Eighth Beijing Intl Film Festival jury includes Wong Kar

Chair welcome ("PearG"):
Note well / Wear masks, in person.

Draft updates (5 mins)

• RG draft statuses

  • IP Address Privacy Considerations:

    • No recent updates since the last meeting, but updates coming
      soon

  • Censorship:

    • Recent update

  • Numeric IDs

    • Sent to RFC editor

  • Safe Internet measurements:

    • Review
    • Maybe interesting for PPM, as well

Presentations (100 mins)

• Interoperable Private Attribution (Martin Thomson) - 30 mins

  • Attribtion: important piece of the ad industry
  • Trains!
  • Let's talk about the Tokyo subway system
  • Actually, let's talk about identifiers, like access cards (e.g.,
    PASMO)
  • Using passenger tracking for the purpose of capacity planning,
    performance, etc.
  • Specifically, for systems that track when a person enters the
    system and when the person exits
  • But logs are a privacy risk and can be used for other purposes,
    even if they are inherently pseudonymous - identities could be
    linked.
  • Can we create a design that aggregates the data that's
    interesting, and provides individual privacy?
  • One design is using tokens with buckets

  • Tokens need to be:

    • anonymous
    • authenticated
    • time-delayed "opening"/redemption
    • ephemeral

  • Moving on to advertising

  • Attribution: information from one context and linking it in a
    different context
  • Answer a question: "How many people saw the ad, then came to the
    show?"

  • Understanding whether certain advertising is working:

    • good placment
    • creatives
    • how much to spend
    • how long to run campaigns

  • Current, cross-context attribution allows linking people across
    contexts

  • With advertising, the context is everything:

    • Whether an ad was shown, and if that ad was clicked
    • Was a product puchased, or not
    • where was the ad shown

  • Interoperable Private Attribtion (IPA)

    • People have an identifier (significant protections against
      revealing the identifier)
    • Sites can request an encrypted and secret-share of that
      identifier
    • Sites have a view of the identifier, but it's not linkable
      cross-site

  • Attribution in MPC (multi-party computation)

    • sites gather events
    • MPC decrypts identifiers and performs attribution
    • aggregated results are the output (histogram)

  • MPC does not, itself, see the original query

  • MPC:

    • Any computation if you only need addition and multiplication
    • It can be expensive
    • IPA uses a three-party, honest-majority threat model

  • Differential Privacy

    • (epsilon, delta)-DP for hiding individual contributions
    • Every site gets a query budget that renews each epoch (e.g.,
      week)
    • This does provide leakage across time (epochs), more
      research needed in this area
    • Parameters are not fixed yet

  • Client's encrypted identifiers are bound to a site, they are
    bound to:

    • the site that requested them
    • the epoch/week they are requested
    • the type of event: source (ad), trigger (purchase)

  • IPA: advances and challenges

    • IPA's flexibility provides somewhat of a drop-in replacment
      for current anti-fraud systems

    • IPA's flexibility hurts accountability

      • Existing challenge in making the system auditable

    • MPC performance is a challenge, especially at the scale of
      10s of billions

  • Status: Good progress, overall, but still requires research in
    some areas

  • Currently running some synthetic trials

  • Ongoing work in W3C working groups, protocol may come to PPM in
    the future

  • Brian Trammel: MPC performance is a challenge. Computation or
    communication complexity?

  • MT: A lot is algorithmic (linear), but some of that will likely
    improved, but much of it is communication cost. Originally,
    records were working on the order of ~40GB, but it's still
    mutli-gigabytes in size
  • Chris Wood: 1) What was the MPC functionality you needed (as
    defined by the existing adtech industry), 2) Now that
    functionality is defined, and how you implement. How did you
    reach this design?
  • MT: Need more time. Lots of people took the steps to get here.
    Apple's PCM took an initial approach. This is mostly about
    understanding how the advertising industry uses measurement as a
    core part of their processes. There is a "need" vs. "want"
    different of perspective by different parties, and those
    discussions are on-going. If you add cross-device attribution,
    it gets more complicated.
  • CW: There is an academic research community that has spent a lot
    of time designing MPC protocols. There seems to be some overlap
    and collaboration opportunity here.
  • Shivan: Who would run the servers in the MPC protocol?
  • MT: We need to trust them to not collude - to be determined
  • Jonathan Hoyland: If it's run by a third-party that is running
    an auction, what are the guarantees that they're actually
    running the MPC protocol
  • MT: Currently leaning on the oversight / auditing.
  • JH: Can the response include a proof?
  • MT: Recently asked if Verifiable MPC was considered - but VMPC
    is not ready yet. So, "trust and verify" is the current approach

• Secure Partitioning Protocols (Phillipp Schoppmann) - 20 mins

  • Let's go more into details for scaling aggregation computations
  • Billions of impressions from billions of clients
  • ALl clients submit their reports to the MPC cluster
  • MPC outputs the aggregate results

  • Goals

    • When sharding the MPC cluster, every client must use the
      same shard
    • We need a private mechanism for mapping one client to the
      same shard
    • This should have low communication cost
    • "correctness" must not be affected

  • Assumptions:

    • Bound on the number of contribitions
    • Many clients, fewer shards

  • Blueprint: partitioning from distributed OPRFs

    • client has an index (i), and payload (v)
    • One server has an OPRF key (server 1)
    • Other server (server 2) will learn the result of OPRF
      computation
    • server 1 must add some padding queries
    • Server 2's output of OPRF is used for mapping client to
      target partition

  • Dense Partitioning: OPRF Output = Shard ID

  • If there are only a small set of shards, then this is reasonable

  • Sparse Partitioning: OPRF Output = Random Client ID

    • Can the client's reports be aggregated before the MPC
      computation?
    • This doesn't result in creating a client identifier because
      server 1 pads the set of known client identifier if dummy
      values, so server 2 can't distinguish between real users and
      fake users

  • How can the sparse histogram be private without seeing the
    actual histogram?

