Argh, I'm really sorry, I thought I'd reached the end of the proposal
but my questions were addressed further down. Sorry for the noise.
Cheers,
Michael
On 05/02/2019 17:42, Michael Rogers wrote:
> I'm very happy to see this proposal! Two quick questions about relay
> selection:
>
> * Can a client specify that it wants an exit node whose policy allows
> something unusual, e.g. exiting to a port that's not allowed by the
> default policy? If not, does the client need to keep picking exit nodes
> until it gets a SNIP with a suitable policy?
>
> * Similarly, if a client has restrictions on the guard nodes it can
> connect to (fascist firewall or IPv4/v6 restrictions, for example), does
> it need to keep picking guards via the directory fallback circuit until
> it gets a suitable one?
>
> In both cases, perhaps a client with unusual requirements could first
> download the consensus, find a relay matching its requirements, then
> send that relay's index in its extend cell, so the relay receiving the
> extend cell wouldn't know whether the index was picked randomly by a
> client with no special requirements, or non-randomly by a client with
> special requirements?
>
> I think this would allow the majority of clients to save bandwidth by
> not downloading the consensus, without allowing relays to distinguish
> the minority of clients with unusual exit/guard requirements. (The
> presence of the full consensus on disk would indicate that the client
> had unusual exit/guard requirements at some point, however.)
>
> Cheers,
> Michael
>
> On 05/02/2019 17:02, Nick Mathewson wrote:
>> Filename: 300-walking-onions.txt
>> Title: Walking Onions: Scaling and Saving Bandwidth
>> Author: Nick Mathewson
>> Created: 5-Feb-2019
>> Status: Draft
>>
>> 0. Status
>>
>> This proposal describes a mechanism called "Walking Onions" for
>> scaling the Tor network and reducing the amount of client bandwidth
>> used to maintain a client's view of the Tor network.
>>
>> This is a draft proposal; there are problems left to be solved and
>> questions left to be answered. Once those parts are done, we can
>> fill in section 4 with the final details of the design.
>>
>> 1. Introduction
>>
>> In the current Tor network design, we assume that every client has a
>> complete view of all the relays in the network. To achieve this,
>> clients download consensus directories at regular intervals, and
>> download descriptors for every relay listed in the directory.
>>
>> The substitution of microdescriptors for regular descriptors
>> (proposal 158) and the use of consensus diffs (proposal 140) have
>> lowered the bytes that clients must dedicate to directory operations.
>> But we still face the problem that, if we force each client to know
>> about every relay in the network, each client's directory traffic
>> will grow linearly with the number of relays in the network.
>>
>> Another drawback in our current system is that client directory
>> traffic is front-loaded: clients need to fetch an entire directory
>> before they begin building circuits. This places extra delays on
>> clients, and extra load on the network.
>>
>> To anonymize the world, we will need to scale to a much larger number
>> of relays and clients: requiring clients to know about every relay in
>> the set simply won't scale, and requiring every new client to download
>> a large document is also problematic.
>>
>> There are obvious responses here, and some other anonymity tools have
>> taken them. It's possible to have a client only use a fraction of
>> the relays in a network--but doing so opens the client to _epistemic
>> attacks_, in which the difference in clients' views of the
>> network is used to partition their traffic. It's also possible to
>> move the problem of selecting relays from the client to the relays
>> themselves, and let each relay select the next relay in turn--but
>> this choice opens the client to _route capture attacks_, in which a
>> malicious relay selects only other malicious relays.
>>
>> In this proposal, I'll describe a design for eliminating up-front
>> client directory downloads. Clients still choose relays at random,
>> but without ever having to hold a list of all the relays. This design
>> does not require clients to trust relays any more than they do today,
>> or open clients to epistemic attacks.
>>
>> I hope to maintain feature parity with the current Tor design; I'll
>> list the places in which I haven't figured out how to do so yet.
>>
>> I'm naming this design "walking onions". The walking onion (Allium x
>> proliferum) reproduces by growing tiny little bulbs at the
>> end of a long stalk. When the stalk gets too top-heavy, it flops
>> over, and the little bulbs start growing somewhere new.
>>
>> The rest of this document will run as follows. In section 2, I'll
>> explain the ideas behind the "walking onions" design, and how they
>> can eliminate the need for regular directory downloads. In section 3, I'll
>> answer a number of follow-up questions that arise, and explain how to
>> keep various features in Tor working. Section 4 (not yet written)
>> will elaborate all the details needed to turn this proposal into a
>> concrete set of specification changes.
