Network Working Group                                       T. Geoghegan
Internet-Draft                                                      ISRG
Intended status: Standards Track                               C. Patton
Expires: 12 January 2023                                      Cloudflare
                                                             E. Rescorla
                                                                 Mozilla
                                                              C. A. Wood
                                                              Cloudflare
                                                            11 July 2022


  Distributed Aggregation Protocol for Privacy Preserving Measurement
                         draft-ietf-ppm-dap-01

Abstract

   There are many situations in which it is desirable to take
   measurements of data which people consider sensitive.  In these
   cases, the entity taking the measurement is usually not interested in
   people's individual responses but rather in aggregated data.
   Conventional methods require collecting individual responses and then
   aggregating them, thus representing a threat to user privacy and
   rendering many such measurements difficult and impractical.  This
   document describes a multi-party distributed aggregation protocol
   (DAP) for privacy preserving measurement (PPM) which can be used to
   collect aggregate data without revealing any individual user's data.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://ietf-wg-
   ppm.github.io/draft-ietf-ppm-dap/draft-ietf-ppm-dap.html.  Status
   information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-ppm-dap/.

   Discussion of this document takes place on the Privacy Preserving
   Measurement Working Group mailing list (mailto:ppm@ietf.org), which
   is archived at https://mailarchive.ietf.org/arch/browse/ppm/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-ppm/draft-ietf-ppm-dap.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.




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Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   4
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  System Architecture . . . . . . . . . . . . . . . . . . .   6
     2.2.  Validating Inputs . . . . . . . . . . . . . . . . . . . .   9
   3.  Message Transport . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  HTTPS Sender Authentication . . . . . . . . . . . . . . .  12
   4.  Protocol Definition . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Task Configuration  . . . . . . . . . . . . . . . . . . .  14
     4.2.  Uploading Reports . . . . . . . . . . . . . . . . . . . .  16
       4.2.1.  HPKE Configuration Request  . . . . . . . . . . . . .  16
       4.2.2.  Upload Request  . . . . . . . . . . . . . . . . . . .  17
       4.2.3.  Upload Extensions . . . . . . . . . . . . . . . . . .  19
     4.3.  Verifying and Aggregating Reports . . . . . . . . . . . .  19
       4.3.1.  Aggregate Initialization  . . . . . . . . . . . . . .  21
       4.3.2.  Aggregate Continuation  . . . . . . . . . . . . . . .  26
     4.4.  Collecting Results  . . . . . . . . . . . . . . . . . . .  29
       4.4.1.  Collection Initialization . . . . . . . . . . . . . .  29
       4.4.2.  Collection Aggregation  . . . . . . . . . . . . . . .  31



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       4.4.3.  Collection Finalization . . . . . . . . . . . . . . .  33
       4.4.4.  Aggregate Share Encryption  . . . . . . . . . . . . .  33
       4.4.5.  Validating Batch Parameters . . . . . . . . . . . . .  34
       4.4.6.  Anti-replay . . . . . . . . . . . . . . . . . . . . .  35
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  35
     5.1.  Protocol participant capabilities . . . . . . . . . . . .  36
       5.1.1.  Client capabilities . . . . . . . . . . . . . . . . .  36
       5.1.2.  Aggregator capabilities . . . . . . . . . . . . . . .  36
       5.1.3.  Collector capabilities  . . . . . . . . . . . . . . .  37
     5.2.  Data resolution limitations . . . . . . . . . . . . . . .  37
     5.3.  Aggregation utility and soft batch deadlines  . . . . . .  37
     5.4.  Protocol-specific optimizations . . . . . . . . . . . . .  38
       5.4.1.  Reducing storage requirements . . . . . . . . . . . .  38
   6.  Compliance Requirements . . . . . . . . . . . . . . . . . . .  38
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  39
     7.1.  Threat model  . . . . . . . . . . . . . . . . . . . . . .  40
       7.1.1.  Client/user . . . . . . . . . . . . . . . . . . . . .  40
       7.1.2.  Aggregator  . . . . . . . . . . . . . . . . . . . . .  41
       7.1.3.  Leader  . . . . . . . . . . . . . . . . . . . . . . .  43
       7.1.4.  Collector . . . . . . . . . . . . . . . . . . . . . .  43
       7.1.5.  Aggregator collusion  . . . . . . . . . . . . . . . .  44
       7.1.6.  Attacker on the network . . . . . . . . . . . . . . .  44
     7.2.  Client authentication or attestation  . . . . . . . . . .  45
     7.3.  Anonymizing proxies . . . . . . . . . . . . . . . . . . .  45
     7.4.  Batch parameters  . . . . . . . . . . . . . . . . . . . .  46
     7.5.  Differential privacy  . . . . . . . . . . . . . . . . . .  46
     7.6.  Robustness in the presence of malicious servers . . . . .  46
     7.7.  Infrastructure diversity  . . . . . . . . . . . . . . . .  47
     7.8.  System requirements . . . . . . . . . . . . . . . . . . .  47
       7.8.1.  Data types  . . . . . . . . . . . . . . . . . . . . .  47
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
     8.1.  Protocol Message Media Types  . . . . . . . . . . . . . .  47
       8.1.1.  "application/dap-hpke-config" media type  . . . . . .  48
       8.1.2.  "application/dap-report" media type . . . . . . . . .  49
       8.1.3.  "application/dap-aggregate-continue-req" media
               type  . . . . . . . . . . . . . . . . . . . . . . . .  50
       8.1.4.  "application/dap-aggregate-initialize-resp" media
               type  . . . . . . . . . . . . . . . . . . . . . . . .  50
       8.1.5.  "application/dap-aggregate-continue-req" media
               type  . . . . . . . . . . . . . . . . . . . . . . . .  51
       8.1.6.  "application/dap-aggregate-continue-resp" media
               type  . . . . . . . . . . . . . . . . . . . . . . . .  52
       8.1.7.  "application/dap-aggregate-share-req" media type  . .  53
       8.1.8.  "application/dap-aggregate-share-resp" media type . .  54
       8.1.9.  "application/dap-collect-req" media type  . . . . . .  55
       8.1.10. "application/dap-collect-req" media type  . . . . . .  55
     8.2.  Upload Extension Registry . . . . . . . . . . . . . . . .  56
     8.3.  URN Sub-namespace for DAP (urn:ietf:params:ppm:dap) . . .  57



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   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  57
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  57
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  57
     10.2.  Informative References . . . . . . . . . . . . . . . . .  58
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  59

1.  Introduction

   This document describes a distributed aggregation protocol for
   privacy preserving measurement.  The protocol is executed by a large
   set of clients and a small set of servers.  The servers' goal is to
   compute some aggregate statistic over the clients' inputs without
   learning the inputs themselves.  This is made possible by
   distributing the computation among the servers in such a way that, as
   long as at least one of them executes the protocol honestly, no input
   is ever seen in the clear by any server.

