[untitled]

6 comments

The code in this post has a major bug.

The data that’s hashed doesn’t include the hash from the previous block, so the chain can be tampered with by modifying only blocks n and n+1.

dale_glass3년 전

Seems buggy.

In a proper blockchain each successive block depends on the previous ones. Thus, breaking 'foo' by replacing it with 'blah' should have also broken 'qux'.

teo_zero3년 전

Yes. The mistake is to only hash the data of the previous block. You should hash the whole previous block, including prev.prev.

sfkeller3년 전

Here's an implementation with lines of sql and pl/pgsql:

Given a table mytable with an pk id and say the fields 'block' and 'hash', replace it with mytable_changes and add an "internal" field called created_at of type "timestamp not null default now()".

Then create this trigger function (e.g. PostgreSQL):

  create function mytable_changes_trigger() returns trigger as $$
  begin
    if (tg_op = 'insert') then
      insert into mytable_changes (block, hash)
      values (new.block, hash);
      return new;
    if (tg_op = 'delete') then
      insert into mytable_changes (block, hash)
      values (old.block, old.hash);
      return old;
    elsif (tg_op = 'update') then
      insert into mytable_changes (block, hash)
      values (old.block, old.hash);
      return new;
    end if;
  end;
  $$ language plpgsql;

And create this trigger:

  create trigger mytable_changes
  after insert, update or delete on mytable
  for each row
  execute function mytable_changes_trigger();

What's left as an exercise is that the hash of the prev. block is being preprended.

capableweb3년 전

Here is a correct (OPs is slightly but importantly wrong) and shorter one in JavaScript:

    const crypto = require('crypto');

    function Block(index, data, prevHash) {
      this.index = index;
      this.data = data;
      this.prevHash = prevHash;
      this.hash = crypto.createHash('sha256').update(`${this.index}${this.data}${this.prevHash}`).digest('hex');
    }

    function Blockchain() {
      this.chain = [new Block(0, 'Genesis', '0')];
    }

    Blockchain.prototype.add = function(data) {
      const prevBlock = this.chain[this.chain.length - 1];
      this.chain.push(new Block(prevBlock.index + 1, data, prevBlock.hash));
    };

Any ways we can make it even shorter?

jonahx3년 전

    const crypto = require('crypto')

    const Block = (index, data, prevHash) => ({
      index,
      data,
      prevHash,
      hash: crypto.createHash('sha256').update(index + data + prevHash).digest('hex'),
    })

    const Blockchain = () => ({
      chain: [Block(0, 'Genesis', '0')],
      add(data) {
        return this.chain.push(
          Block(this.chain.at(-1).index + 1, data, this.chain.at(-1).hash)
        )
      },
    })