>>
>> 2. Overview
>>
>> 2.1. Recapping proposal 141
>>
>> Back in Proposal 141 ("Download server descriptors on demand"), Peter
>> Palfrader proposed an idea for eliminating ahead-of-time descriptor
>> downloads. Instead of fetching all the descriptors in advance, a
>> client would fetch the descriptor for each relay in its path right
>> before extending the circuit to that relay. For example, if a client
>> has a circuit from A->B and wants to extend the circuit to C, the
>> client asks B for C's descriptor, and then extends the circuit to C.
>>
>> (Note that the client needs to fetch the descriptor every time it
>> extends the circuit, so that an observer can't tell whether the
>> client already had the descriptor or not.)
>>
>> There are a couple of limitations for this design:
>> * It still requires clients to download a consensus.
>> * It introduces a extra round-trip to each hop of the circuit
>> extension process.
>>
>> I'll show how to solve these problems in the two sections below.
>>
>> 2.2. An observation about the ntor handshake.
>>
>> I'll start with an observation about our current circuit extension
>> handshake, ntor: it should not actually be necessary to know a
>> relay's onion key before extending to it.
>>
>> Right now, the client sends:
>> NODEID (The relay's identity)
>> KEYID (The relay's public onion key)
>> CLIENT_PK (a diffie-hellman public key)
>>
>> and the relay responds with:
>> SERVER_PK (a diffie-hellman public key)
>> AUTH (a function of the relay's private keys and
>> *all* of the public keys.)
>>
>> Both parties generate shared symmetric keys from the same inputs
>> that are are used to create the AUTH value.
>>
>> The important insight here is that we could easily change
>> this handshake so that the client sends only CLIENT_PK, and receives
>> NODEID and KEYID as part of the response.
>>
>> In other words, the client needs to know the relay's onion key to
>> _complete_ the handshake, but doesn't actually need to know the
>> relay's onion key in order to _initiate_ the handshake.
>>
>> This is the insight that will let us save a round trip: When the
>> client goes to extend a circuit from A->B to C, it can send B a
>> request to extend to C and retrieve C's descriptor in a single step.
>> Specifically, the client sends only CLIENT_PK, and relay B can include C's
>> keys as part of the EXTENDED cell.
>>
>> 2.3. Extending by certified index
>>
>> Now I'll explain how the client can avoid having to download a
>> list of relays entirely.
>>
>> First, let's look at how a client chooses a random relay today.
>> First, the client puts all of the relays in a list, and computes a
>> weighted bandwidth for each one. For example, suppose the relay
>> identities are R1, R2, R3, R4, and R5, and their bandwidth weights
>> are 50, 40, 30, 20, and 10. The client makes a table like this:
>>
>> Relay Weight Range of index values
>> R1 50 0..49
>> R2 40 50..89
>> R3 30 90..119
>> R4 20 120..139
>> R5 10 140..149
>>
>> To choose a random relay, the client picks a random index value
>> between 0 and 149 inclusive, and looks up the corresponding relay in
>> the table. For example, if the client's random number is 77, it will
>> choose R2. If its random number is 137, it chooses R4.
>>
>> The key observation for the "walking onions" design is that the
>> client doesn't actually need to construct this table itself.
>> Instead, we will have this table be constructed by the authorities
>> and distributed to all the relays.
>>
>> Here's how it works: let's have the authorities make a new kind of
>> consensus-like thing. We'll call it an Efficient Network Directory
>> with Individually Verifiable Entries, or "ENDIVE" for short. This
>> will differ from the client's index table above in two ways. First,
>> every entry in the ENDIVE is normalized so that the bandwidth
>> weights maximum index is 2^32-1:
>>
>> Relay Normalized weight Range of index values
>> R1 0x55555546 0x00000000..0x55555545
>> R2 0x44444438 0x55555546..0x9999997d
>> R3 0x3333332a 0x9999997e..0xcccccca7
>> R4 0x2222221c 0xcccccca8..0xeeeeeec3
>> R5 0x1111113c 0xeeeeeec4..0xffffffff
>>
>> Second, every entry in the ENDIVE is timestamped and signed by the
>> authorities independently, so that when a client sees a line from the
>> table above, it can verify that it came from an authentic ENDIVE.
>> When a client has chosen a random index, one of these entries will
>> prove to the client that a given relay corresponds to that index.