1.1.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are used:

   Aggregation function:  The function computed over the users' inputs.

   Aggregator:  An endpoint that runs the input-validation protocol and
      accumulates input shares.

   Batch:  A set of reports that are aggregated into an output.

   Batch duration:  The time difference between the oldest and newest
      report in a batch.

   Batch interval:  A parameter of the collect or aggregate-share
      request that specifies the time range of the reports in the batch.

   Client:  The endpoint from which a user sends data to be aggregated,
      e.g., a web browser.

   Collector:  The endpoint that receives the output of the aggregation
      function.

   Input:  The measurement (or measurements) emitted by a client, before
      any encryption or secret sharing scheme is applied.



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   Input share:  An aggregator's share of the output of the VDAF [VDAF]
      sharding algorithm.  This algorithm is run by each client in order
      to cryptographically protect its measurement.

   Measurement:  A single value (e.g., a count) being reported by a
      client.  Multiple measurements may be grouped into a single
      protocol input.

   Minimum batch duration:  The minimum batch duration permitted for a
      DAP task, i.e., the minimum time difference between the oldest and
      newest report in a batch.

   Minimum batch size:  The minimum number of reports in a batch.

   Leader:  A distinguished aggregator that coordinates input validation
      and data collection.

   Aggregate result:  The output of the aggregation function over a
      given set of reports.

   Aggregate share:  A share of the aggregate result emitted by an
      aggregator.  Aggregate shares are reassembled by the collector
      into the final output.

   Output share:  An aggregator's share of the output of the VDAF [VDAF]
      preparation step.  Many output shares are combined into an
      aggregate share via the VDAF aggregation algorithm.

   Proof:  A value generated by the client and used by the aggregators
      to verify the client's input.

   Report:  Uploaded to the leader from the client.  A report contains
      the secret-shared and encrypted input and proof.

   Server:  An aggregator.

   This document uses the presentation language of [RFC8446] to define
   messages in the DAP protocol.  Encoding and decoding of these
   messages as byte strings also follows [RFC8446].

2.  Overview

   The protocol is executed by a large set of clients and a small set of
   servers.  We call the servers the _aggregators_. Each client's input
   to the protocol is a set of measurements (e.g., counts of some user
   behavior).  Given the input set of measurements x_1, ..., x_n held by
   n users, the goal of a protocol for privacy preserving measurement is
   to compute y = F(p, x_1, ..., x_n) for some function F while



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   revealing nothing else about the measurements.

   This protocol is extensible and allows for the addition of new
   cryptographic schemes that implement the VDAF interface specified in
   [VDAF].  Candidates include:

   *  Prio3, which allows for aggregate statistics such as sum, mean,
      histograms, etc.  This class of VDAFs is based on Prio [CGB17] and
      includes improvements described in [BBCGGI19].

   *  Poplar1, which allows for finding the most popular strings among a
      collection of clients (e.g., the URL of their home page) as well
      as counting the number of clients that hold a given string.  This
      VDAF is the basis of the Poplar protocol of [BBCGGI21], which is
      designed to solve the heavy hitters problem in a privacy
      preserving manner.

   This protocol is designed to work with schemes that use secret
   sharing.  Rather than send its input in the clear, each client shards
   its measurements into a sequence of _input shares_ and sends an input
   share to each of the aggregators.  This provides two important
   properties:

   *  It's impossible to deduce the measurement without knowing _all_ of
      the shares.

   *  It allows the aggregators to compute the final output by first
      aggregating up their measurements shares locally, then combining
      the results to obtain the final output.

2.1.  System Architecture

   The overall system architecture is shown in Figure 1.


















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                       +------------+
                       |            |
   +--------+          |   Helper   |
   |        |          |            |
   | Client +----+     +-----^------+
   |        |    |           |
   +--------+    |           |
                 |           |
   +--------+    |     +-----v------+         +-----------+
   |        |    +----->            |         |           |
   | Client +---------->   Leader   <---------> Collector |
   |        |    +----->            |         |           |
   +--------+    |     +-----^------+         +-----------+
                 |           |
   +--------+    |           |
   |        |    |           |
   | Client +----+     +-----V------+
   |        |          |            |
   +--------+          |   Helper   |
                       |            |
                       +------------+

                       Figure 1: System Architecture

   [[OPEN ISSUE: This shows two helpers, but the document only allows
   one for now. https://github.com/ietf-wg-ppm/draft-ietf-ppm-dap/
   issues/117]]

   The main participants in the protocol are as follows:

   Collector:  The entity which wants to take the measurement and
      ultimately receives the results.  Any given measurement will have
      a single collector.

   Client(s):  The endpoints which directly take the measurement(s) and
      report them to the DAP protocol.  In order to provide reasonable
      levels of privacy, there must be a large number of clients.

   Aggregator:  An endpoint which receives report shares.  Each
      aggregator works with the other aggregators to compute the final
      aggregate.  This protocol defines two types of aggregators:
      Leaders and Helpers.  For each measurement, there is a single
      leader and helper.

   Leader:  The leader is responsible for coordinating the protocol.  It
      receives the encrypted shares, distributes them to the helpers,
      and orchestrates the process of computing the final measurement as
      requested by the collector.



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   Helper:  Helpers are responsible for executing the protocol as
      instructed by the leader.  The protocol is designed so that
      helpers can be relatively lightweight, with most of the state held
      at the leader.

   The basic unit of DAP is the "task" which represents a single
   measurement (though potentially taken over multiple time windows).
   The definition of a task includes the following parameters:

   *  The type of each measurement.

   *  The aggregation function to compute (e.g., sum, mean, etc.).

   *  The set of aggregators and necessary cryptographic keying material
      to use.

   *  The VDAF to execute, which to some extent is dictated by the
      previous choices.

   *  The minimum "batch size" of reports which can be aggregated.

   *  The rate at which measurements can be taken, i.e., the "minimum
      batch window".

