "middle ware" OR "datafeed" OR "data feed" OR "data-feed"))

Search query

Systematic Literature Review

7

Findings

  1. Town Crier [ZCC+16] - Utilizes trusted hardware to serve source authenticated.
  2. Astrae [ABV+18] - Decentralized incentive-based network with submitters, voters and certifiers
  3. Gilroy Gordon [Gor17] - Protocol for sensor data authenticity and integrity in a IoT network
  4. Francisco Monroy [Mon18]  - Gambling protocol based on incentives using a pool of 7 oracles.
  5. J. Eberhardt [EH18] - Proposes a systematic classification of off-chain computation oracles.
2

Town Crier Architecture

Aestrae Architecture

8

Commercial products and projects

 
  1. Oraclize.it (provable) [Ora18] - Utilizes authenticity proofs to guarantee oracle honesty
  2. ChanLink [EJN17], - Decentralized network of oracles to guarantee availability and query multiple data sources
  3. Schelling Coin [Vit14] - Decentralized network that rewards participants that submit answers closest to the median of all submitted
  4. TrueBit [TR17] - System of solvers and verifiers, monetarily incentivized to check on each other's answers.

9

RQ1: What kind of blockchain oracles have been proposed?

  1. Software-based Oracles - Mostly use authenticity proofs to prove their honesty

  2. Hardware-based Oracles - Similar to software-based oracles but take advantage of hardware trusted execution environments, such as Intel SGX

  3. Consensus-based Oracles - Utilize a network of peers and to achieve consensus by wisdom-of-the-crowd solutions or incentive-based ones.

10

RQ2: What are the research trends on blockchain oracles?

  1. Academic research focuses mostly on consensus-based or hardware-based solutions, ex: Astrae and Town Crier.

  2. Whereas the industry researches and develops authenticity proofs levaring both software and hardware-solutions, ex: TLSNotary, Android Proof, Ledger Proof, etc.

11

Analysis of Authenticity Proofs

Proof Requires specific hardware Can query the web Future-proof Currently used in production
TLSNotary x x
TLS-N x x
Ledger Proof x x x
Android Proof x x x x
Town Crier x x x x

12

Systematization of Oracle Architectures

13

Desiderata

  • Fast time-to-market
  • Keeping trust standards
  • Data-feed fault tolerance
  • Data veracity
  • Lower smart contract costs
  • Lower oracle complexity
  • Oracle decentralization
  • Oracle ownership decentralization

14

Analysing trust

Trusting oracle behavior

Trusting the data-feed

Off-chain

On-chain

15

Oracle as a Service w/ Single Data Feed

Forces

  1. Fast time-to-market
  2. Trust

Resulting context

  1. No data-feed availability guarantees
  2. Single source of data
  3. Centralized oracle node
  4. Single oracle owner
  5. Burden of checking proofs

Known uses

Oraclize.it (provable.xyz)

16

Oracle as a Service w/ Multiple Data Feeds

Forces

  1. Fast time-to-market
  2. Trust
  3. Data-feed increased availability
  4. Multiple sources of truth
 

Resulting context

  1. Centralized oracle node
  2. Single oracle owner
  3. Burden of checking proofs

Known uses

ChainLink

17

Single-Party Self-Hosted Oracle

Forces

  1. Cost optimization
  2. Trust
  3. Data-feed increased availability
  4. Multiple sources of truth
 

Resulting context

  1. Centralized oracle node
  2. Single oracle owner

Known uses

gardeneroracle.io

18

Multi-Party Self Hosted Oracle

Forces

  1. Cost optimization (no proofs)
  2. Trust
  3. Data-feed increased availability
  4. Multiple sources of truth
  5. Decentralized oracle nodes
 

Resulting context

  1. Increased costs when using multiple nodes

Known uses

n/a

19

Summary

OaaS w/SDF OaaS w/MDF SP-SHO MP-SHO
Fast time-to-market X X
Keeping trust standards X X X X
Data-feed fault tolerance X X
Data veracity X X
Lower smart contract costs X X
Lower oracle complexity X X
Oracle decentralization X
Oracle ownership decentralization X

20

Implementing a Self-hosted Oracle

21

The Unlicense
A license with no conditions whatsoever which dedicates works to the public domain.