>> Because of this property, we'll be calling these entries "Separable
>> Network Index Proofs", or "SNIP"s for short.
>>
>> For example, a single SNIP from the table above might consist of:
>> * A range of times during which this SNIP is valid
>> * R1's identity
>> * R1's ntor onion key
>> * R1's address
>> * The index range 0x00000000..0x55555545
>> * A signature of all of the above, by a number of authorities
>>
>> Let's put it together. Suppose that the client has a circuit from
>> A->B, and it wants to extend to a random relay, chosen randomly
>> weighted by bandwidth.
>>
>> 1. The client picks a random index value between 0 and 2**32 - 1. It
>> sends that index to relay B in its EXTEND cell, along with a
>> g^x value for the ntor handshake.
>>
>> Note: the client doesn't send an address or identity for the next
>> relay, since it doesn't know what relay it has chosen! (The
>> combination of an index and a g^x value is what I'm calling a
>> "walking onion".)
>>
>> 2. Now, relay B looks up the index in its most recent ENDIVE, to
>> learn which relay the client selected.
>>
>> (For example, suppose that the client's random index value is
>> 0x50000001. This index value falls between 0x00000000 and
>> 0x55555546 in the table above, so the relay B sees that the client
>> has chosen R1 as its next hop.)
>>
>> 3. Relay B sends a create cell to R1 as usual. When it gets a
>> CREATED reply, it includes the authority-signed SNIP for
>> R1 as part of the EXTENDED cell.
>>
>> 4. As part of verifying the handshake, the client verifies that the
>> SNIP was signed by enough authorities, that its timestamp
>> is recent enough, and that it actually corresponds to the
>> random index that the client selected.
>>
>> Notice the properties we have with this design:
>>
>> - Clients can extend circuits without having a list of all the
>> relays.
>>
>> - Because the client's random index needs to match a routing
>> entry signed by the authorities, the client is still selecting
>> a relay randomly by weight. A hostile relay cannot choose
>> which relay to send the client.
>>
>>
>> On a failure to extend, a relay should still report the routing entry
>> for the other relay that it couldn't connect to. As before, a client
>> will start a new circuit if a partially constructed circuit is a
>> partial failure.
>>
>>
>> We could achieve a reliability/security tradeoff by letting clients
>> offer the relay a choice of two or more indices to extend to.
>> This would help reliability, but give the relay more influence over
>> the path. We'd need to analyze this impact.
>>
>>
>> In the next section, I'll discuss a bunch of details that we need to
>> straighten out in order to make this design work.
>>
>>
>> 3. Sorting out the details.
>>
>> 3.1. Will these routing entries fit in EXTEND2 and EXTENDED2 cells?
>>
>> The EXTEND2 cell is probably big enough for this design. The random
>> index that the client sends can be a new "link specifier" type,
>> replacing the IP and identity link specifiers.
>>
>> The EXTENDED2 cell is likely to need to grow here. We'll need to
>> implement proposal 249 ("Allow CREATE cells with >505 bytes of
>> handshake data") so that EXTEND2 and EXTENDED2 cells can be larger.
>>
>> 3.2. How should SNIPs be signed?
>>
>> We have a few options, and I'd like to look into the possibilities
>> here more closely.
>>
>> The simplest possibility is to use **multiple signatures** on each
>> SNIP, the way we do today for consensuses. These signatures should
>> be made using medium-term Ed25519 keys from the authorities. At a
>> cost of 64 bytes per signature, at 9 authorities, we would need 576
>> bytes for each SNIP. These signatures could be batch-verified to
>> save time at the client side. Since generating a signature takes
>> around 20 usec on my mediocre laptop, authorities should be able to
>> generate this many signatures fairly easily.
>>
>> Another possibility is to use a **threshold signature** on each SNIP,
>> so that the authorities collaboratively generate a short signature
>> that the clients can verify. There are multiple threshold signature
>> schemes that we could consider here, though I haven't yet found one
>> that looks perfect.
>>
>> Another possibility is to use organize the SNIPs in a **merkle tree
>> with a signed root**. For this design, clients could download the
>> signed root periodically, and receive the hash-path from the signed
>> root to the SNIP. This design might help with
>> certificate-transparency-style designs, and it would be necessary if we
>> ever want to move to a postquantum signature algorithm that requires
>> large signatures.