   These parameters are distributed out of band to the clients and to
   the aggregators.  They are distributed by the collecting entity in
   some authenticated form.  Each task is identified by a unique 32-byte
   ID which is used to refer to it in protocol messages.

   During the duration of the measurement, each client records its own
   value(s), packages them up into a report, and sends them to the
   leader.  Each share is separately encrypted for each aggregator so
   that even though they pass through the leader, the leader is unable
   to see or modify them.  Depending on the measurement, the client may
   only send one report or may send many reports over time.

   The leader distributes the shares to the helpers and orchestrates the
   process of verifying them (see Section 2.2) and assembling them into
   a final measurement for the collector.  Depending on the VDAF, it may
   be possible to incrementally process each report as it comes in, or
   may be necessary to wait until the entire batch of reports is
   received.









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2.2.  Validating Inputs

   An essential task of any data collection pipeline is ensuring that
   the data being aggregated is "valid".  In DAP, input validation is
   complicated by the fact that none of the entities other than the
   client ever sees the values for individual clients.

   In order to address this problem, the aggregators engage in a secure,
   multi-party computation specified by the chosen VDAF [VDAF] in order
   to prepare a report for aggregation.  At the beginning of this
   computation, each aggregator is in possession of an input share
   uploaded by the client.  At the end of the computation, each
   aggregator is in posession of either an "output share" that is ready
   to be aggregated or an indication that a valid output share could not
   be computed.

   To facilitiate this computation, the input shares generated by the
   client include information used by the aggregators during aggregation
   in order to validate their corresponding output shares.  For example,
   Prio3 includes a distributed zero-knowledge proof of the input's
   validity [BBCGGI19] which the aggregators can jointly verify and
   reject the report if it cannot be verified.  However, they do not
   learn anything about the individual report other than that it is
   valid.

   The specific properties attested to in the proof vary depending on
   the measurement being taken.  For instance, if we want to measure the
   time the user took performing a given task the proof might
   demonstrate that the value reported was within a certain range (e.g.,
   0-60 seconds).  By contrast, if we wanted to report which of a set of
   N options the user select, the report might contain N integers and
   the proof would demonstrate that N-1 were 0 and the other was 1.

   It is important to recognize that "validity" is distinct from
   "correctness".  For instance, the user might have spent 30s on a task
   but the client might report 60s.  This is a problem with any
   measurement system and DAP does not attempt to address it; it merely
   ensures that the data is within acceptable limits, so the client
   could not report 10^6s or -20s.

3.  Message Transport

   Communications between DAP entities are carried over HTTPS [RFC2818].
   HTTPS provides server authentication and confidentiality.  When
   client authenticaiton is also required, the client uses the mechanism
   described in Section 3.2.





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3.1.  Errors

   Errors can be reported in DAP both at the HTTP layer and within
   challenge objects as defined in Section 8.  DAP servers can return
   responses with an HTTP error response code (4XX or 5XX).  For
   example, if the client submits a request using a method not allowed
   in this document, then the server MAY return HTTP status code 405
   Method Not Allowed.

   When the server responds with an error status, it SHOULD provide
   additional information using a problem document [RFC7807].  To
   facilitate automatic response to errors, this document defines the
   following standard tokens for use in the "type" field (within the DAP
   URN namespace "urn:ietf:params:ppm:dap:error:"):





































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     +============================+==================================+
     | Type                       | Description                      |
     +============================+==================================+
     | unrecognizedMessage        | The message type for a response  |
     |                            | was incorrect or the payload was |
     |                            | malformed.                       |
     +----------------------------+----------------------------------+
     | unrecognizedTask           | An endpoint received a message   |
     |                            | with an unknown task ID.         |
     +----------------------------+----------------------------------+
     | unrecognizedAggregationJob | An endpoint received a message   |
     |                            | with an unknown aggregation job  |
     |                            | ID.                              |
     +----------------------------+----------------------------------+
     | outdatedConfig             | The message was generated using  |
     |                            | an outdated configuration.       |
     +----------------------------+----------------------------------+
     | reportTooLate              | Report could not be processed    |
     |                            | because it arrived too late.     |
     +----------------------------+----------------------------------+
     | reportTooEarly             | Report could not be processed    |
     |                            | because its timestamp is too far |
     |                            | in the future.                   |
     +----------------------------+----------------------------------+
     | batchInvalid               | A collect or aggregate-share     |
     |                            | request was made with invalid    |
     |                            | batch parameters.                |
     +----------------------------+----------------------------------+
     | insufficientBatchSize      | There are not enough reports in  |
     |                            | the batch interval to satisfy    |
     |                            | the task's minimum batch size.   |
     +----------------------------+----------------------------------+
     | batchLifetimeExceeded      | The batch lifetime has been      |
     |                            | exceeded for one or more reports |
     |                            | included in the batch interval.  |
     +----------------------------+----------------------------------+
     | batchMismatch              | Aggregators disagree on the      |
     |                            | report shares that were          |
     |                            | aggregated in a batch.           |
     +----------------------------+----------------------------------+
     | unauthorizedRequest        | Authentication of an HTTP        |
     |                            | request failed (see              |
     |                            | Section 3.2).                    |
     +----------------------------+----------------------------------+

                                  Table 1





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   This list is not exhaustive.  The server MAY return errors set to a
   URI other than those defined above.  Servers MUST NOT use the DAP URN
   namespace for errors not listed in the appropriate IANA registry (see
   Section 8.3).  Clients SHOULD display the "detail" field of all
   errors.  The "instance" value MUST be the endpoint to which the
   request was targeted.  The problem document MUST also include a
   "taskid" member which contains the associated DAP task ID (this value
   is always known, see Section 4.1), encoded in Base 64 using the URL
   and filename safe alphabet with no padding defined in sections 5 and
   3.2 of [RFC4648].

   In the remainder of this document, we use the tokens in the table
   above to refer to error types, rather than the full URNs.  For
   example, an "error of type 'unrecognizedMessage'" refers to an error
   document with "type" value
   "urn:ietf:params:ppm:dap:error:unrecognizedMessage".

   This document uses the verbs "abort" and "alert with [some error
   message]" to describe how protocol participants react to various
   error conditions.

3.2.  HTTPS Sender Authentication

   Some HTTP requests in the DAP protocol require the sender to
   authenticate its request.  It does so as described here.

   Prior to the start of the protocol, the sender and receiver arrange
   to share a secret sender-specific API token, which MUST be suitable
   for representation in an HTTP header.

