[tor-commits] [torspec/master] Add proposal 307 (onionbalance for v3 onion services)
nickm at torproject.org
nickm at torproject.org
Fri Sep 6 12:02:50 UTC 2019
commit 09611eaf46028f0610824412c547f9450ff81bc5
Author: Nick Mathewson <nickm at torproject.org>
Date: Fri Sep 6 08:02:46 2019 -0400
Add proposal 307 (onionbalance for v3 onion services)
---
proposals/000-index.txt | 2 +
proposals/307-onionbalance-v3.txt | 180 ++++++++++++++++++++++++++++++++++++++
2 files changed, 182 insertions(+)
diff --git a/proposals/000-index.txt b/proposals/000-index.txt
index 2c3c620..69ae099 100644
--- a/proposals/000-index.txt
+++ b/proposals/000-index.txt
@@ -227,6 +227,7 @@ Proposals by number:
304 Extending SOCKS5 Onion Service Error Codes [ACCEPTED]
305 ESTABLISH_INTRO Cell DoS Defense Extension [DRAFT]
306 A Tor Implementation of IPv6 Happy Eyeballs [OPEN]
+307 Onion Balance Support for Onion Service v3 [DRAFT]
Proposals by status:
@@ -238,6 +239,7 @@ Proposals by status:
300 Walking Onions: Scaling and Saving Bandwidth
303 When and how to remove support for protocol versions
305 ESTABLISH_INTRO Cell DoS Defense Extension
+ 307 Onion Balance Support for Onion Service v3
NEEDS-REVISION:
212 Increase Acceptable Consensus Age [for 0.2.4.x+]
219 Support for full DNS and DNSSEC resolution in Tor [for 0.2.5.x]
diff --git a/proposals/307-onionbalance-v3.txt b/proposals/307-onionbalance-v3.txt
new file mode 100644
index 0000000..8716f13
--- /dev/null
+++ b/proposals/307-onionbalance-v3.txt
@@ -0,0 +1,180 @@
+Filename: 307-onionbalance-v3.txt
+Title: Onion Balance Support for Onion Service v3
+Author: Nick Mathewson
+Created: 03-April-2019
+Status: Draft
+
+0. Draft Notes
+
+ 2019-07-25:
+
+ At this point in time, the cross-certification is not implemented
+ correctly in >= tor-0.3.2.1-alpha. See https://trac.torproject.org/29583
+ for more details.
+
+ This proposal assumes that this bug is fixed.
+
+1. Introduction
+
+ The OnionBalance tool allows several independent Tor instances to host an
+ onion service, while clients can access that onion service without having
+ to take its distributed status into account. OnionBalance works by having
+ each instance run a separate onion service. Then, a management server
+ periodically downloads the descriptors from those onion services, and
+ generates a new descriptor containing the introduction points from each
+ instance's onion service.
+
+ OnionBalance is used by several high-profile onion services, including
+ Facebook and The Tor Project.
+
+ Unfortunately, because of the cross-certification features in v3 onion
+ services, OnionBalance no longer works for them. To a certain extent, this
+ breakage is because of a security improvement: It's probably a good thing
+ that random third parties can no longer grab a onion service's introduction
+ points and claim that they are introduction points for a different service.
+ But nonetheless, a lack of a working OnionBalance remains an obstacle for
+ v3 onion service migration.
+
+ This proposal describes extensions to v3 onion service design to
+ accommodate OnionBalance.
+
+2. Background and Solution
+
+ If an OnionBalance management server wants to provide an aggregate
+ descriptor for a v3 onion service, it faces several obstacles that it
+ didn't have in v2.
+
+ When the management server goes to construct an aggregated descriptor, it
+ will have a mismatch on the "auth-key", "enc-key-cert", and
+ "legacy-key-cert" fields: these fields are supposed to certify the onion
+ service's current descriptor-signing key, but each of these keys will be
+ generated independently by each instance. Because they won't match each
+ other, there is no possible key that the aggregated descriptor could use
+ for its descriptor signing key.
+
+ In this design, we require that each instance should know in advance about
+ a descriptor-signing public key that the aggregate descriptor will use for
+ each time period. (I'll explain how they can do this later, in section 3
+ below.) They don't have to know the corresponding private key.