>>
>> Another possibility (due to a conversation among Chelsea Komlo, Sajin
>> Sasy, and Ian Goldberg), is to *use SNARKs*. (Why not? All the cool
>> kids are doing it!) For this, we'd have the clients download a
>> signed hash of the ENDIVE periodically, and have the authorities
>> generate a SNARK for each SNIP, proving its presence in that
>> document.
>>
>> 3.3. How can we detect authority misbehavior?
>>
>> We might want to take countermeasures against the possibility that a
>> quorum of corrupt or compromised authorities give some relays a
>> different set of SNIPs than they give other relays.
>>
>> If we incorporate a merkle tree or a hash chain in the design, we can
>> use mechanisms similar to certificate transparency to ensure that the
>> authorities have a consistent log of all the entries that they have
>> ever handed out.
>>
>> 3.4. How many types of weighted node selection are there, and how do we
>> handle them?
>>
>> Right now, there are multiple weights that we use in Tor:
>> * Weight for exit
>> * Weight for guard
>> * Weight for middle node
>>
>> We also filter nodes for several properties, such as flags they have.
>>
>> To reproduce this behavior, we should enumerate the various weights
>> and filters that we use, and (if there are not too many) create a
>> separate index for each. For example, the Guard index would weight
>> every node for selection as guard, assigning 0 weight to non-Guard
>> nodes. The Exit index would weight every node for selection as an
>> exit, assigning 0 weight to non-Exit nodes.
>>
>> When choosing a relay, the client would have to specify which index
>> to use. We could either have a separate (labeled) set of SNIPs
>> entries for each index, or we could have each SNIP have a separate
>> (labeled) index range for each index.
>>
>> REGRESSION: the client's choice of which index to use would leak the
>> next router's position and purpose in the circuit. This information
>> is something that we believe relays can infer now, but it's not a
>> desired feature that they can.
>>
>> 3.5. Does this design break onion service introduce handshakes?
>>
>> In rend-spec-v3.txt section 3.3.2, we specify a variant of ntor for
>> use in INTRODUCE2 handshakes. It allows the client to send encrypted
>> data as part of its initial ntor handshake, but requires the client
>> to know the onion service's onion key before it sends its initial
>> handshake.
>>
>> That won't be a problem for us here, though: we still require clients
>> to fetch onion service descriptors before contacting a onion
>> service.
>>
>> 3.6. How does the onion service directory work here?
>>
>> The onion service directory is implemented as a hash ring, where
>> each relay's position in the hash ring is decided by a hash of its
>> identity, the current date, and a shared random value that the
>> authorities compute each day.
>>
>> To implement this hash ring using walking onions, we would need to
>> have an extra index based not on bandwidth, but on position in the
>> hash ring. Then onion services and clients could build a circuit,
>> then extend it one more hop specifying their desired index in the
>> hash ring.
>>
>> We could either have a command to retrieve a trio of hashring-based
>> routing entries by index, or to retrieve (or connect to?) the n'th item
>> after a given hashring entry.
>>
>> 3.7. How can clients choose guard nodes?
>>
>> We can reuse the fallback directories here. A newly bootstrapping
>> client would connect to a fallback directory, then build a three-hop
>> circuit, and finally extend the three-hop circuit by indexing to a
>> random guard node. The random guard node's SNIP would
>> contain the information that the client needs to build real circuits
>> through that guard in the future. Because the client would be
>> building a three-hop circuit, the fallback directory would not learn
>> the client's guards.
>>
>> (Note that even if the extend attempt fails, we should still pick the
>> node as a possible guard based on its router entry, so that other
>> nodes can't veto our choice of guards.)
>>
>> 3.8. Does the walking onions design preclude postquantum circuit handshakes?
>>
>> Not at all! Both proposal 263 (ntru) and proposal 270 (newhope) work
>> by having the client generate an ephemeral key as part of its initial
>> handshake. The client does not need to know the relay's onion key to
>> do this, so we can still integrate those proposals with this one.
>>
>> 3.9. Does the walking onions design stop us from changing the network
>> topology?
>>
>> For Tor to continue to scale, we will someday need to accept that not
>> every relay can be simultaneously connected to every other relay.
>> Therefore, we will need to move from our current clique topology
>> assumption to some other topology.
>>
>> There are also proposals to change node selection rules to generate
>> routes providing better performance, or improved resistance to local
>> adversaries.
