Word of Wisdom Service with Proof Of Work DDOS Protection

Task

Design and implement a “Word of Wisdom” tcp server

• TCP server should be protected from DDOS attacks with the Proof of Work, the challenge-response protocol should be used.
• The choice of the POW algorithm should be explained
• After Prof Of Work verification, server should send one of the quotes from “word of wisdom” book or any other collection of the quotes
• Docker file should be provided both for the server and for the client that solves the POW challenge

Notes

• Libraries like zerolog or zap can be used for nicer logging. Skipped because of testing task goals

How to run

Run server

docker-compose up -d server
// You can set env ADDRESS. Default - 0.0.0.0:8001

Run client

docker-compose up -d client
// You can set env ADDRESS. Default - server:8001

Implementation

• The client establishes a new TCP connection with the server
• The server generates a new challenge(time-based) with a difficulty level of 2
• The server sends the message to the client
• The client parses the challenge and the difficulty
• The client calculates the right nonce to solve it
• The client sends the solution with the challenge
• If nonce is valid, the server sent a quote from a "word of wisdom" book
• If nonce is invalid, the server closes the stream

PoW Algorithm

I used the Hashcash with SHA256 hashing PoW algorithm to set DDoS protection

• It is quick to implement, and many libraries provide SHA256 hashing
• It is easy to check the solution
• It doesn't consume much memory, so even clients with limited memory can use this service
• In this and many cases sha256 is sufficient to protect from DDoS attacks
• Using Hashcash is a common approach to limit spam and provide DDOS protection

Tests and benchmarks can be found in the common/pow_test.rs file

Difficulty(leading zeros)	Average time to solve
1	21.71 µs
2	4.27 ms
3	2.00 s

Problems and possible solutions

• TCP connection keep alive while waiting for solution from clients. It can be the reason of server stop.

Solution: Close current tcp connection to free server resources, GRPC protocol can be used

• Connection multi opening with one challenge in case of closing connection

Solution: Store session of each client, delete challenge after usage