[tor-dev] Proposal 300: Walking Onions: Scaling and Saving Bandwidth

Michael Rogers michael at briarproject.org
Tue Feb 5 17:42:19 UTC 2019

I'm very happy to see this proposal! Two quick questions about relay

* 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.)


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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