>>
>> We can, I think, implement this kind of proposal by changing the way
>> that ENDIVEs are generated. Instead giving every relay the same
>> ENDIVE, the authorities would generate different ENDIVEs for
>> different relays, depending on the probability distribution of which
>> relay should be chosen after which in the network topology. In the
>> extreme case, this would produce O(n) ENDIVEs and O(n^2) SNIPs. In
>> practice, I hope that we could do better by having the network
>> topology be non-clique, and by having many relays share the same
>> distribution of successors.
>>
>>
>> 3.10. How can clients handle exit policies?
>>
>> This is an unsolved challenge. If the client tells the middle relay
>> its target port, it leaks information inappropriately.
>>
>> One possibility is to try to gather exit policies into common
>> categories, such as "most ports supported" and "most common ports
>> supported".
>>
>> Another (inefficient) possibility is for clients to keep trying exits
>> until they find one that works.
>>
>> Another (inefficient) possibility is to require that clients who use
>> unusual ports fall back to the old mechanism for route selection.
>>
>>
>> 3.11. Can this approach support families?
>>
>> This is an unsolved challenge.
>>
>> One (inefficient) possibility is for clients to generate circuits and
>> discard those that use multiple relays in the same family.
>>
>> One (not quite compatible) possibility is for the authorities to sort
>> the ENDIVE so that relays in the same family are adjacent to
>> one another. The index-bounds part of each SNIP would also
>> have to include the bounds of the family. This approach is not quite
>> compatible with the status quo, because it prevents relays from
>> belonging to more than one family.
>>
>> One interesting possibility (due to Chelsea Komlo, Sajin Sasy, and
>> Ian Goldberg) is for the middle node to take responsibility for
>> family enforcement. In this design, the client might offer the middle
>> node multiple options for the next relay's index, and the middle node
>> would choose the first such relay that is neither in its family nor
>> its predecessor's family. We'd need to look for a way to make sure
>> that the middle node wasn't biasing the path selection.
>>
>> (TODO: come up with more ideas here.)
>>
>> 3.12. Can walking onions support IP-based and country-based restrictions?
>>
>> This is an unsolved challenge.
>>
>> If the user's restrictions do not exclude most paths, one
>> (inefficient) possibility is for the user to generate paths until
>> they generate one that they like. This idea becomes inefficient
>> if the user is excluding most paths.
>>
>> Another (inefficient and fingerprintable) possibility is to require
>> that clients who use complex path restrictions fall back to the old
>> mechanism for route selection.
>>
>> (TODO: come up with better ideas here.)
>>
>> 3.13. What scaling problems have we not solved with this design?
>>
>> The walking onions design doesn't solve (on its own) the problem that
>> the authorities need to know about every relay, and arrange to have
>> every relay tested.
>>
>> The walking onions design doesn't solve (on its own) the problem that
>> relays need to have a list of all the relays. (But see section 3.9
>> above.)
>>
>> 3.14. Should we still have clients download a consensus when they're
>> using walking onions?
>>
>> There are some fields in the current consensus directory documents
>> that the clients will still need, like the list of supported
>> protocols and network parameters. A client that uses walking onions
>> should download a new flavor of consensus document that contains only
>> these fields, and does not list any relays. In some signature
>> schemes, this consensus would contain a digest of the ENDIVE -- see
>> 3.2 above.
>>
>> (Note that this document would be a "consensus document" but not a
>> "consensus directory", since it doesn't list any relays.)
>>
>>
>> 4. Putting it all together
>>
>> [This is the section where, in a later version of this proposal, I
>> would specify the exact behavior and data formats to be used here.
>> Right now, I'd say we're too early in the design phase.]
>>
>>
>> A.1. Acknowledgments
>>
>> Thanks to Peter Palfrader for his original design in proposal 141,
>> and to the designers of PIR-Tor, both of which inspired aspects of
>> this Walking Onions design.
>>
>> Thanks to Chelsea Komlo, Sajin Sasy, and Ian Goldberg for feedback on
>> an earlier version of this design.
>>
>> Thanks to David Goulet, Teor, and George Kadianakis for commentary on
>> earlier versions of this draft.
>>
>> A.2. Additional ideas
>>
>> Teor notes that there are ways to try to get this idea to apply to
>> one-pass circuit construction, something like the old onion design.
>> We might be able to derive indices and keys from the same seeds,
>> even. I don't see a way to do this without losing forward secrecy,
>> but it might be worth looking at harder.
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