A generic “Claim and Endorse” Contract

Did you already endorse someone at LinkedIn? For instance, someone claims that he knows C++ and you endorse this claim because you know it’s true.

A large number of processes can be modelled in this way:

A simple Solidity contract for managing claims and endorsements could look like below.

contract ClaimAndEndorse {
  struct ENDORSEMENT {
  uint creationTime;
 }
 
 struct CLAIM {
  uint creationTime;
  uint claimHash;
  mapping (address => ENDORSEMENT) endorsements;
 }
 
 mapping (address => 
  mapping (uint /* CLAIM GUID */ => CLAIM)) claims;
 
 function setClaim(uint claimGuid, uint claimHash) {
  CLAIM c = claims[msg.sender][claimGuid];
  if(c.claimHash > 0) throw; // unset first!
  c.creationTime = now;
  c.claimHash = claimHash;
 }
 
 function unsetClaim(uint claimGuid) {
  delete claims[msg.sender][claimGuid];
 }
 
 function setEndorsement(
  address claimer, uint claimGuid, uint expectedClaimHash
 ) {
  CLAIM c = claims[claimer][claimGuid];
  if(c.claimHash != expectedClaimHash) throw;
  ENDORSEMENT e = c.endorsements[msg.sender];
  e.creationTime = now;
 }
 
 function unsetEndorsement(address claimer, uint claimGuid) {
  delete claims[claimer][claimGuid]
          .endorsements[msg.sender];
 }
 
 function checkClaim(
  address claimer, uint claimGuid, uint expectedClaimHash
 ) constant returns (bool) {
  return claims[claimer][claimGuid].claimHash 
         == expectedClaimHash;
 }
 
 function checkEndorsement(
  address claimer, uint claimGuid, address endorsedBy
 ) constant returns (bool) {
  return claims[claimer][claimGuid]
   .endorsements[endorsedBy].creationTime > 0;
 }
}

The fantastic thing about this very simple contract is that we now can answer the following question:

Who claims what and who endorses it?

Usecase — Skill/Degree Endorsements

John claims that he holds a PhD in computer science at the Stanford university.

contract.setClaim(
 3000010 /* GUID for PhD in CS */,
 HASH("PhD in Computer Science at Stanford University"))

The Stanford university confirms this fact.

contract.setEndorsement(JOHN, 3000010)

Usecase — Identity Verification

Step 1. John claims facts about his personal data by binding hashes to his ethereum address. The corresponding pseudo code is:

// RND is a random sequence introduced for making it impossible to restore the initial data from the hash by brute force lookups.
contract.setClaim(1000010 /*guid firstname*/, 
 HASH("John"+RND))
contract.setClaim(1000011 /*guid surename */, 
 HASH("Smith"+RND))
contract.setClaim(1000012 /*guid bday */, 
 HASH("1965-01-01"+RND))
contract.setClaim(1000013 /*gender */, 
 HASH("m"+RND"))

Step 2. John visits his bank, which endorses these facts about his identity:

contract.setEndorsement(JOHN, 1000010 /*guid firstname*/);
contract.setEndorsement(JOHN, 1000011 /*guid surename */);
contract.setEndorsement(JOHN, 1000012 /*guid bday */);
contract.setEndorsement(JOHN, 1000013 /*guid gender */);

Now consider that JOHN wants to open A) an account at ACME Inc, B) buy alcohol in the bar and C) register at a dating site. If all three trust John’s bank, he’ll be able to digitally prove his claims on his personal data. Moreover, he only needs to present the relevant pieces of data. For instance at the bar, he only has to prove the claim that he’s older than 18.

Usecase — Approving existence of documents

ACME Inc. wants to publish a new financial product. The hashes of the required documents are stored on the blockchain. The authorities and the exchanges are confirming the existence and the correctness of these documents.

Related: Luxembourg Stock Exchange OAM

Usecase — Managing Memberships

John wants to become a member in his local bowling club. He stores this fact on the blockchain and the club confirms this fact. With the first step, John manifests his will to enter the club. In the second step, the club confirms that they are accepting John as a member.

Try it yourself on Ropsten Testnet

https://testnet.etherscan.io/address/0x65ec6e00e336a96972987ee25386a3090f38a27d#code

Originally published at http://blockchainers.org on November 27, 2016.