   For requests requiring authentication, the sender includes a "DAP-
   Auth-Token" header in its HTTP request containing the API token.

   To authenticate the request, the receiver looks up the token for the
   sender as determined by the task configuration.  (See Section 4.1.)
   If the value of the "DAP-Auth-Token" header does not match the token,
   then the receiver MUST abort with error "unauthorizedRequest" and
   HTTP status code 403 Forbidden.

   [OPEN ISSUE: This simple bearer-token scheme is meant to unblock
   interop testing.  Eventually it should be replaced with a more secure
   authentication mechanism, e.g., TLS client certificates.  See
   https://mailarchive.ietf.org/arch/msg/ppm/
   z65FK8kOU27Dt38WNhpI6apc2so/ for details.]







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4.  Protocol Definition

   DAP has three major interactions which need to be defined:

   *  Uploading reports from the client to the aggregators, specified in
      Section 4.2

   *  Computing the results of a given measurement, specified in
      Section 4.3

   *  Collecting aggregated results, specified in Section 4.4

   We start with some basic type definitions used in other messages.






































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/* ASCII encoded URL. e.g., "https://example.com" */
opaque Url<1..2^16-1>;

Duration uint64; /* Number of seconds elapsed between two instants */

Time uint64; /* seconds elapsed since start of UNIX epoch */

/* An interval of time of length duration, where start is included and (start +
duration) is excluded. */
struct {
  Time start;
  Duration duration;
} Interval;

/* A nonce used to uniquely identify a report in the context of a DAP task. It
includes the timestamp of the current batch and a random 16-byte value. */
struct {
  Time time;
  uint8 rand[16];
} Nonce;

/* The various roles in the DAP protocol. */
enum {
  collector(0),
  client(1),
  leader(2),
  helper(3),
} Role;

/* Identifier for a server's HPKE configuration */
uint8 HpkeConfigId;

/* An HPKE ciphertext. */
struct {
  HpkeConfigId config_id;    // config ID
  opaque enc<1..2^16-1>;     // encapsulated HPKE key
  opaque payload<1..2^16-1>; // ciphertext
} HpkeCiphertext;

4.1.  Task Configuration

   Prior to the start of execution of the protocol, each participant
   must agree on the configuration for each task.  A task is uniquely
   identified by its task ID:

   opaque TaskId[32];





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   A TaskId is a globally unique sequence of bytes.  It is RECOMMENDED
   that this be set to a random string output by a cryptographically
   secure pseudorandom number generator.  Each task has the following
   parameters associated with it:

   *  aggregator_endpoints: A list of URLs relative to which an
      aggregator's API endpoints can be found.  Each endpoint's list
      MUST be in the same order.  The leader's endpoint MUST be the
      first in the list.  The order of the encrypted_input_shares in a
      Report (see Section 4.2) MUST be the same as the order in which
      aggregators appear in this list.

   *  max_batch_lifetime: The maximum number of times a batch of reports
      may be used in collect requests.

   *  min_batch_size: The minimum number of reports that appear in a
      batch.

   *  min_batch_duration: The minimum time difference between the oldest
      and newest report in a batch.  This defines the boundaries with
      which the batch interval of each collect request must be aligned.
      (See Section 4.4.5.)

   *  A unique identifier for the VDAF instance used for the task,
      including the type of measurement associated with the task.

   In addition, in order to facilitate the aggregation and collect
   protocols, each of the aggregators is configured with following
   parameters:

   *  collector_config: The [HPKE] configuration of the collector
      (described in Section 4.2.1); see Section 6 for information about
      the HPKE configuration algorithms.

   *  vdaf_verify_key: The VDAF verification key shared by the
      aggregators.  This key is used in the aggregation sub-protocol
      (Section 4.3).  [OPEN ISSUE: The manner in which this key is
      distributed may be relevant to the VDAF's security.  See
      issue#161.]

   The helper stores a bearer token used to authenticate HTTP requests
   from the leader.  Likewise, the leader stores a bearer token to
   authenticate HTTP request from the collector.  The authentication
   mechanism is described in Section 3.2.

   Finally, the collector is configured with the HPKE secret key
   corresponding to collector_hpke_config.




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4.2.  Uploading Reports

   Clients periodically upload reports to the leader, which then
   distributes the individual shares to each helper.

4.2.1.  HPKE Configuration Request

   Before the client can upload its report to the leader, it must know
   the HPKE configuration of each aggregator.  See Section 6 for
   information on HPKE algorithm choices.

   Clients retrieve the HPKE configuration from each aggregator by
   sending an HTTP GET request to
   [aggregator]/hpke_config?task_id=[task-id], where [aggregator] is the
   aggregator's endpoint URL, obtained from the task parameters, and
   [task-id] is the task ID obtained from the task parameters, encoded
   in Base 64 with URL and filename safe alphabet with no padding, as
   specified in sections 5 and 3.2 of [RFC4648].  If the aggregator does
   not recognize the task ID, then it responds with HTTP status code 404
   Not Found and an error of type unrecognizedTask.  The aggregator
   responds to well-formed requests with HTTP status code 200 OK and an
   HpkeConfig value:

   [TODO: Allow aggregators to return HTTP status code 403 Forbidden in
   deployments that use authentication to avoid leaking information
   about which tasks exist.]

   struct {
     HpkeConfigId id;
     HpkeKemId kem_id;
     HpkeKdfId kdf_id;
     HpkeAeadKdfId aead_id;
     HpkePublicKey public_key;
   } HpkeConfig;

   opaque HpkePublicKey<1..2^16-1>;
   uint16 HpkeAeadId; // Defined in [HPKE]
   uint16 HpkeKemId;  // Defined in [HPKE]
   uint16 HpkeKdfId;  // Defined in [HPKE]

   [OPEN ISSUE: Decide whether to expand the width of the id, or support
   multiple cipher suites (a la OHTTP/ECH).]

   The client MUST abort if any of the following happen for any HPKE
   config request:

   *  the client and aggregator failed to establish a secure,
      aggregator-authenticated channel;



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   *  the GET request failed or didn't return a valid HPKE
      configuration; or

   *  the HPKE configuration specifies a KEM, KDF, or AEAD algorithm the
      client doesn't recognize.

