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Proof of Work protected TCP server.

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# powtcp

This project is a simple example of [Proof of work (PoW)](https://en.wikipedia.org/wiki/Proof_of_work) protected TCP server. It implements challenge-response protocol and uses [hashcash](https://en.wikipedia.org/wiki/Hashcash) algorithm.

## Messaging

The server and client communicate using an internal messaging protocol. Each message ends with the `\n` character. It's used to separeate messages from each other.

A message consists of a command and a payload. They are separated by the `:` character. The payload can be any string without `\n` character. It's not very convenient in real life, but in this project all payloads are fixed and don't contain `\n` character.

Supported commands:
* `0` - *`Error`* (server -> client);
* `1` - *`RequestPuzzle`* (client -> server);
* `2` - *`ResponsePuzzle`* (server -> client);
* `3` - *`RequestResource`* (client -> server);
* `4` - *`ResponseResource`* (server -> client).

A messaging is implemented in the [`message`](./internal/pkg/lib/message/message.go) package.

## PoW

**PoW** is implemented with a challenge-response protocol:

1. The client establishes a tcp connection with the server. The server starts to listening to client messages.
2. The client sends the *`RequestPuzzle`* command to receive a puzzle from server. 
   
   Message: `1:\n`.
3. The server generates a new puzzle using a hashcash algorithm, stores a puzzle in the cache with some TTL and sends the *`ResponsePuzzle`* command with this puzzle to the client. 
   
   Message: `2:puzzle\n`.
4. The client receives a puzzle and tries to compute a puzzle hash with enough number of zero bits in the beggining. Than the client requests a resource sending a solved puzzle in the *`RequestResource`* command. 
   
   Message: `3:solved-puzzle\n`.
5. The server receives the solved puzzle, checks TTL and sends *`ResponseResource`* command with some resource if that puzzle was solved correctly. 
   
   Message: `4:some-resource\n`.
   
**Implementation**:

* [`hashcash algorithm`](./internal/pkg/lib/hashcash/hashcash.go);
* [`server service`](./internal/pkg/service/service_server.go);
* [`client service`](./internal/pkg/service/service_client.go).

## How To

### Requirements

* [Go 1.21+](https://go.dev/doc/install);
* [Docker](https://docs.docker.com/engine/install/) and [docker-compose](https://docs.docker.com/compose/install/).

### Docker

```bash
# Run client and server just for demo
$ docker-compose up

# Run server listening on 8080 port
$ docker-compose up -d server
```

### Makefile

```bash
$ make help

Usage: make [command]

Commands:

build-server          Build server app
 build-client          Build client app

run-server            Run server app
 run-client            Run client app

test                  Run tests
 fmt                   Format code
```

### Go

```bash
# Build server
$ go build -o ./bin/server ./cmd/server/*.go

# Build client
$ go build -o ./bin/client ./cmd/client/*.go

# Run server
$ ./bin/server

# Run client
$ ./bin/client
```

### Configuration

Server and client applications support configuration from `.yaml` or `.env` files or from environment variables. Applications use [default configuration](./internal/pkg/lib/config/config.go) if a custom configuration not passed.

**Server**

```bash
# Run server passing yaml config file
$ ./bin/server --config config.yaml

# Run server passing env config file
$ ./bin/server --config config.env

# Or run server using environment variables
$ ./bin/server
```

**Client**

```bash
# Run client passing yaml config file
$ ./bin/client --config config.yaml

# Run client passing env config file
$ ./bin/client --config config.env

# Or run client using environment variables
$ ./bin/client
```

**Templates** are available in the [config](./config/) folder.

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