Task for Server Engineer

Design and implement “Word of Wisdom” tcp server

• TCP server should be protected from DDOS attacks with the Prof of Work POW - wiki, 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

---

Requirements

• docker
• go 1.18 (local development)

Build

docker build -t proofofwork .

Run application

docker run -p <port>:8001 proofofwork e.g. docker run -p 9010:8001 proofofwork runs the application on 9010 of the caller system

full example (run in the project directory)
docker build -t proofofwork . && docker run -p 9010:8001 proofofwork

Supported application Args[]

-port=8001 // server application port
-protocol=tcp // server application protocol (tcp is the only available option for now)
-timeout=600 // timeout in seconds before client gets disconnected by server
-clients=100 // number of clients participating in proof of work challenge in pow package integration tests
-log_level=debug // system wide logging level

---

Local development

Main app

go run main.go starts the application on 8001 port

Mock server with clients

cd ./pow && go run main.go

---

Manual testing

Send TCP message

netcat 127.0.0.1 9010 establish connection

then use stubs listed below, protocol mutually json encoded

Message stubs

Challenge controller

{
  "controller": "challenge"
}

Example response

{
  "action": "challenge",
  "message": "c1419d1224dba805efb4c0397db229db747a56ea|bb6f6c336e94819f99a64b8ab3b03161a298be43"
}

Prove controller

Prove suggest must append one of the number encoded in SHA-1, in range [1-10] Use Online sha-1 encode tool for manual testing

{
  "controller": "prove",
  "message": {
    "suggest": "c1419d1224dba805efb4c0397db229db747a56ea|bb6f6c336e94819f99a64b8ab3b03161a298be43|c1419d1224dba805efb4c0397db229db747a56ea"
  }
}

Example success response

{
  "action": "prove",
  "success": true,
  "message": "You could make it a singleton too, but friends don’t let friends create singletons.\" ― Robert Nystrom"
}

Example failed response

{
  "action": "prove",
  "success": false,
  "message": "try again"
}

---

POW Algorithm

• Client ip address registered in the system after client login, server generates random nonce. Client IP address encrypted in SHA-1 in pattern [date|ip-address|nonce]. All parts are sha-1 encoded
• Server sends to client hash but without nonce part [date|ip-address]
• Client prepares header [date|ip-address|nonce] and try to suggest nonce (incrementally from 0)
• Client try to prove [date|ip-address|client_incr_nonce]