   Aggregators SHOULD use HTTP caching to permit client-side caching of
   this resource [RFC5861].  Aggregators SHOULD favor long cache
   lifetimes to avoid frequent cache revalidation, e.g., on the order of
   days.  Aggregators can control this cached lifetime with the Cache-
   Control header, as follows:

     Cache-Control: max-age=86400

   Clients SHOULD follow the usual HTTP caching [RFC7234] semantics for
   key configurations.

   Note: Long cache lifetimes may result in clients using stale HPKE
   configurations; aggregators SHOULD continue to accept reports with
   old keys for at least twice the cache lifetime in order to avoid
   rejecting reports.

4.2.2.  Upload Request

   Clients upload reports by using an HTTP POST to [leader]/upload,
   where [leader] is the first entry in the task's aggregator endpoints.
   The payload is structured as follows:

   struct {
     TaskID task_id;
     Nonce nonce;
     Extension extensions<0..2^16-1>;
     HpkeCiphertext encrypted_input_shares<1..2^16-1>;
   } Report;

   This message is called the client's report.  It contains the
   following fields:

   *  task_id is the task ID of the task for which the report is
      intended.

   *  nonce is the report nonce generated by the client.  This field is
      used by the aggregators to ensure the report appears in at most
      one batch.  (See Section 4.4.6.)

   *  extensions is a list of extensions to be included in the Upload
      flow; see Section 4.2.3.




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   *  encrypted_input_shares contains the encrypted input shares of each
      of the aggregators.  The order in which the encrypted input shares
      appear MUST match the order of the task's aggregator_endpoints
      (i.e., the first share should be the leader's, the second share
      should be for the first helper, and so on).

   To generate the report, the client begins by initializing the report
   nonce.  Specifically, the client first sets Nonce.rand to 16 random
   bytes output from a cryptographically secure random number generator.
   It then sets Nonce.time to the number of seconds elapsed since the
   start of the UNIX epoch, rounded down to the nearest multiple of
   min_batch_duration.  This truncation is done to ensure that the
   report's timestamp cannot be used to link the report back to the
   originating client.

   The client then finishes the report generation by sharding its
   measurement into a sequence of input shares as specified by the VDAF
   in use.  To encrypt an input share, the client generates an [HPKE]
   ciphertext for the aggregator by running

enc, payload = SealBase(pk, "dap-01 input share" || 0x01 || server_role,
    task_id || nonce || extensions, input_share)

   where pk is the aggregator's public key; server_role is the Role of
   the intended recipient (0x02 for the leader and 0x03 for the helper);
   task_id, nonce, and extensions are the corresponding fields of
   Report; and input_share is the aggregator's input share.

   The leader responds to well-formed requests to [leader]/upload with
   HTTP status code 200 OK and an empty body.  Malformed requests are
   handled as described in Section 3.1.  Clients SHOULD NOT upload the
   same measurement value in more than one report if the leader responds
   with HTTP status code 200 OK and an empty body.

   The leader responds to requests whose leader encrypted input share
   uses an out-of-date HpkeConfig.id value, indicated by
   HpkeCiphertext.config_id, with HTTP status code 400 Bad Request and
   an error of type 'outdatedConfig'.  Clients SHOULD invalidate any
   cached aggregator HpkeConfig and retry with a freshly generated
   Report.  If this retried report does not succeed, clients MUST abort
   and discontinue retrying.










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   The leader MUST ignore any report whose nonce contains a timestamp
   that falls in a batch interval for which it has received at least one
   collect request from the collector.  (See Section 4.4.)  Otherwise,
   comparing the aggregate result to the previous aggregate result may
   result in a privacy violation.  (Note that the helpers enforce this
   as well; see Section 4.4.)  In addition, the leader SHOULD abort the
   upload protocol and alert the client with error "reportTooLate".

   Leaders can buffer reports while waiting to aggregate them.  The
   leader SHOULD NOT accept reports whose timestamps are too far in the
   future.  Implementors MAY provide for some small leeway, usually no
   more than a few minutes, to account for clock skew.  If the leader
   rejects a report for this reason, it SHOULD abort the upload protocol
   and alert the client with error "reportTooEarly".

4.2.3.  Upload Extensions

   Each Report carries a list of extensions that clients may use to
   convey additional, authenticated information in the report.  [OPEN
   ISSUE: The extensions aren't authenticated.  It's probably a good
   idea to be a bit more clear about how we envision extensions being
   used.  Right now this includes client attestation for defeating Sybil
   attacks.  See issue#89.]  Each extension is a tag-length encoded
   value of the following form:

     struct {
         ExtensionType extension_type;
         opaque extension_data<0..2^16-1>;
     } Extension;

     enum {
         TBD(0),
         (65535)
     } ExtensionType;

   "extension_type" indicates the type of extension, and
   "extension_data" contains information specific to the extension.

4.3.  Verifying and Aggregating Reports

   Once a set of clients have uploaded their reports to the leader, the
   leader can send them to the helpers to be verified and aggregated.
   In order to enable the system to handle very large batches of
   reports, this process can be performed incrementally.  Verification
   of a set of reports is referred to as an aggregation job.  Each
   aggregation job is associated with exactly one DAP task, and a DAP
   task can have many aggregation jobs.  Each job is associated with an
   ID that is unique within the context of a DAP task in order to



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   distinguish different jobs from one another.  Each aggregator uses
   this ID as an index into per-job storage, e.g., to keep track of
   report shares that belong to a given aggregation job.

   To run an aggregation job, the leader sends a message to each helper
   containing the report shares in the job.  The helper then processes
   them (verifying the proofs and incorporating their values into the
   ongoing aggregate) and replies to the leader.

   The exact structure of the aggregation job flow depends on the VDAF.
   Specifically:

   *  Some VDAFs (e.g., Prio3) allow the leader to start aggregating
      reports proactively before all the reports in a batch are
      received.  Others (e.g., Poplar1) require all the reports to be
      present and must be initiated by the collector.

   *  Processing the reports -- especially validating them -- may
      require multiple round trips.

   Note that it is possible to aggregate reports from one batch while
   reports from the next batch are coming in.  This is because each
   report is validated independently.

   This process is illustrated below in Figure 2.  In this example, the
   batch size is 20, but the leader opts to process the reports in sub-
   batches of 10.  Each sub-batch takes two round-trips to process.

