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Can't governments who want to censor Tor just block the directory authorities? Or does the client reach the directory authorities to fetch the consensus only after building a circuit so that we have {user}<->{guard}<->{middle}<->{exit}<->{directory-authority}? And if so, how can he make a circuit without the consensus?

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When a normal client first starts it fetches the directory information from a directory authority (or a fall-back). Since it does not know any relays it has no choice but to fetch them directly. At that point a censor could indeed stop the connection being made. However a censor can also just download the directory information and block all relays, which would include the directory authorities and the fall-backs.

This is why censored users use bridges. A user uses the bridge as a directory guard: they fetch the directory information from the bridge, not directly from the directory authorities or fall-backs. They also fetch it over an encrypted connection, not plain-text to avoid the contents of the directory request being detectable.

Any user who might be censored should be using bridges and preferably also some form of pluggable transport.

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