Hi,
Some time ago, like 1,5 years, this list received a proposal for how to build Tor Consensus Transparency. I got some good feedback and then i got busy with other things.
Here's an updated version with feedback incorporated, change in wordings, more of an actual specification, some changes in data structures and over all an easier read, I hope.
I'll follow up shortly with a separate email describing the status of an experimental implementation.
--8<---------------cut here---------------start------------->8--- Filename: xxx-tor-consensus-transparency.txt Title: Tor Consensus Transparency Author: Linus Nordberg Created: 2014-06-28 Status: Draft
0. Introduction
This document describes how to provide and use public, append-only, verifiable logs containing Tor consensus and vote status documents, much like what Certificate Transparency [CT] does for TLS certificates, making it possible for log monitors to detect false consensuses and votes.
Tor clients and relays can refuse using a consensus not present in a set of logs of their choosing, as well as provide possible evidence of misissuance by submitting such a consensus to any number of logs.
1. Overview
Tor status documents, consensuses as well as votes, are stored in one or more public, append-only, externally verifiable log using a history tree like the one described in [CrosbyWallach].
Consensus-users, i.e. Tor clients and relays, expect to receive one or more "proof of inclusions" with new consensus documents. A proof of inclusion is a hash sum representing the tree head of a log, signed by the logs private key, and an audit path listing the nodes in the tree needed to recreate the tree head. Consensus-users are configured to use one or more logs by listing a log address and a public key for each log. This is enough for verifying that a given consensus document is present in a given log.
Submission of status documents to a log can be done by anyone with an internet connection (and the Tor network, in case of logs only on a .onion address). The submitter gets a signed tree head and a proof of inclusion in return. Directory authorities are expected to submit to one or more logs and include the proofs when serving consensus documents. Directory caches and consensus-users receiving a consensus not including a proof of inclusion may submit the document and use the proof they receive in return.
Auditing log behaviour and monitoring the contents of logs is performed in cooperation between the Tor network and external services. Relays act as log auditors with help from Tor clients gossiping about what they see. Directory authorities are good candidates for monitoring log content since they know what votes they have sent and received as well as what consensus documents they have issued. Anybody can run both an auditor and a monitor though, which is an important property of the proposed system.
2. Motivation
Popping a handful of boxes (currently five) or factoring the same number of RSA keys should not be ruled out as a possible attack against a subset of Tor users. An attacker controlling a majority of the directory authorities signing keys can, using man-in-the-middle or man-on-the-side attacks, serve consensus documents listing relays under their control. If mounted on a small subset of Tor users on the internet, the chance of detection is probably low. Implementation of this proposal increases the cost for such an attack by raising the chances of it being detected.
Note that while the proposed solution gives each individual some degree of protection against using a false consensus this is not the primary goal but more of a nice side effect. The primary goal is to detect correctly signed consensus documents which differ from the consensus of the directory authoritites. This raises the risk of exposure of an attacker capable of producing a consensus and feed it to users.
The complexity of the proposed solution is motivated by the fact that the log key is not just another key on top of the directory authority keys since the log doesn't have to be trusted. Another value is the decentralisation given -- anybody can run their own log and use it. Anybody can audit all existing logs and verify their correct behaviour. This empowers people outside the group of Tor directory authority operators and the people who trust them for one reason or the other.
3. Design
Communication with logs is done over HTTP using TLS or Tor onion services for transport, similar to what is defined in [rfc6962-bis-12]. Parameters for POSTs and all responses are encoded as name/value pairs in JSON objects [RFC4627].
Summary of proposed changes to Tor:
- Configuration is added for listing known logs and for describing policy for using them.
- Directory authorities start submitting newly created consensuses to at least one public log.
- Tor clients and relays receiving a consensus not accompanied by a proof of inclusion start submitting that consensus to at least one public log.
- Consensus-users start rejecting consensuses accompanied by an invalid proof of inclusion.