   Leader                                                 Helper

   Aggregate request (Reports 1-10, Job = N) --------------->  \
   <----------------------------- Aggregate response (Job = N) | Reports
   Aggregate request (continued, Job = N) ------------------>  | 1-10
   <----------------------------- Aggregate response (Job = N) /


   Aggregate request (Reports 11-20, Job = M) -------------->  \
   <----------------------------- Aggregate response (Job = M) | Reports
   Aggregate request (continued, Job = M) ------------------>  | 11-20
   <----------------------------- Aggregate response (Job = M) /

     Figure 2: Aggregation Flow (batch size=20).  Multiple aggregation
                  flows can be executed at the same time.

   [OPEN ISSUE: Should there be an indication of whether a given
   aggregate request is a continuation of a previous sub-batch?]





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   The aggregation flow can be thought of as having three phases for
   transforming each valid input report share into an output share:

   *  Initialization: Begin the aggregation flow by sharing report
      shares with each helper.  Each aggregator, including the leader,
      initializates the underlying VDAF instance using these report
      shares and the VDAF configured for the corresponding measurement
      task.

   *  Continuation: Continue the aggregation flow by exchanging messages
      produced by the underlying VDAF instance until aggregation
      completes or an error occurs.  These messages do not replay the
      shares.

   *  Completion: Finish the aggregate flow, yielding an output share
      corresponding for each input report share in the batch.

4.3.1.  Aggregate Initialization

   The leader begins an aggregation job by choosing a set of candidate
   reports that pertain to the same DAP task and a unique job ID.  The
   job ID is a 32-byte value, structured as follows:

   opaque AggregationJobID[32];

   The leader can run this process for many candidate reports in
   parallel as needed.  After choosing the set of candidates, the leader
   begins aggregation by splitting each report into "report shares", one
   for each aggregator.  The leader and helpers then run the aggregate
   initialization flow to accomplish two tasks:

   1.  Recover and determine which input report shares are invalid.

   2.  For each valid report share, initialize the VDAF preparation
       process.

   An invalid report share is marked with one of the following errors:

   enum {
     batch-collected(0),
     report-replayed(1),
     report-dropped(2),
     hpke-unknown-config-id(3),
     hpke-decrypt-error(4),
     vdaf-prep-error(5),
   } ReportShareError;

   The leader and helper initialization behavior is detailed below.



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4.3.1.1.  Leader Initialization

   The leader begins the aggregate initialization phase with the set of
   candidate report shares as follows:

   1.  Generate a fresh AggregationJobID.  This ID MUST be unique within
       the context of the corresponding DAP task.  It is RECOMMENDED
       that this be set to a random string output by a cryptographically
       secure pseudorandom number generator.

   2.  Decrypt the input share for each report share as described in
       Section 4.3.1.3.

   3.  Check that the resulting input share is valid as described in
       Section 4.3.1.4.

   4.  Initialize VDAF preparation as described in Section 4.3.1.5.

   If any step yields an invalid report share, the leader removes the
   report share from the set of candidate reports.  Once the leader has
   initialized this state for all valid candidate report shares, it then
   creates an AggregateInitializeReq message for each helper to
   initialize the preparation of this candidate set.  The
   AggregateInitializeReq message is structured as follows:

   struct {
     Nonce nonce;
     Extension extensions<0..2^16-1>;
     HpkeCiphertext encrypted_input_share;
   } ReportShare;

   struct {
     TaskID task_id;
     AggregationJobID job_id;
     opaque agg_param<0..2^16-1>;
     ReportShare report_shares<1..2^16-1>;
   } AggregateInitializeReq;

   [[OPEN ISSUE: consider sending report shares separately (in parallel)
   to the aggregate instructions.  RIght now, aggregation parameters and
   the corresponding report shares are sent at the same time, but this
   may not be strictly necessary. ]]

   The nonce and extensions fields of each ReportShare match that in the
   Report uploaded by the client.  The encrypted_input_share field is
   the HpkeCiphertext whose index in Report.encrypted_input_shares is
   equal to the index of the aggregator in the task's
   aggregator_endpoints to which the AggregateInitializeReq is being



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   sent.  The agg_param field is an opaque, VDAF-specific aggregation
   parameter provided during a collection flow.  The job_id parameter
   contains the leader's chosen AggregationJobID.

   [[OPEN ISSUE: Check that this handling of agg_param is appropriate
   when the definition of Poplar is done.]]

   Let [aggregator] denote the helper's API endpoint.  The leader sends
   a POST request to [aggregator]/aggregate with its
   AggregateInitializeReq message as the payload.  The media type is
   "message/dap-aggregate-initialize-req".  In addition, this request
   MUST be authenticated as described in Section 3.2.

4.3.1.2.  Helper Initialization

   Each helper begins their portion of the aggregate initialization
   phase with the set of candidate report shares obtained in an
   AggregateInitializeReq message from the leader.  It attempts to
   recover and validate the corresponding input shares similar to the
   leader, and eventually returns a response to the leader carrying a
   VDAF-specific message for each report share.

   To begin this process, the helper first checks that the nonces in
   AggregateInitializeReq.report_shares are all distinct.  If two
   ReportShare values have the same nonce, then the helper MUST abort
   with error "unrecognizedMessage".  If this check succeeds, the helper
   then attempts to recover each input share in
   AggregateInitializeReq.report_shares as follows:

   1.  Decrypt the input share for each report share as described in
       Section 4.3.1.3.

   2.  Check that the resulting input share is valid as described in
       Section 4.3.1.4.

   3.  Initialize VDAF preparation and initial outputs as described in
       Section 4.3.1.5.

   [[OPEN ISSUE: consider moving the helper nonce check into #input-
   share-batch-validation]]

   Once the helper has processed each valid report share in
   AggregateInitializeReq.report_shares, the helper then creates an
   AggregateInitializeResp message to complete its initialization.  This
   message is structured as follows:






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enum {
  continued(0),
  finished(1)
  failed(2),
} PrepareStepResult;

struct {
  Nonce nonce;
  PrepareStepResult prepare_step_result;
  select (PrepareStep.prepare_step_result) {
    case continued: opaque prep_msg<0..2^16-1>; // VDAF preparation message
    case finished:  Empty;
    case failed:    ReportShareError;
  }
} PrepareStep;

struct {
  PrepareStep prepare_steps<1..2^16-1>;
} AggregateInitializeResp;

   The message is a sequence of PrepareStep values, the order of which
   matches that of the ReportShare values in
   AggregateInitializeReq.report_shares.  Each report that was marked as
   invalid is assigned the PrepareStepResult failed.  Otherwise, the
   PrepareStep is either marked as continued with the output prep_msg,
   or is marked as finished if the VDAF preparation process is finished
   for the report share.

