give a certain date to a document

Question

I was asked to "timestamp" document in a "trusted way". I first begin by RFC 3161 and TSA, then I meet blockchains.

I would like to test my undestranding.

Let's say I deploy the following contract:

pragma solidity ^0.4.0;

contract SimpleTimeStamp {
    uint lastHash;

    function set(uint x) public {
        lastHash = x;
    }
}

I will get, in return, a contract addrees.

Then for a doc, "all" I have to do is:

• calculate the sha 256
• convert it to an uint
• call the set function of the contract
• get a transaction hash in return
• store this transaction hash in relation with the doc.

If I want the date I "only" have to use a chain broswer to seek the transaction by his hash and I will get:

• the transaction date
• the unint associated <=> the file sha 256

Am I correct ?

Answer 1

Or an alternative using events which gives us a historical log. We can combine it with Lauri Peltonen's example to make it so a document can only be set once.

contract Timestamping {
 event logTimestamp(bytes32 indexed _documentReference, uint256 timestamp, address _from);
 function timestamp(bytes32 _documentReference) {
   emit logTimestamp(_documentReference, now, msg.sender);
 }
}

Using the web3 javascript api we can then get the historical log from the deployed contract: c

onst Timestamping = artifacts.require('Timestamping') //contract name, not file name
const timestamping = new web3.eth.Contract(Timestamping, <contractaddress>, [, options])

timestamping.getPastEvents('MyEvent', {
    filter: {_documentReference: <documenthash>} //optional, to filter by document reference
    fromBlock: 0,
    toBlock: 'latest'
}, function(error, events){ console.log(events); })
.then(function(events){
    console.log(events) // same results as the optional callback above
});

Answer 2

Well, yes. But.

As you seem to be aware of, all information in the blockchain is permanent. With that in mind, your approach is quite complicated. Let me suggest an easier approach.

1) Get hash of the document

2) Store the hash in a contract which is something like this:

pragma solidity ^0.4.24;
contract Timestamping {
    mapping(string => uint256) timestamps;

    function setTimestamp(string hash) {
        // Do not allow timestamping a document which is already timestamped
        require(timestamps[hash] == 0);
        timestamps[hash] = now;
    }
}

(you shouldn't use strings but byte arrays and so on, but you get the idea)

3) When you want to know the document's timestamp, just ask the blockchain's most recent block's version of the contract data. The contract will always contain only one timestamp entry per document hash and it can't be modified.