How does misconfigured time affect Tor?

Question

Tor users and Tor relays across the world have different time zones.

How does misconfigured time affects Tor operation either on client or server side? Is there a time skew margin in which Tor still operates well ? Could a misconfigured time zone in client distinguish her from other Tor users?

Answer 1

• Denial of Service: Tor will be unable to connect, if the system clock is more than 1 hour in the past or more than 3 hour in the future. (ref) Also hidden services can only be accessed if the clock is no more than 30 minutes ahead or behind. (ref)
• Tracking: Probably it is possible for guards (or bridges or anybody else who can see the initial connection) to probabilistically track a client with a skewed clock even as it changes IPs. (ref) (A fix is in the works.)
• Fingerprinting: Destination servers (also web tracking services such as google analytics) could reduce anonymity to pseudonymity when the clock skew is unique. (ref) (A fix for TBB might be in the works, I am unsure about other applications.)
• De-anonymiziation: An ISP watching the clock skew on one end - for example by checking TLS timestamps of misc applications such as software updaters - and cooperating with a destination server on the other end, can make educated guesses, when the clock skew is significant enough. Since most systems use unauthenticated network time synchronization (ntp), it might even be possible for an ISP to actively introduce the skew. I am not aware of any research papers, that explore how much clock skew is required to pull off this attack. (ref)