   The helper's response to the leader is an HTTP status code 200 OK
   whose body is the AggregateInitializeResp and media type is "message/
   dap-aggregate-initialize-resp".

   Upon receipt of a helper's AggregateInitializeResp message, the
   leader checks that the sequence of PrepareStep messages corresponds
   to the ReportShare sequence of the AggregateInitializeReq.  If any
   message appears out of order, is missing, has an unrecognized nonce,
   or if two messages have the same nonce, then the leader MUST abort
   with error "unrecognizedMessage".

   [[OPEN ISSUE: the leader behavior here is sort of bizarre -- to whom
   does it abort?]]










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4.3.1.3.  Input Share Decryption

   Each report share has a corresponding task ID, nonce, list of
   extensions, and encrypted input share.  Let task_id, nonce,
   extensions, and encrypted_input_share denote these values,
   respectively.  Given these values, an aggregator decrypts the input
   share as follows.  First, the aggregator looks up the HPKE config and
   corresponding secret key indicated by
   encrypted_input_share.config_id.  If not found, then it marks the
   report share as invalid with the error hpke-unknown-config-id.
   Otherwise, it decrypts the payload with the following procedure:

input_share = OpenBase(encrypted_input_share.enc, sk, "dap-01 input share" ||
    0x01 || server_role, task_id || nonce || extensions,
    encrypted_input_share.payload)

   where sk is the HPKE secret key, and server_role is the role of the
   aggregator (0x02 for the leader and 0x03 for the helper).  If
   decryption fails, the aggregator marks the report share as invalid
   with the error hpke-decrypt-error.  Otherwise, it outputs the
   resulting input_share.

4.3.1.4.  Input Share Validation

   Validating an input share will either succeed or fail.  In the case
   of failure, the input share is marked as invalid with a corresponding
   ReportShareError error.

   The validation checks are as follows.

   1.  Check if the report has never been aggregated but is contained by
       a batch that has been collected.  If this check fails, the input
       share is marked as invalid with the error batch-collected.  This
       prevents additional reports from being aggregated after its batch
       has already been collected.

   2.  Check if the report has already been aggregated with this
       aggregation parameter.  If this check fails, the input share is
       marked as invalid with the error report-replayed.  This is the
       case if the report was used in a previous aggregate request and
       is therefore a replay.  An aggregator may also choose to mark an
       input share as invalid with the error report-dropped under the
       conditions prescribed in Section 4.4.6.

   If both checks succeed, the input share is not marked as invalid.






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4.3.1.5.  Input Share Preparation

   Input share preparation consists of running the preparation-state
   initialization algorithm for the VDAF associated with the task and
   computes the first state transition.  This produces three possible
   values:

   1.  An error, in which case the input report share is marked as
       invalid.

   2.  An output share, in which case the aggregator stores the output
       share for future collection as described in Section 4.4.

   3.  An initial VDAF state and preparation message, denoted
       (prep_state, prep_msg).

   Each aggregator runs this procedure for a given input share with
   corresponding nonce as follows:

   prep_state = VDAF.prep_init(vdaf_verify_key,
                               agg_id,
                               agg_param,
                               nonce,
                               input_share)
   out = VDAF.prep_next(prep_state, None)

   vdaf_verify_key is the VDAF verification key shared by the
   aggregators; agg_id is the aggregator ID (0x00 for the Leader and
   0x01 for the helper); and agg_param is the opaque aggregation
   parameter distributed to the aggregtors by the collector.

   If either step fails, the aggregator marks the report as invalid with
   error vdaf-prep-error.  Otherwise, the value out is interpreted as
   follows.  If this is the last round of the VDAF, then out is the
   aggregator's output share.  Otherwise, out is the pair (prep_state,
   prep_msg).

4.3.2.  Aggregate Continuation

   In the continuation phase, the leader drives the VDAF preparation of
   each share in the candidate report set until the underlying VDAF
   moves into a terminal state, yielding an output share for all
   aggregators or an error.  This phase may involve multiple rounds of
   interaction depending on the underlying VDAF.  Each round trip is
   initiated by the leader.






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4.3.2.1.  Leader Continuation

   The leader begins each round of continuation for a report share based
   on its locally computed prepare message and the previous PrepareStep
   from the helper.  If PrepareStep is of type "failed", then the leader
   marks the report as failed and removes it from the candidate report
   set and does not process it further.  If the type is "finished", then
   the leader aborts with "unrecognizedMessage".  [[OPEN ISSUE: This
   behavior is not specified.]] If the type is "continued", then the
   leader proceeds as follows.

   Let leader_outbound denote the leader's prepare message and
   helper_outbound denote the helper's.  The leader computes the next
   state transition as follows:

inbound = VDAF.prep_shares_to_prep(agg_param, [leader_outbound, helper_outbound])
out = VDAF.prep_next(prep_state, inbound)

   where [leader_outbound, helper_outbound] is a vector of two elements.
   If either of these operations fails, then the leader marks the report
   as invalid.  Otherwise it interprets out as follows.  If this is the
   last round of the VDAF, then out is the aggregator's output share, in
   which case the aggregator finishes and stores its output share for
   further processing as described in Section 4.4.  Otherwise, out is
   the pair (new_state, prep_msg), where new_state is its updated state
   and prep_msg is its next VDAF message (which will be leader_outbound
   in the next round of continuation).  For the latter case, the helper
   sets prep_state to new_state.

   The leader then sends each PrepareStep to the helper in an
   AggregateContinueReq message, structured as follows:

   struct {
     TaskID task_id;
     AggregationJobID job_id;
     PrepareStep prepare_shares<1..2^16-1>;
   } AggregateContinueReq;

   For each aggregator endpoint [aggregator] in
   AggregateContinueReq.task_id's parameters except its own, the leader
   sends a POST request to [aggregator]/aggregate with
   AggregateContinueReq as the payload and the media type set to
   "message/dap-aggregate-continue-req".  In addition, this request MUST
   be authenticated as described in Section 3.2.







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4.3.2.2.  Helper Continuation

   The helper continues with preparation for a report share by combining
   the leader's input message in AggregateContinueReq and its current
   preparation state (prep_state).  This step yields one of three
   outputs:

   1.  An error, in which case the input report share is marked as
       invalid.

   2.  An output share, in which case the helper stores the output share
       for future collection as described in Section 4.4.

