[Ethereum] Sample showing how to store and retrieve a list of IPFS hash on Ethereum

Using web3 v1.0 I sent this transaction

eth.sendTransaction({
  from: account0,
  to: account1,
  value: 0,
  data: web3.utils.toHex("Hello World")
});

Now with the returned hash I can query the transaction (after it is mined) } eth.getTransaction("0x6c2135c..");`. And we get the resulting transaction

{
  blockHash: "0xb69212f..",
  blockNumber: 510,
  from: "0x9bc62f..",
  gas: 90000,
  gasPrice: 100000000000,
  hash: "0x6c2135c..",
  input: "0x48656c6c6f20576f726c64",
  nonce: 11,
  r: "0x49097a7..",
  s: "0x2b6ce95..",
  to: "0x7fe0197..",
  transactionIndex: 0,
  v: "0xec",
  value: 0
}

The field you have to look is input you can decode it web3.utils.toAscii("0x48656c6c6f20576f726c64"), it will return

"Hello World"

The contract might look something like the sketch below.

There is a struct to contain everything but the id. A mapping from hash => Cert for random access (using the hash as id) and a certList to enumerate the certificates in case the hash isn't known. That should not happen because it emits events for each important state change. I left out access control to keep it brief. You would probably want to protect the newCert() function with onlyOwner, a whiteList or role-based access control.

To "confirm" a cert, the recipient confirms by signing a transaction. The existence of a true in this field shows that the user mentioned did indeed sign because there is no other way for this to occur.

pragma solidity 0.5.1;

contract Certificates {

    struct Cert {
        address recipient;
        bool confirmed;
    }

    mapping(bytes32 => Cert) public certs;
    bytes32[] public certList;

    event LogNewCert(address sender, bytes32 cert, address recipient);
    event LogConfirmed(address sender, bytes32 cert);

    function isCert(bytes32 cert) public view returns(bool isIndeed) {
        if(cert == 0) return false;
        return certs[cert].recipient != address(0);
    }

    function createCert(bytes32 cert, address recipient) public {
        require(recipient != address(0));
        require(!isCert(cert));
        Cert storage c = certs[cert];
        c.recipient = recipient;
        certList.push(cert);
        emit LogNewCert(msg.sender, cert, recipient);
    }

    function confirmCert(bytes32 cert) public {
        require(certs[cert].recipient == msg.sender);
        require(certs[cert].confirmed == false);
        certs[cert].confirmed = true;
        emit LogConfirmed(msg.sender, cert);
    }

    function isUserCert(bytes32 cert, address user) public view returns(bool) {
        if(!isCert(cert)) return false;
        if(certs[cert].recipient != user) return false;
        return certs[cert].confirmed;
    }
}

Hope it helps.