Naivecoin Addendum

2 minute read

Currently I am building a cryptocurrency in C++. The project can be found here. This is not a project where I’ve forked another code base, instead the purpose of this project is to write a brand new protocol from scratch.

Much of what I’m learning comes from this Javascript tutorial. However, like any tutorial, the author assumes a foundation of knowledge that may not exist, or leaves out important details. This is not a knock on the author, for the tutorial is well written, is excellent source material, and has helped me immensely. This post is just an attempt to fill in some of the gaps.

This post will be updated periodically to cover all items in the tutorial that I believe deserve futher explanation.

Chapter 3 - Transactions

Transaction Inputs

Here the reader is introduced to TxIn which is given the following structure:

class TxIn 
{
    public txOutId: string;
    public txOutIndex: number;
    public signature: string;
}

The signature field is explained but the txOutId and txOutIndex fields never are! Here is what I could find out from both of them.

The txIn is the part of the transaction that says “the money being sent in this transaction comes from this txtOut”. That txOut is located by two components:

txOutIndex

This is the block index that contains the transaction in which the originating TxOut exists.

txOutId

This is the transaction id in which the originating TxOut exists.

Example

Consider the following JSON:

{
  "index": 57,
  /* lots of other Block fields */
  "transactions": [
    {
      "id'": "transaction0",
      "inputs": [
        {
          "txOutId": "",
          "txOutIndex": 0,
          "signature": ""
        }
      ],
      "outputs": [
        {
          "address": "Stefan",
          "amount": 10
        }
      ]
    },
    {
      "id'": "transaction1",
      "inputs": [
        {
          "txOutId": "transaction0",
          "txOutIndex": 57,
          "signature": ""
        }
      ],
      "outputs": [
        {
          "address": "Henry",
          "amount": 4
        },
        {
          "address": "Addy",
          "amount": 3
        },
        {
          "address": "Stefan",
          "amount": 1
        }
      ]
    }
  ]
}

Here we have Block #57 in our chain, which contains two transactions transaction0 and transaction1. In practice the fields id, address and signature would have hashes, for purposes of this explanation they have been simplified.

You may notice that transaction0 has no txIn information, which would only happen with a coinbase transaction (i.e. the reward that happens when a miner has solved a block). In this example the receiving address is Stefan who has balance of 10.

In transaction1 we see that Stefan has sent 4 crypto to Henry and 3 to Addy, with the remainder being send back to Stefan.

Also, notice how the txOutIndex value is 57 meaning that the source of this txIn is located in Block #57. Likewise, the value of txOut tells you exactly which transaction in the block from which this txIn comes.