    • View the output of the OPRF as a histogram
    • Make sure frequency can't be linked to specific users
    • Choose a threshold, below threshold add dummy values, above
      threshold [..] (?)

  • Conclusion: efficient for these use cases

  • Next steps: Is there general interest? Are there other protocols
    where this might be useful? Are there other properties that are
    needed?

  • Chris Patton: Definitely interesting, but maybe not as an
    independent draft

  • PS: So, add this into individual drafts, instead of making a
    general purpose protocol
  • CP: Yes
  • Martin Thomson: The bounds seem to be fundemental. How confident
    are you that these are required costs?
  • PS: The numbers are not the absolute lower bound, they are based
    on the curent design described in this presentation
  • MT: IPA may not be able to set an upper bound on the number of
    contributions, for example due to a Sybil attack
  • PS: While any party can create reports, but fraudulent reports
    may be able to be filtered downstream

• DP3T: Deploying decentralized, privacy-preserving proximity tracing
  (Wouter Lueks) - 25 mins

  • D3-PT, started back in March 2020, first draft in May 2020,
    September 2020 - Summer 2021 working on presence tracing
  • Non-traditional academic environment - scaling to millions of
    users on a small timescale
  • Relying on existing infrastructure had a large impact
  • The system was designed that they were purpose-built and
    couldn't be re-used for other purposes

  • Risks associated with digital contact tracing:

    • Must embed social contact / graph
    • location tracing
    • medical information
    • social interactions
    • social control risk

  • Time has shown what can go wrong with designs/deployments like
    this

    • Police departments in crime solving
    • data leaks
    • harassment of specific subgroups

  • It is very important that systems should be designed with
    purpose-limitations in mind, so they can't be easily abused in
    other ways

  • Relying on existing infrastructure, using phones with BTLE
    sending beacons
  • Proximity can be derived based on the beacons they saw
  • Exposure notification works by the set intersection of beacons
    the person (who tested positive) saw and all of the identifiers
    that another person broadcast
  • The design of these beacon broadcasts required that the OS
    vendor must be involved
  • While the design was relatively simple, relying on existing
    hardware made the situation more difficult/complicated
  • The result of collaboration with Google/Apple, was the
    Google/Apple Exposure Notification (GAEN) Framework/API

  • For full effect, you need privacy at all layers of the stack,
    including the bluetooth protocl stack

    • MAC address must rotate at the same time as the beacons

  • Similarly, at the network layer, a network adversary can detect
    uploading the report of seen beacon identifiers (when reporting
    covid positive) - CH used dummy uploads to hide

  • Lessons learned:

    • Purpose limitations
    • context matters (how/where they are deployed)
    • Privacy at all layers

  • Tommy Pauly: More comment than questions: for privacy at all
    layers, Apple is routing upload report through iCPR

  • WL: While this is great, there might be other sidechannels we
    need to look at
  • XXX: How do you authenticate IDs?
  • WL: There isn't any binding, but the upload requires knowing the
    underlying seed from which the beacon was derived
  • Chris Wood: What would've an ideal interface looked like, and
    how would you've designed it differently?
  • WL: The strictness provided protections, but it introduced
    challenges, as well. There isn't an easy answer.

• LogPicker: Strengthening Certificate Transparency Against Covert
  Adversaries (Alexandra Dirksen) - 25 mins

  • HTTPS is mostly a default now (90%+ of all page loads are http
    in chrome)
  • CAs are the trust anchors of the Web PKI

  • There are recent illicit certificate creations, and seemingly
    increasing

    • WoSign
    • Digicert
    • Diginotar
    • Comodo
    • TurkTrust

  • For rogue certificates, where you get a certificate for a domain
    that you don't own (e.g., HTTPS interception)

  • In the attacker scenario, a covert attacker obtaining a rogue
    certificate
  • Certificate transparency overview

  • CT is still vulnerable to this attack

    • All logs belong to a CA vendor
    • First compromise was in 2020
    • vulnerable to collaboration attacks
    • vulnerable to split view attack

  • Gossip is proposed as a mitigation for Split View attacks

  • LogPicker: a decentralized approach

    • CA contacts one log (leader) from a large set of logs (log
      pool)
    • Leader then contacts the other logs in the pool
    • the pool then selects one log, at random
    • The selected log includes the certificate in its merkle tree
    • The logs that participated in choosing the log create a
      proof, and that proof is aggregated and sent back to the CA
      for inclusion in the certificate

  • This design meets the goals

  • Chris Wood: The log pool uses an election protocol?
  • AD: Yes, two protocols
  • CW: Have you looked at alternative solutions that use threshold
    signing?
  • AD: The aggregated signature uses BLS, but which signature
    scheme is used is not strictly defined