BLOCKCHAINS

March 25, 2019

Introduction to

slides available at https://slides.com/chenevert

Gabriel Chênevert

Math teacher at ISEN Lille

Computer Science department

Information theory:

  • compression
  • error correction
  • cryptography

What is a blockchain?

...and why should you care?

Some history

Birth of Bitcoin

Term "blockchain"

S. Nakamoto

oct. 2008

So... what is a blockchain?

A blockchain is an authenticated, immutable and distributed ledger

Data structures

Information can be stored in many ways inside computers, depending on how it is meant to be used and accessed

Example: Array

Johnny 06 12 34 56 78
Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Tommy 06 02 83 64 40

For efficient access: sort keys

Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40

But: insertion involves a lot of copying

Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40
Gabriel 06 87 93 03 45

But: insertion involves a lot of copying

Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40
Gabriel 06 87 93 03 45

But: insertion involves a lot of copying

Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40
Gabriel 06 87 93 03 45

But: insertion involves a lot of copying

Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40
Gabriel 06 87 93 03 45

But: insertion involves a lot of copying

Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40
Gabriel 06 87 93 03 45

But: insertion involves a lot of copying

Dee Dee 06 43 04 46 24
Gabriel 06 87 93 03 45
Joey
06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40

indeed!

Solution: use a chained list

Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40

Each entry includes a pointer to the next

Inserting is now easy

Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40
Gabriel 06 87 93 03 45

Inserting is now easy

Dee Dee 06 43 04 46 24
Joey 06 78 23 84 92
Johnny 06 12 34 56 78
Tommy 06 02 83 64 40
Gabriel 06 87 93 03 45

Ledgers

Chained lists used to keep track of financial transactions

Personal ledger

Income +200,00 €
Sandwich -     6,50 €
Book -   12,98 €
Found on the floor +    5,00 €
Flowers for mom -  15,00 €

Given the initial amount, one can go through the ledger to find out the current amount at any time

Income +200,00 €
Sandwich -     6,50 €
Book -   12,98 €
Found on the floor +    5,00 €
Flowers for mom -  15,00 €
25,00 €
225,00 €
218,50 €
205,52 €
210,52 €
190,52 €

Multi-user ledger

me boss resto library floor florist
+200 € -200 €
-6,50 € +6,50 €
-12,98 € +12,98 €
+5,00 € -5,00 €
-15,00 € +15,00 €

Now imagine a gigantic ledger keeping track of all the transactions made with a given currency since its introduction...

...so that is it possible to know precisely what amount is owned by any user at any time...

...and we have just reinvented Bitcoin!

Blocks

May contain one or more transactions + associated data

From: Yncréa

To: Gabriel

Amount: 200,00 €

 

From: Gabriel

To: restaurant

Amount: 6,50 €

 

 

From: Gabriel

To: bookstore

Amount: 12,98 €

 

 

From: floor

To: Gabriel

Amount: 5 €

 

From: Gabriel

To: florist

Amount: 15 €

So now we have a chain of blocks

Recall

A blockchain is an authenticated, immutable and distributed ledger

Distributed: no central authority controls the ledger

Can be stored and accessed across a network of independent (untrustworthy) nodes

So anyone in the network can add a new block to the chain

Authentication: every block is digitally signed by its issuer

To authenticate the order of the blocks:

Each block contains a digest (hash) of the previous block, thus authenticating the pointer as well

Digest: 0af4ec6

Transactions:

From: Alice

To: Bob

Amount: 200 €

...

Digest: 56f34a9

Transactions:

From: Carol

To: David

Amount: 30 €

...

Digest: 38ba3e2

Transactions:

From: Elliott

To: Felix

Amount: 15 €

...

Every node stores a complete copy of the blockchain and can verify its integrity at any time

But who decides which blocks are legit and can be added to the chain?

Private blockchain: managed by a central authority

 

(others might have read access)

 

Public blockchain: no one / anyone / everyone!

Adding a block

Digest: 0af4ec6

Transactions:

From: Alice

To: Bob

Amount: 200 €

...

In order to validate / sign a block, the node has to solve a computational challenge ensuring that adding a block to the chain is a non-trivial operation

proof of work

(other mechanisms are possible, e.g. proof of stake)

Extending the chain

Whenever two different versions of the chain are encountered, the longest one is selected

To change a block once committed, one would need to control > 50% of the computing resources of the network

Applications

 

... rise of the World Computer?

  • Anchoring documents (patents, diplomas, notarial acts, ...)
  • Smart contracts and distributed programming (see: Ethereum, CryptoKitties, ...)

 

  • Cryptocurrencies (Bitcoin, Zcash, Monero, ...)
  • Digital payment systems (Ripple, ...)
  • Royalties distribution system (eMusic, ...)

Further reading

  • ...

these slides available at https://slides.com/chenevert

Introduction to blockchains

By Gabriel Chênevert

Introduction to blockchains

Introduction to blockchain technology for 4th year ISA students on February 11 / March 25, 2019

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