Blockchain is a way of storing data in a way that makes it immutable and prevents tampering. It works by storing data in blocks that are chained together using cryptography. Each block contains a cryptographic hash of the previous block, linking the blocks together. This makes it very difficult to tamper with past data, as any change would require recalculating all subsequent hashes. Blockchains use a peer-to-peer network where each node maintains a copy of the chain. When a new block is created, it is distributed across the network and nodes must reach consensus on its validity before it is added to the chain. This process of proof-of-work secures the blockchain and distributed ledger.

1. Blockchain

2. • What’s a blockchain?
• How do they work, what problems do they solve and how can they be
used?

3. • Everyone is talking about blockchains and cryptocurrencies these days. But
what is this blockchain thing?
• A blockchain is a way of storing data so that cannot be changed
anymore. This is called immutability and a very useful feature when dealing
with very important data like bank records or transactions.

4. • Like the name indicates, A blockchain is a chain of blocks that contains
information.
• This technique was originally described in in 1991 by a group of researchers
(Stuart Haber and W. Scott Stornetta) and was originally intended to
timestamp digital documents so that it’s not possible to backdate them or to
tamper with them.

5. How do they work?

6. 1- The data that inserted inside a block it depends on the
type of blockchain.
The Bitcoin blockchain for example stores the details
about the transaction in here, such as the sender,
receiver and amount of coins.
Data

7. You can compare a hash to fingerprint.
It identifies a block and all of it’s contents and it’s
always unique like a fingerprint.
Once a block is created, it’s hash is being
calculated
Hash

8. • Changes something inside the block will cause the
hash to change
• Hashes is very useful when you want to detect
changes to block
• If the fingerprint of a block changes, it no longer the
same is the same block
Detect changes

9. Hash of previous block
• The third elements inside each block the hash of the
previous block
• This effectively creates a chain of blocks and it’s this
technique that makes a blockchain so secure

10. • Computers can calculate hundreds of thousands of
hashes per second
• Recalculate all the hashes of other block to make
your blockchain valid again
How every block changes
according the previous hash

11. Proof of work
It’s a mechanism that slows down the creation of new blocks
the proof-of-work difficulty is determined by a moving
average targeting an average number of blocks per hour. If
they're generated too fast, the difficulty increases.

12. Example
• In bitcoins case: it takes 10 minutes to calculate the proof-of-
work and add new block to the chain
• This mechanism makes it very hard to tamper with the blocks,
because you tamper with 1 block, you need to calculate all
the following blocks
• so the security of a blockchain comes from its creative use of
hashing and the proof-of-work mechanism

13. Peer-to-peer (p2p)
• Instead of using central entity to manage the chain,
blockchain use peer-to-peer network and every one is
allow to join
• Files can be shared directly between systems on the
network without the need of a central server
• The only requirements for a computer to join a peer-to-
peer network are an Internet connection and P2P software
• When someone join the networks, he gets full copy of the
blockchain

14. Create new block
• Each node then verifies the block to make sure that it hasn’t
been tampered with
• If everything checks out, each node add this block to there own
block

15. nodes
• They agree about what blocks are valid and which aren’t
• Blocks that tamper with will be rejected by other nodes in
the network
• So to successfully tamper with a chain you will need to
tamper with all the blocks on the chain

16. • Blockchain are also constantly evolving
• One of the more recent developments is the creation of smart
contract
• This contract are simple programs that are stored on blockchain
and can be used to automatically exchange coins based on
certain conditions

17. Bitcoin dashboard