+
+ When generating their own onion service descriptors for a given time
+ period, the instances generate these additional fields to be used for the
+ aggregate descriptor:
+
+ "meta-auth-key"
+ "meta-enc-key-cert"
+ "meta-legacy-key-cert"
+
+ These fields correspond to "auth-key", "enc-key-cert", and
+ "legacy-key-cert" respectively, but differ in one regard: the
+ descriptor-signing public key that they certify is _not_ the instance's own
+ descriptor-signing key, but rather the aggregate public key for the time
+ period.
+
+ Ordinary clients ignore these new fields.
+
+ When the management server creates the aggregate descriptor, it checks that
+ the signing key for each of these "meta" fields matches the signing key for
+ its corresponding non-"meta" field, and that they certify the correct
+ descriptor-signing key-- and then uses these fields in place of their
+ corresponding non-"meta" variants.
+
+2.1. A quick note on synchronization
+
+ In the design above, and in the section below, I frequently refer to "the
+ current time period". By this, I mean the time period for which the
+ descriptor is encoded, not the time period in which it is generated.
+
+ Instances and management servers should generate descriptors for the two
+ closest time periods, as they do today: no additional synchronization
+ should needed here.
+
+3. How to distribute descriptor-signing keys
+
+ The design requires that every instance of the onion service knows about
+ the public descriptor-signing key that will be used for the aggregate onion
+ service. Here I'll discuss how this can be achieved.
+
+3.1. If the instances are trusted.
+
+ If the management server trusts each of the instances, it can distribute a
+ shared secret to each one of them, and use this shared secret to derive
+ each time period's private key.
+
+ For example, if the shared secret is SK, then the private descriptor-
+ signing key for each time period could be derived as:
+
+ H("meta-descriptor-signing-key-deriv" |
+ onion_service_identity
+ INT_8(period_num) |
+ INT_8(period_length) |
+ SK )
+
+ (Remember that in the terminology of rend-spec-v3, INT_8() denotes a 64-bit
+ integer, see section 0.2 in rend-spec-v3.txt.)
+
+ If shared secret is ever compromised, then an attacker can impersonate the
+ onion service until the shared secret is changed, and can correlate all
+ past descriptors for the onion service.
+
+3.2. If the instances are not trusted: Option One
+
+ If the management server does not trust the instances with
+ descriptor-signing public keys, another option for it is to simply
+ distribute a load of public keys in advance, and use them according to a
+ schedule.
+
+ In this design, the management server would pre-generate the
+ "descriptor-signing-key-cert" fields for a long time in advance, and
+ distribute them to the instances offline. Each one would be
+ associated with its corresponding time period.
+
+ If these certificates were revealed to an attacker, the attacker
+ could correlate descriptors for the onion service with one another,
+ but could not impersonate the service.
+
+3.3. If the instances are not trusted: Option Two
+
+ Another option for the trust model of 3.2 above is to use the same
+ key-blinding method as used for v3 onion services. The management server
+ would hold a private descriptor-signing key, and use it to derive a
+ different private descriptor-signing key for each time period. The instance
+ servers would hold the corresponding public key, and use it to derive a
+ different public descriptor-signing key for each time period.
+
+ (For security, the key-blinding function in this case should use a
+ different nonce than used in the)
+
+ This design would allow the instances to only be configured once, which
+ would be simpler than 3.2 above-- but at a cost. The management server's
+ use of a long-term private descriptor-signing key would require it to keep
+ that key online. (It could keep the derived private descriptor-signing keys
+ online, but the parent key could be derived from them.)
+
+ Here, if the instance's knowledge were revealed to an attack, the attacker
+ could correlate descriptors for the onion service with one another, but
+ could not impersonate the service.
+
+4. Some features of this proposal
+
+ We retain the property that each instance service remains accessible as a
+ working onion service. However, anyone who can access it can identify it as
+ an instance of an OnionBalance service, and correlate its descriptor to the
+ aggregate descriptor.
+
+ Instances could use client authorization to ensure that only the management
+ server can decrypt their introduction points. However, because of the
+ key-blinding features of v3 onion services, nobody who doesn't know the
+ onion addresses for the instances can access them anyway: It would be
+ sufficient to keep these addresses secret.
+
+ Although anybody who successfully accesses an instance can correlate its
+ descriptor to the meta-descriptor, this only works for two descriptors
+ within a single time period: You can't match an instance descriptor from
+ one time period to a meta-descriptor from another.
+
+A. Acknowledgments
+
+ Thanks to the network team for helping me clarify my ideas here, explore
+ options, and better understand some of the implementations and challenges
+ in this problem space.
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