- A new cell type LOG_STH is defined, for clients and relays to exchange information about seen tree heads and their validity.
- Consensus-users send seen tree heads to relays acting as log auditors.
- Relays acting as log auditors validate tree heads (section 3.2.2) received from consensus-users and send results back.
- Consensus-users start rejecting consensuses for which valid proofs of inclusion can not be obtained.
Definitions:
- Log id: The SHA-256 hash of the log's public key, to be treated as an opaque byte string identifying the log.
3.1. Consensus submission
Logs accept consensus submissions from anyone as long as the consensus is signed by a majority of the Tor directory authorities of the Tor network that it's logging.
Consensus documents are POST:ed to a well-known URL as defined in section 5.2.
The output is what we call a proof of inclusion.
3.2. Verification
3.2.1. Log entry membership verification
Calculate a tree head from the hash of the received consensus and the audit path in the accompanying proof. Verify that the calculated tree head is identical to the tree head in the proof. This can easily be done by consensus-users for each received consensus.
We now know that the consensus is part of a tree which the log claims to be The Tree. Whether this tree is the same tree that everybody else see is unknown at this point.
3.2.2. Log consistency verification
Ask the log for a consistency proof between the tree head to verify and a previously known good tree head from the pool. Section 5.3 specifies how to fetch a consistency proof.
[[TBD require auditors to fetch and store the tree head for the empty tree as part of bootstrapping, in order to avoid the case where there's no older tree to verify against?]]
[[TODO description of verification of consistency goes here]]
Relays acting as auditors cache results to minimise calculations and communication with log servers.
[[TBD have clients verify consistency as well? NOTE: we still want relays to see tree heads in order to catch a lying log (the split-view attack)]]
We now know that the verified tree is a superset of a known good tree.
3.3. Log auditing
A log auditor verifies two things:
- A logs append-only property, i.e. that no entries once accepted by a log are ever altered or removed.
- That a log presents the same view to all of its users [[TODO describe the Tor networks role in auditing more than what's found in section 3.2.2]]
A log auditor typically doesn't care about the contents of the log entries, other than calculating their hash sums for auditing purposes.
Tor relays should act as log auditors.
3.4. Log monitoring
A log monitor downloads and investigates each entry in a log searching for anomalies according to its monitoring policy.
This document doesn't define monitoring policies but does outline a few strategies for monitoring in section [[TBD]].
Note that there can be more than one valid consensus documents for a given point in time. One reason for this is that the number of signatures can differ due to consensus voting timing details. [[TODO Are there more reasons?]]
[[TODO expand on monitoring strategies -- even if this is not part of the proposed extensions to the Tor network it's good for understanding. a) dirauths can verify consensus documents byte for byte; b) anyone can look for diffs larger than D per time T, where "diffs" certainly can be smarter than a plain text diff]]
3.5. Consensus-user behaviour
[[TODO move most of this to section 5]]
Keep an on-disk cache of consensus documents. Mark them as being in one of three states:
LOG_STATE_UNKNOWN -- don't know whether it's present in enough logs or not LOG_STATE_LOGGED -- have seen good proof(s) of inclusion LOG_STATE_LOGGED_GOOD -- confident about the tree head representing a good tree
Newly arrived consensus documents start in UNKNOWN or LOGGED depending on whether they are accompanied by enough proofs or not. There are two possible state transitions:
- UNKNOWN --> LOGGED: When enough correctly verifying proofs of inclusion (section 3.2.1) have been seen. The number of good proofs required is a policy setting in the configuration of the consensus-user.
- LOGGED --> LOGGED_GOOD: When the tree head in enough of the inclusion proofs have been verified (section 3.2.2) or enough LOG_STH cells vouching for the same tree heads have been seen. The number of verifications required is a policy setting in the configuration of the consensus-user.
Consensuses in state UNKNOWN are not used but are instead submitted to one or more logs. If the submission succeeds, this will take the consensus to state LOGGED.
