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What is A block?
A block is the basic unit of the Bitcoin protocol. Like other cryptocurrencies, a block is a digitally signed collection of transactions and a link to the previous block in the chain. Blocks may be produced by any node on the network, but are usually produced every 10 minutes by miners. Blocks are not only useful for directing transaction processing on the network, they also act as proof of work for the currency.: 263 : 271

What is A transaction?
A transaction is the process of recording the transfer of bitcoin from one party to another, and is broadcast over the network. Specifically, it consists of: 

1. The sender's public key (Bitcoin address).
2. The receiver's public key (Bitcoin address).
3. The amount of bitcoin being transferred.
4. An optional message (usually a comment) to explain the transaction to other users.
5. A digital signature that binds the transaction to the sender's private key.: 

Controversies
Vulnerabilities in the Bitcoin protocol have been known since 2013, when it was revealed that it was possible to create a double-spend using only six blocks out of thousands in the blockchain.

Attacks
The most well-known attack that exploits the blockchain is the 51% attack, whereby an attacker could overwrite the blockchain so that all transactions are reversed. This can occur if more than half of the computing power on the network is operated by a single entity. In this case, it is then possible for that entity to spend coins it did not actually create, and to double-spend them by replacing the existing blockchain with its own altered record, which would appear as valid and permanent. This is known as a chain split.

The first recording of a malicious blockchain occurred in October 2010. Greg Maxwell publicized a greedy block-refiner attack, which allows an attacker to gain 51% of the network's computing power through economically viable and (very) slow changes in difficulty. Due to the valid nature of the double-spending created by this transaction, the blockchain never recovered and the attacker kept all the coins.: 

As the transaction that launched the attack was validated by a network node, the fork in the blockchain was forced to include the double-spent input, and nearly all of its associated outputs. The attacker was able to force this forking action because the miner that produced the block containing the invalid transaction was mining
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