Blockchain is a distributed database that records transactions in chronological order in digitally signed blocks. Each block contains a cryptographic hash linking it to the previous block, forming a chain. Miners on the network verify and record new transactions in blocks, which are then broadcast to the network. While branching can occur, the blockchain resolves it automatically by continuing on the longest branch. Tampering with past transactions requires overcoming the main branch through computational power. The first blockchain application was Bitcoin, which uses this structure to record ownership of digital currency through public/private key cryptography.

1. Blockchain for dummies
or how to make $127 million in 20 minutes…
Thibault Debatty
Cyber Defense Lab, Royal Military Academy
www.cylab.be
BeMilCIS2019

2. Like GO : simple principle, complex
implications!
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3. Blockchain
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4. Blockchain
Each block contains
•A link to previous block (the chain)
•A timestamp
•Data
•A hard to compute hash
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5. Miners
•Keep a copy of the complete chain (BTC: 160GB)
•Create (and solve) new blocks…
•… starting from the end of the chain
•Broadcast the new block to other miners
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6. Branching
•Can happen
•Will solve automatically
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7. Branching
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8. Branching
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9. Blockchain tampering
Modifying the past requires
•To create a new branch
•And to overtake the main branch
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10. Blockchain tampering
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11. Blockchain
•A replicated database
•That records time related data
•With eventual consistency
•Protects integrity (non-repudiation):
• With no central authority
• Even with untrusted participants
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12. But why so much hype?
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13. Bitcoin
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14. Bitcoin
Data of a block is a list of transactions
•E.g. 0,01 BTC from A to B
Difficulty adapts : 1 new block every 10 minutes
Transaction considered as final after 6 blocks (1h)
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15. Bitcoin
Uses asymetric cryptography
•Private key used to sign transactions
•Public key is the address of account
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16. Bitcoin wallet
•Stores public and private keys
•Create and sign transactions
•Send transaction to miners
•Can be online, desktop, app
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17. Online wallets
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18. Online wallets
As safe as the website
https://www.theregister.co.uk/2019/06/07/komodo_npm_wallets/
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19. Desktop wallets and apps
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20. Desktop wallets and apps
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21. 21Blockchain for dummies - BeMilCIS2019
Miners consume a LOT of power (5MW)

22. How do miners make money?
•Controlled supply:
• 12,5 new reward bitcoins per block
• (originally 50)
•Transaction fee:
• $2/transaction on average
• $8000/block
But what when reward bitcoins disappear?
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23. How can youget BTC?
•Mine
•Buy
•As a payment
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24. What makes the value of BTC?
Just like gold :
•Supply and demand
•Mining cost
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25. Other (useful) applications of
Blockchain?
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26. obvious: cryptocurrencies
Currently 26 active cryptocurrencies!
•Ethereum (ETH)
•Bitcoin Cash (BCH)
•Ripple (XRP)
Totalling more than $100bn
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27. possible: smart contracts
Record transactions with no central authority
•Finance and trade (e.g. real estate)
•Authorship
•NATO NCIA : contracts handling
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28. possible: smart contracts
Replace DNS authorities and SSL/TLS certificate
authorities
•Blockchain DNS
•okTurtles DNSChain
But what about proof of work, reward and price?
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29. Doubtful…
•US DoD messaging system
•Health records management
Why not a centralized system?
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30. In conclusion, Blockchain
•Is a simple chain of blocks
•Offers integrity protection in a network with
no central authority
•Has a cost in time and power
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31. Thank you!
Find this presentation and more resources at
www.cylab.be
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