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Re: [tor-talk] Revoking a hidden service key



A client receiving a revocation descriptor would want to remember not to trust that hidden service for as long as possible, so there's still going to be long-term storage somewhere in the chain. Putting it in the directories would mean that as many client as possible could be notified of the hidden service's revocation, even long after the initial revocation is published, in cases where the hidden service operator is unwilling or unable to continue to announce the revocation.

Consider that for long-validity revocations, there would actually be less load placed on the network than for a regular short term descriptor. The hidden service would not need to frequently publish a new descriptor about itself. Once a client knows a hidden service is revoked, they do not need to ask about it again. Old revocations could conceivably be stored to disk.

The need to revoke hidden service keys is real. It doesn't take long to dig up anecdotes and news reports of .onion sites that have been compromised, but even when detected there is no reliable way for a legitimate hidden service operator to notify the network his service cannot be trusted. Detecting if someone has stolen your hidden service key is easy and is hijacking your traffic is easy, you only have to look out for hidden service descriptors for your service that you did not publish, but there is currently not much that can be done with this information. The hidden service operator could include a notice on his hidden website warning of the compromise and telling users to divert to a different .onion address, but a user has no way of knowing if that warning was published by the attacker and directs to another malicious site.

On 2015-03-03 5:19 AM, Donncha O'Cearbhaill wrote:
Alternatively the original hidden service operator could publish hidden
service descriptors with a normal validity period which contain a
revocation field. A HSDir which receives a descriptor containing the
revocation could replace the (potentially malicious) HS descriptor
stored in its cache.

A client could be show an alert that the hidden service they are
attempting to access has been compromised/revoked and should not be used
in future. A HS operator would then keep broadcasting the revocation
descriptor until such time that all clients are likely to have been
notified. This kind of replacement approach would allow revocation
without placing any more load or memory demands on the network.

In practice do HS operators have a need to revoke hidden service keys?

On 03/03/15 03:05, Adrien Johnson wrote:
An solution might be to allow hidden service revocation descriptors to
expire after a long time, and to be updated by the hidden service
operator, but only as a new revocation descriptor which has a later
expiration date. That way the Tor network can eventually forget about
revoked hidden services which are no longer used and where the operator
no longer feels there is a threat of impersonation.

On 2015-03-02 9:50 PM, Max Bond wrote:
It seems like the only way this scheme could work is if the directories
remembered which services had issued revocations, making compromises
expensive for the whole network and opening the door for
denial-of-service
attacks that effect hidden services as a whole.

I would counter propose that you set up a Twitter account which tweets
about the status of your hidden service, where you could make an
emergency
announcement. Perhaps you could have a passcode required to enter the
site
that changes on a daily basis and is announced from twitter, so that your
users get in the habit of checking twitter before logging in to your
site.

On Mon, Mar 2, 2015 at 6:40 PM, Adrien Johnson <adrienj@adrienj.com>
wrote:

Deleting your key and taking down your service would prevent further
compromise of your system, but if your private key was already
stolen, it
wouldn't stop an attacker from continuing to announce your key and
running
an imposter service.

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