Architecture

22

contract Oracle {
      Request[] requests;        //list of requests made to the contract
      uint currentId = 0;        //increasing request id
      uint minQuorum = 2;        //minimum number of responses
      uint totalOracleCount = 3; //hardcoded oracle count

      // defines a general api request
      struct Request {
        uint id;                            //request id
        string urlToQuery;                  //API url
        string attributeToFetch;            //json attribute (key)
        string agreedValue;                 //value from key
        mapping(uint => string) anwers;     //answers provided by the oracles
        mapping(address => uint) quorum;    //oracles which will query the answer 
                                            //(1=oracle hasn't voted, 2=oracle has voted)
      }

      ...
}

On-chain oracle contract

23

  function createRequest (
    string memory _urlToQuery,
    string memory _attributeToFetch
  )
  public  {
    uint length = requests.push(Request(currentId, _urlToQuery, _attributeToFetch, ""));
    Request storage r = requests[length-1];

    // Hardcoded oracle addresses
    r.quorum[address(0x6c2339b46F41a06f09CA0051ddAD54D1e582bA77)] = 1;
    r.quorum[address(0xb5346CF224c02186606e5f89EACC21eC25398077)] = 1;
    r.quorum[address(0xa2997F1CA363D11a0a35bB1Ac0Ff7849bc13e914)] = 1;

    // launch an event to be detected by oracle outside of blockchain
    emit NewRequest (
      currentId,
      _urlToQuery,
      _attributeToFetch
    );

    // increase request id
    currentId++;
  }

Making a request

24

  function updateRequest (
    uint _id,
    string memory _valueRetrieved
  ) public {

    Request storage currRequest = requests[_id];
    
    //check if oracle is in the list of trusted oracles
    //and if the oracle hasn't voted yet
    if(currRequest.quorum[address(msg.sender)] == 1){
      
      currRequest.quorum[msg.sender] = 2; //marking that this address has voted
    
      //iterate through "array" of answers until a position if free and save the retrieved value
      uint tmpI = 0;
      bool found = false;
      while(!found) {
        //find first empty slot
        if(bytes(currRequest.anwers[tmpI]).length == 0){
          found = true;
          currRequest.anwers[tmpI] = _valueRetrieved;
        }
        tmpI++;
      }
   

Reaching consensus (1)

25

  uint currentQuorum = 0;

  //iterate through oracle list and check if enough oracles(minimum quorum)
  //have voted the same answer has the current one
  for(uint i = 0; i < totalOracleCount; i++){
    bytes memory a = bytes(currRequest.anwers[i]);
    bytes memory b = bytes(_valueRetrieved);

    if(keccak256(a) == keccak256(b)){
      currentQuorum++;
      if(currentQuorum >= minQuorum){
        currRequest.agreedValue = _valueRetrieved;
        emit UpdatedRequest (
          currRequest.id,
          currRequest.urlToQuery,
          currRequest.attributeToFetch,
          currRequest.agreedValue
        );
      }
    }
  }
}

Reaching consensus (2)

26

Implementation attributes

  1. Fast time-to-market ?
  2. Keeping trust standards
  3. Data-feed fault tolerance
  4. Data veracity
  5. Lower smart contract costs
  6. Lower oracle complexity (in comparison to using proofs)
  7. Oracle decentralization
  8. Oracle ownership decentralization

27

Contributions

  1. Systematic literature review (paper to be submitted)
  2. Analysis of authenticity proofs
  3. Systematization of oracle architectures
  4. Implementing a multi-party self-hosted oracle

28

References

[Gor17] Gilroy Gordon. Provenance and authentication of oracle sensor data with blockchain lightweight wireless network authentication scheme for constrained oracle sensors. 2017.

 

[Mon18] Francisco Javier Andrés Montoto Monroy. Bitcoin gambling using distributed oracles in the blockchain. 2018.

 

[Ora18] Oraclize.it. Oraclize Documentation, 2018.

 

[ABV+18] John Adler, Ryan Berryhill, Andreas Veneris, Zissis Poulos, Neil Veira, and Anastasia Kastania. Astraea: A Decentralized Blockchain Oracle. aug 2018.

 

[ZCC+16] Fan Zhang, Ethan Cecchetti, Kyle Croman, Ari Juels, and Elaine Shi. Town Crier: An Authenticated Data Feed for Smart Contracts. Technical report, 2016.

 

[EH18] Jacob Eberhardt and Jonathan Heiss. Off-chaining Models and Approaches to Offchain Computations. In Proceedings of the 2nd Workshop on Scalable and Resilient Infrastructures for Distributed Ledgers - SERIAL’18, pages 7–12, New York, New York, USA, 2018. ACM Press.

 

[EJN17] Steve Ellis, Ari Juels, and Sergey Nazarov. ChainLink A Decentralized Oracle Network. Technical report, 2017.

 

[TR17] Jason Teutsch and Christian Reitwießner. A scalable verification solution for blockchains. Technical report, 2017.


[Vit14] Vitalik Buterin. SchellingCoin: A Minimal-Trust Universal Data Feed, 2014.

29

Obrigado

Pedro Duarte da Costa

pedro.duartecosta@fe.up.pt

Master Thesis Presentation

By Pedro Costa

Master Thesis Presentation

Master Thesis in Informatics and Computing Engineering Presentation

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