Word of Wisdom solution

Requirements

Design and implement “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 Proof 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.

Overview

Description

• The communication protocol is minimalistic by design. We don't want to uncover things to abuser. -There are a lot of options regarding protection of using the same PoW token. This solution is required to use a client ip as a resource part.
• The server stores challenge resources in a cache, deleting them when a PoW token with the resource is received. It is a tradeoff that keeps us from using a cryptography, or storing a huge amount of received early tokens.

Limitations

• In case of server restarts, all challenges that were received by clients become invalid.
• PoW tokens cache is located in memory, so the server can't scale horizontally (see Further Improvements section below).

How it works

sequenceDiagram

participant C as Tcp Client
participant S as Tcp Server

autonumber

rect rgba(0, 50, 250, .2)
note right of C: Challenge requested
C ->>+ S: connect
S ->>+ S: store_resource_in_cache(resource)
deactivate S
S -->>- C: challenge<resource>
activate C
C ->>+ C: compute_challenge(resource)
deactivate C
end

rect rgba(0, 250, 50, .2)
note right of C: Challenge solved
C ->>- S: request_quote<challenge_payload>
activate S
S ->> S: verify challenge(challenge_payload)
% if step above is succeed:
S ->> S: remove_resource_from_cache(resource)
deactivate S
S -->> C: reply_quote<String>
end

PoW algorithm selection benefits

The hashcash algorithm has been selected because of following reasons:

• Widely Accepted: Hashcash is a well-known and widely adopted proof-of-work protocol. It has been used in various applications, including email systems, to combat spam.
• Computational Resource Intensive: The computational nature of Hashcash puzzles makes it an effective deterrent against DDoS attacks. Solving the puzzle requires significant computational resources, which becomes challenging for attackers attempting to launch large-scale DDoS attacks.
• Adjustable Difficulty: Hashcash allows you to adjust the difficulty level of the proof-of-work puzzle.
• Minimal Client Impact: Hashcash imposes a computational burden on the client by requiring them to solve the proof-of-work puzzle.

Further improvements

This code is provided as a PoC and it could be improved. For instance:

• use the same code style because of currently, a slighlty different ones are used by design.
• store PoW tokens cache in Redis or another persistent storage with auto remove according TTL feature turned on.
• handle errors better (consider transfer some details to a client)
• additinal tests.

Usage

Environment variables

• PORT: used both by client and server
• SERVER_HOST: used by client for connecting to the server

Run locally

📝 Make sure rust and cargo both are installed.

First, run the server instance:

cargo run -p word-of-wisdom-server

and after is started, run the client within a different terminal:

cargo run -p word-of-wisdom-client

Deploy

📝 To detach out of a container push ^P^Q key sequence.

For running inside docker containers, type:

docker network create word-of-wisdom-network
docker build -t word-of-wisdom .

docker run --network word-of-wisdom-network --name word-of-wisdom-server -it -p 44000:44000 -e SERVICE=server word-of-wisdom

docker run --network word-of-wisdom-network -e SERVICE=client -e SERVER_HOST=word-of-wisdom-server word-of-wisdom