05 - Trust and 51% attack

Wooclap Questions

Transactions in Bitcoin require exactly…

• … from 1 to N input accounts and from 1 to N output accounts (N being an integer)

In the Bitcoin protocol, if a node receives a transaction…

• … it makes sure that it is already mined to accept it
• it adds to the ledger

Among the properties of Consistency, Availability and Partition tolerance, the Bitcoin blockchain gives up on guaranteeing…

• Consistency
  • (not serendipity)

In the Bitcoin’s blockchain, every block header keeps…

• … the hash of the previous block’s header
  • (not signature)

Sorting blocks by hashing

Inside a block there is:

• block header
  • hash previous block header
  • timestamp
  • nonce
  • merkle root hash
• hash (block01 header)
• transaction list

The computed hash of the black header is written into the hash previous block header of the next block.

• in the Proof of Work model, the right to publish the next block is granted by solving a computationally intensive puzzle

Trust

• when a node receives a completed block from another one, they are incentivized to include the new block
  • if other mining nodes accept, they start building off it
  • if they refuse, they will be building a shorter chain of blocks
    • by default, the longest valid chain is adopted

If we win at mining game, we are rewarded with coins! Who writes the special value? The miner itself! If we mine a block, we would like that the entire world knows this thing!

If a transaction is not known to any other node in the network but you, then the transaction is not accepted by the network.

So basically a miner wants to:

• mine blocks
• gain consensus by spreading its work
  • if they don’t, the transaction has never happened!

Bitcoin money supply

• puzzles could be adjusted when too much blocks are produced

  • rewards are halved every four years

• puzzles are a trade-off between security and complexity

• it costs electricity

51% attack

• kind of implementation of ^abbd96

Pseudonimity

I don’t wanna let to others people know who I am, but I want to make public that I am sending something from A to B.

For this reason is a bad idea including private stuff in transactions.

Users can have as many accounts as they please (wallets).

Private and public key cryptography is used for signing transactions.

• that’s why using a real hand-written signature is a bad idea: is replicable!

More specifically:

• private keys are used to digitally sign transactions
• public keys are used to derive addresses
  • and they are used to verify signatures generated with private keys

Notice

If you have the private key you can do whatever you like

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