1. Blockchain & Bitcoin Notions fondamentales Lionel Brunie, Omar Hasan Institut National des Sciences Appliquées de Lyon

2. Outils de base (1/2) Fonction de hachage cryptographique
Associe à un objet numérique de taille variable, une chaîne de caractères de taille fixe (appelée résidu, empreinte, hash...)
Sens unique
Calcul du hash rapide
Calcul inverse extrêmement difficile
Probabilité très faible d’obtenir le même hash avec deux données d’entrée différentes (collision)
Petite modification de la donnée d’entrée => modification aléatoire du hash
Hashes différents => données d’entrée différentes avec une quasi certitude
Utilisations
Intégrité : hash non modifié => donnée non modifiée avec une quasi certitude
Masquage d’information, propriété et authenticité (log) : stocker le hash d’une donnée permet de masquer le contenu de celle-ci et de minimiser l’espace de stockage ; cependant, le propriétaire de la donnée d’entrée peut, en dévoilant celle-ci, en restituer le contenu dont l’authenticité est garantie par le hash

3. Outils de base (2/2) Chiffrement asymétrique
Deux clefs associées : clef publique / clef privée
Clef publique... publique (affichée à la vue de tous) !
Clef secrète... secrète (conservée par devers soi et transmise à personne)
Quasiment impossible de déduire l’une des clefs à partir de l’autre
Chiffrement avec une clef, déchiffrement avec l’autre clef
Utilisations
Confidentialité : chiffrer avec la clef publique du destinataire
Authentification : chiffrer avec la clef privée de l’émetteur

4. Bitcoin Address Bitcoin manages transactions i.e., flows of « coins »
A coin « belongs » to an address
An address is the 160-bit hash of the public key of a public/private ECDSA keypair
Bitcoin allows one to create as many addresses as one wants, and use a new one for every transaction

5. Structure of a Block (Bitcoin)
3 fields
Header: 80 Bytes
Number of transaction (Varint) e.g., n
n concatenated transactions
Total size <=1MB

6. Block Header (Bitcoin)
80 Bytes
Version number (4 Bytes)
Hash pointer to the previous block (32 Bytes)
Root of the Merkle tree formed by the n transactions in the block (hash) (32 Bytes)
Timestamp (uint32) (4 Bytes)
nBits: Difficulty to solve the block (uint32) (4 Bytes)
Nonce result of the puzzle (uint32) (4 Bytes)

7. Transaction (Bitcoin)
Header, incl.
Nb of inputs
List of inputs
Nb of outputs
List of outputs
Lock time (block height or timestamp when the transaction can be included into a block

8. Input – Output – Coin (1/2)
A coin is not identified using a fixed id
A transaction transforms inputs (coins) into output (coins)
Once used, input coins cannot be anymore used as inputs
Inputs/Ouputs can represent any amount of satoshis (some miners impose a minimum)
Inputs (coins) must have been produced as outputs of a previous transaction i.e., an input = reference to an output from a previous transaction
The total value of the inputs must be >= total value of the outputs

9. Input – Output – Coin (2/2)
Input format
hash of the previous transaction from which the input was produced
index nb referencing the specific output of this transaction
script length
Script (used notably to check the ownership of the inputs)
sequence nb (allow modifying the transaction when before LockTime expires (may be disabled)
Output format
value : nb of satoshis (1 satoshi = 1/10^8 bitcoins)
Script (includes the Bitcoin address of the recipient of the output (hash of its public key)

10. Examples of Transactions
From en.bitcoin.it

11. Scripts and Verification
Stack-based language (Forth-like)
Pay-to-PubkeyHashTransaction
script on the inputs (scriptSig): <sig> <pubKey>
script on the outputs (scriptPubKey): OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECKSIG
the “owner” of a transaction input (coin) provides its public key + her signature of the transaction which created this coin (remember a coin is created as output of a transaction)
the combination of the scripts allows verifying that the hash of the public key matches the address attached to the “coin” (i.e. to the recipient of the output corresponding to the “coin”) and that the signature is correctly decrypted by this public key
Pay to script hash Transaction
allows sending coins to a script hash instead of a public key hash
to spend these coins, the recipient must provide a script that matches the script has and data which makes the script evaluate to true
Used to implement “(smart) contracts”

12. OP-RETURN Script Operator
OP_RETURN, when placed in an output script (scriptPubKey), makes the script fail => the coin to which the script is attached is unspendable (“burnt”)
Data placed after the OP_RETURN operator are not read by the miner (but they are stored in the ledger)
Usually, one places less than 40 Bytes after OP_RETURN
Used to create “colored” coins

13. Multisignature
m of n: n addresses are attached to an output coin; at least m of them must sign in order to use this coin
Often use to introduce an escrow (signature 2 of 3)
May be use to store information (“fake” address of a 1 of 2 signature)

