Tor 的 Onion 導入防禦機制,在遭受 DoS 的時候要求用戶端執行 PoW 任務

在「Introducing Proof-of-Work Defense for Onion Services」這邊看到 0.4.8 的新機制,當 Onion 服務受到 DoS 時,會需要 client 提供 PoW 證明,有證明的會優先處理:

Tor's PoW defense is a dynamic and reactive mechanism, remaining dormant under normal use conditions to ensure a seamless user experience, but when an onion service is under stress, the mechanism will prompt incoming client connections to perform a number of successively more complex operations. The onion service will then prioritize these connections based on the effort level demonstrated by the client.

主要原因是傳統遇到 DoS 時可以透過 IP address 之類的資訊設計阻擋機制,但在 Onion 服務裡面沒有這個資訊,所以需要其他方式阻擋:

The inherent design of onion services, which prioritizes user privacy by obfuscating IP addresses, has made it vulnerable to DoS attacks and traditional IP-based rate limits have been imperfect protections in these scenarios. In need of alternative solutions, we devised a proof-of-work mechanism involving a client puzzle to thwart DoS attacks without compromising user privacy.

這個 PoW 機制的說明可以在「torspec/proposals/327-pow-over-intro.txt」這邊看到,看起來是三年前 (2020/04/02) 就提出來了,直到 0.4.8 才推出。

裡面有提到 PoW 的演算法是用 Equi-X

For our proof-of-work function we will use the Equi-X scheme by tevador [REF_EQUIX].

看起來是個方法,而且從 cryptocurrency 後大家對 PoW 的用法愈來愈熟悉了,在這邊用還不錯...

聯邦政府僱用 CMU 的人研究攻擊 Tor

先前 Tor Project 有提到外面報導 FBI 在 2014 年時僱用 CMU 的人研究 Tor 的弱點並且加以攻擊:「Did the FBI Pay a University to Attack Tor Users?」。

這的確是錯的,因為不是 FBI,而是 DoD (美國國防部) 僱用的 (瞬間噴出茶來):「Judge confirms what many suspected: Feds hired CMU to break Tor」。Tor Project 的反應在「Statement from the Tor Project re: the Court's February 23 Order in U.S. v. Farrell」這邊。

A federal judge in Washington has now confirmed what has been strongly suspected: that Carnegie Mellon University (CMU) researchers at its Software Engineering Institute were hired by the federal government to do research into breaking Tor in 2014.