Consensuses in state LOGGED are used despite not being fully verified with regard to logging. LOG_STH cells containing tree heads from received proofs are being sent to relays for verification. Clients send to all relays that they have a circuit to, i.e. their guard relay(s). Relays send to three random relays that they have a circuit to.
3.6. Relay behaviour when acting as an auditor
In order to verify the append-only property of a log, relays acting as log auditors verify the consistency of tree heads received in LOG_STH cells. An auditor keeps a copy of 2+N known good tree heads in a pool stored on persistent media [[TBD where N is either a fixed number in the range 32-128 or is a function of the log size]]. Two of them are the oldest and newest tree heads seen, respectively. The rest, N, are randomly chosen from the tree heads seen.
[[TODO describe or refer to an algorithm for "randomly chosen", hopefully not subjective to flushing attacks (or other attacks)]].
3.7. Notable differences from Certificate Transparency
- The data logged is "strictly time-stamped", i.e. ordered.
- Much shorter lifetime of logged data -- a day rather than a year. Is the effects of this difference of importance only for "one-shot attacks"?
- Directory authorities have consensus about what they're signing -- there are no "web sites knowing better".
- Submitters are not in the same hurry as CA:s and can wait minutes rather than seconds for a proof of inclusion.
4. Security implications
TODO
5. Specification
5.0. Data structures
Data structures are defined as described in [RFC5246] section 4, i.e. TLS 1.2 presentation language. While it is tempting to try to avoid yet another format, the cost of redefining the data structures in [rfc6962-bis-12] outweighs this consideration. The burden of redefining, reimplementing and testing is extra true for those structures which need precise definitions because they are to be signed.
5.1. Signed Tree Head (STH)
An STH is a TransItem structure of type "signed_tree_head" as defined in [rfc6962-bis-12] section 5.8.
5.2. Submitting a consensus document to a log
POST https://<log server>/tct/v1/add-consensus
Input:
consensus: A consensus status document as defined in [dir-spec] section 3.4.1 [[TBD gziped and base64 encoded to save 50%?]]
Output:
sth: A signed tree head as defined in section 5.1 refering to a tree in which the submitted document is included.
inclusion: An inclusion proof as specified for the "inclusion" output in [rfc6962-bis-12] section 6.5.
5.3. Getting a consistency proof from a log
GET https://<log server>/tct/v1/get-sth-consistency
Input and output as specified in [rfc6962-bis-12] section 6.4.
5.x. LOG_STH cells
A LOG_STH cell is a variable-length cell with the following fields:
TBDname [TBD octets] TBDname [TBD octets] TBDname [TBD octets]
6. Compatibility
TBD
7. Implementation
TBD
8. Performance and scalability notes
TBD
A. Open issues / TODOs
- TODO: Add SCTs from CT, at least as a practical "cookie" (i.e. no need to send them around or include them anywhere). Logs should be given more time for distributing than we're willing to wait on an HTTP response for.
- TODO: explain why no hash function and signing algorithm agility, [[rfc6962-bis-12] section 10
- TODO: add a blurb about the values of publishing logs as onion services
- TODO: discuss compromise of log keys
B. Acknowledgements
This proposal leans heavily on [rfc6962-bis-12]. Some definitions are copied verbatim from that document. Valuable feedback has been received from Ben Laurie, Karsten Loesing and Ximin Luo.
C. References
[CrosbyWallach] http://static.usenix.org/event/sec09/tech/full_papers/crosby.pdf [dir-spec] https://gitweb.torproject.org/torspec.git/blob/HEAD:/dir-spec.txt [RFC4627] https://tools.ietf.org/html/rfc4627 [rfc6962-bis-12] https://datatracker.ietf.org/doc/draft-ietf-trans-rfc6962-bis/12 [CT] https://https://www.certificate-transparency.org/ --8<---------------cut here---------------end--------------->8---