14. Blockchain Block = data (transactions in Bitcoin)
Hash of a block = address (id) of this block. In Bitcoin, one considers the hash of the header. Note: the header contains the root of the Merkle tree formed by the transactions stored in the block
Blocks are chained (ordered)

15. Mining Blocks and Consensus Protocol (1/3)
Transactions are broadcasted to the Bitcoin P2P network
Mining = adding transaction records to Bitcoin's public ledger of past transactions
This ledger is used to log legitimate transactions and prevent double spending
This ledger = blockchain
Miners are rewarded for their work
In each block, a reward transaction (“coinbase transaction”) is included. This transaction creates new coins which are given to the miner. This reward halves every blocks. It’s now 12,5 bitcoins
Transactions may (actually should) include “transaction fees” that are given to the miner
Mining is the only way to create coins

16. Mining Blocks and Consensus Protocol (2/3
Proof of work (PoW)
deter abuses e.g., DoS, spam, etc.
PoW = piece of data hard to produce but whose validity can be easily verified
Bitcoin PoW: find a value (nonce) such that the hash of the concatenation “header of the block + nonce” is lower that a specified threshold (the “target”) (i.e. the hash starts with a certain number of ‘0’)
as each miner mines a “personalized” block, each miner solves a different puzzle
Unpredictability of the puzzle => a miner with x% of the total computing power has x% chance to be the first to solving the puzzle
The difficulty of the puzzle is set to such a difficulty (target) that a block is mined approx. every 10 mn
PoWs are verified by the other miners

17. Mining Blocks and Consensus Protocol (3/3)
The P2P Bitcoin network is an asynchronous network => multiple miners may solve their own puzzle at the same time; each of them then broadcasts its block to the network => the chain is now “forked”
A consensus protocol is required to fix this issue and make a decision on which block is the new block
Protocol: choosing the longest chain
If a node receives two blocks (thus facing a chain with 2 branches), it stores both of them and and try to append a new block to one of the 2 branches (typically, the first it receives)
In the meantime, if it receives a new block for one of the branches, it discards the other one (i.e., the shortest one)
Eventually, all miners agree on the same chain (actually, as the chain always grows, they agree on the same prefix)
When a miner receives a longer chain, it needs to “roll back” the blocks (called “orphan blocks”) down to the fork, then to add the newly received blocks
Rewards attached to orphan blocks are nor spendable (actually, not spendable on the longest blockchain)
Transactions logged in an orphan block return back to the memory pool of the miners
It is usually considered that a block is confirmed after 6 other blocks are added to the blockchain

18. Some figures (March 7, 2017) (1/2)
Total bitcoins in circulation: 16,200,950
Total bitcoins to be ever produced: 21,000,000
Exchange rate : 1 BTC~ 1280 USD
Bitcoin capitalization: 20,7 billion USD (19,6 billion €)
Nb of transactions per hour: (for 80,121 BTC)
Total blocks: 456,075
Nb of blocks generated per day: 144
Computing power: 3.3 exahashes/s – 42,454,023 PFLOPS (#1 top500 supercomputer: 125 PFLOPS (Rpeak)-93 PFLOPS (Rmax)); sum of top500: 672PFLOPS (Rmax))!!!!! (i7~: a few tens to hundreds of GFLOPS) (use of ASICS)

19. Some figures (March 7, 2017) (2/2)
Total bitcoins in circulation: 16,200,950
Total bitcoins to be ever produced: 21,000,000
Exchange rate : 1 BTC~ 1280 USD
Bitcoin capitalization: 20,7 billion USD (19,6 billion €)
Nb of transactions per hour: (for 80,121 BTC). Note: max speed: 7 transactions per seconds; VISA : average 2000 trans./s, peak 56,000 trans./s!
Total blocks: 456,075
Size of the Bitcoin blockchain: 105 GB
Nb of blocks generated per day: 144
Nb of orphan blocks generated per day: 0-4
Computing power: 3.3 exahashes/s – 42,454,023 PFLOPS (#1 top500 supercomputer: 125 PFLOPS (Rpeak)-93 PFLOPS (Rmax)); sum of top500: 672PFLOPS (Rmax))!!!!! (i7~: a few tens to hundreds of GFLOPS) (use of ASICS)
The 5 more powerful mining pools concentrates 55% of the total computing power
Number of unique addresses: 500,000+

20. Conclusion: Why Using Blockchains?
Some nice properties
No trusted third party, no single point of failure
Reliability and immutability (very high replication factor)
(eventual) Consistency
Non-repudiability
Existing infrastructure
Open infrastructure
Contracts
Some issues
Security (in the ecosystem more than in the Bitcoin protocol itself)
Total transparency; no real anonymity
Hard consensus
Low latency, low throughput, poor transactional scalability (cf. huge computing power for only transactions per day!)
Waste of resources and energy due to a huge replication factor and the solving of useless puzzles
PoW vs Proof of Stake