word-of-wisdom

README

Word of Wisdom

Demo TCP server protected from DDoS with PoW stateless challenge-response protocol

PoW algorithm

In the terminology of the Hashcash - A Denial of Service Counter-Measure white paper, in this demo server hashcash-cookie algorithm is used. The main advantage over other interactive Hashcash algorithms is it's resistance to DDoS, because server doesn't hold the connection while client is solving the challenge. In order to make this communication secure, challenge is signed by the server. Client passes signature together with the solution, and then server checks that the challange solved is exactly the one, which was signed earlier.

SHA256 is used as an interactive cost function.

HMACSHA256 is used for signing/verifying the challenge on the server.

Important distinctive features of this implementation are:

• Protocol is designed to be stateless, just like HTTP request-response, because the number TCP connections is critical for resisting the DDoS attack
• Challenge includes expiration time, so an attacker's ability to accumulate solutions is limited. Moreover, this nice feature lets the server to dynamically change algorithm parameters and loose support of old challenges by just waiting for them to expire.
• Client IP address is added to the challenge signature by the server, so it is impossible to change it between the challenge-response calls, so an attacker won't be able to accumulate solutions from different locations and spend them all in one location with the purpose of DoS
• It has zero dependencies, and quite low amount of source code: 10 files, <600 non-blank non-comment lines

Running

docker compose up --force-recreate --build server --build client

Known problems and limitations

• Server secret is re-generated on every start, so challenges should not survive server restart, or be used with different servers
• It fails with Go 1.21.0, because of the bug, please use Go 1.21.5 or higher
• Message format is determined by encoding/gob, which is not interoperable format. This is done intentionally to simplify the implementation. It is easily replaceable with a more advanced formatters
• Unit-tests are missing, but solution is testable (e.g. see auto-generated draft here: cmd/server/handler_test.go), so it just waits for some more time to be spent on it