Cryptography & Distributed Systems

1. Cryptography &
Distributed Systems
Filip Đurđević
Filip Ivanović
Vanja Milutinović
Mara Bolić
University of Belgrade

2. What is cryptography?
●Encryption
●Decryption
●“Attack at dawn”
●Caesar cipher
● Kerckhoffs's principle

3. What is cryptography?
?
Key
Plaintext Cyphertext ?
Key
Plaintext

4. Symmetric Algorithms
Single key used for encryption and decryption
●Block algorithms
●Stream algorithms
●Why do we need something better?

5. Asymmetric Algorithms
Different keys for encryption and decryption
●Public and private keys
●Confidentiality
●Digital signature
●“Man in the middle” attack

6. RSA
Encryption●Fast to encrypt, slow to decrypt
●“Trapdoor” Ф(n)=(P-1)(Q-1)
●Complexity of prime factorization
●d=(k*Ф(n)+1)/e
c=mᵉ mod n
m=cᵈ mod n
Ex: P=1181 Q=2683⇒ n=3168623,
Ф(n)=3164760
Ф(n)=2*2*2*3*3*5*59*149

7. Hash functions
●Outputs the value of fixed
length
●Hash collision
●Secure Hash Algorithm (SHA)
●SHA-256 (SHA-2)

8. Merkle Trees
●Reduces amount of data
●Blockchain transactions

9. What are distributed systems?
●A distributed system represents a system of multiple nodes (computers) that
communicate and coordinate actions in order to appear as a single coherent system to
the end-user.
●Examples: Google, Dropbox, Email servers, Cloud computing, Banking systems, Massive
multiplayer online video games, Telephone networks, Sensor networks, IoT...

10. Distributed Systems
●Unlimited Horizontal Scaling
●Low Latency
●Fault Tolerance
●Performance
●Data Integration & Consistency
●Network & Communication
Failure
●Management Overhead
Advantages Disadvantages

11. Peer to Peer Networks
●Distributed
●All nodes have equal power and perform the same task.
●There is no “server” and hierarchy within the network.
●The core of bitcoin.
●Why do we use it in blockchain?
●Security

12. Centralization

13. Bitcoin Network

14. Bitcoin nodes
Types of bitcoin nodes
●Full node
●Pruning node
●Lightweight node
●Mining node

15. Bitcoin nodes
How do they function?
●Discovery
●Synchronization
●Controlled flooding protocol
●Pull data synchronization protocol

16. “Bitcoin represents the culmination of
decades of research in cryptography and
distributed systems.”

17. Do you have any questions?

18. Thank You!