   3.  An updated VDAF state and preparation message, denoted
       (prep_state, prep_msg).

   To carry out this step, for each PrepareStep in
   AggregateContinueReq.prepare_shares received from the leader, the
   helper performs the following check to determine if the report share
   should continue being prepared.

   *  If failed, then mark the report as failed and reply with a failed
      PrepareStep to the leader.

   *  If finished, then mark the report as finished and reply with a
      finished PrepareStep to the leader.  The helper then stores the
      output share and awaits for collection; see Section 4.4.

   Otherwise, preparation continues.  In this case, the helper computes
   its updated state and output message as follows:

   out = VDAF.prep_next(prep_state, inbound)

   where inbound is the previous VDAF preapre message sent by the leader
   and prep_state is the helper's current preparation state.  If this
   operation fails, then the helper fails with error vdaf-prep-error.
   Otherwise, it interprets out as follows.  If this is the last round
   of VDAF preparation phase, then out is the helper's output share, in
   which case the helper stores the output share for future collection.
   Otherwise, the helper interpets out as the tuple (new_state,
   prep_msg), where new_state is its updated preparation state and
   prep_msg is its next VDAF message.

   This output message for each report in
   AggregateContinueReq.prepare_shares is then sent to the leader in an
   AggregateContinueResp message, structured as follows:





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   struct {
     PrepareStep prepare_shares<1..2^16-1>;
   } AggregateContinueResp;

   The order of AggregateContinueResp.prepare_shares MUST match that of
   the PrepareStep values in AggregateContinueReq.prepare_shares.  The
   helper's response to the leader is an HTTP status code 200 OK whose
   body is the AggregateContinueResp and media type is "message/dap-
   aggregate-continue-resp".  The helper then awaits the next message
   from the leader.

   [[OPEN ISSUE: consider relaxing this ordering constraint.  See
   issue#217.]]

4.4.  Collecting Results

   In this phase, the collector requests aggregate shares from each
   aggregator and then locally combines them to yield a single,
   aggregate output.  In particular, the collector asks the leader to
   collect and return the results for a given DAP task over a given time
   period.  The aggregate shares are encrypted to the collector so that
   it can decrypt and combine them to yield the aggregate output.  This
   entire process is composed of two interactions:

   1.  Collect request and response between the collector and leader,
       specified in Section 4.4.1

   2.  Aggregate share request and response between the leader and each
       aggregator, specified in Section 4.4.2

   Once complete, the collector computes the final aggregate result as
   specified in Section 4.4.3.

4.4.1.  Collection Initialization

   To initiate collection, the collector issues a POST request to
   [leader]/collect, where [leader] is the leader's endpoint URL.  The
   request MUST be authenticated as described in Section 3.2.  The body
   of the request is structured as follows:

   [OPEN ISSUE: Decide if and how the collector's request is
   authenticated.  If not, then we need to ensure that collect job URIs
   are resistant to enumeration attacks.]








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   struct {
     TaskID task_id;
     Interval batch_interval;
     opaque agg_param<0..2^16-1>; // VDAF aggregation parameter
   } CollectReq;

   The named parameters are:

   *  task_id, the DAP task ID.

   *  batch_interval, the request's batch interval.

   *  agg_param, an aggregation parameter for the VDAF being executed.
      This is the same value as in AggregateInitializeReq (see
      Section 4.3.1.1).

   Depending on the VDAF scheme and how the leader is configured, the
   leader and helper may already have prepared all the reports falling
   within batch_interval and be ready to return the aggregate shares
   right away, but this cannot be guaranteed.  In fact, for some VDAFs,
   it is not be possible to begin preparing inputs until the collector
   provides the aggregation parameter in the CollectReq.  For these
   reasons, collect requests are handled asynchronously.

   Upon receipt of a CollectReq, the leader begins by checking that the
   request meets the requirements of the batch parameters using the
   procedure in Section 4.4.5.  If so, it immediately sends the
   collector a response with HTTP status 303 See Other and a Location
   header containing a URI identifying the collect job that can be
   polled by the collector, called the "collect job URI".

   The leader then begins working with the helper to prepare the shares
   falling into CollectReq.batch_interval (or continues this process,
   depending on the VDAF) as described in Section 4.3.

   After receiving the response to its CollectReq, the collector makes
   an HTTP GET request to the collect job URI to check on the status of
   the collect job and eventually obtain the result.  If the collect job
   is not finished yet, the leader responds with HTTP status 202
   Accepted.  The response MAY include a Retry-After header field to
   suggest a pulling interval to the collector.

   If the leader has not yet obtained an aggregator share from each
   aggregator, the leader invokes the aggregate share request flow
   described in Section 4.4.2.  Otherwise, when all aggregator shares
   are successfully obtained, the leader responds to subsequent HTTP GET
   requests to the collect job's URI with HTTP status code 200 OK and a
   body consisting of a CollectResp:



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   struct {
     HpkeCiphertext encrypted_agg_shares<1..2^16-1>;
   } CollectResp;

   The encrypted_agg_shares field is the vector of encrypted aggregate
   shares.  They MUST appear in the same order as the aggregator
   endpoints list of the task parameters.

   If obtaining aggregate shares fails, then the leader responds to
   subsequent HTTP GET requests to the collect job URI with an HTTP
   error status and a problem document as described in Section 3.1.

   The leader MUST retain a collect job's results until the collector
   sends an HTTP DELETE request to the collect job URI, in which case
   the leader responds with HTTP status 204 No Content.

   [OPEN ISSUE: Allow the leader to drop aggregate shares after some
   reasonable amount of time has passed, but it's not clear how to
   specify that.  ACME doesn't bother to say anything at all about this
   when describing how subscribers should fetch certificates:
   https://datatracker.ietf.org/doc/html/rfc8555#section-7.4.2]

   [OPEN ISSUE: Describe how intra-protocol errors yield collect errors
   (see issue#57).  For example, how does a leader respond to a collect
   request if the helper drops out?]

4.4.2.  Collection Aggregation

   The leader obtains each helper's encrypted aggregate share in order
   to respond to the collector's collect response.  To do this, the
   leader first computes a checksum over the set of output shares
   included in the batch window identified by the collect request.  The
   checksum is computed by taking the SHA256 hash of each nonce from the
   client reports included in the aggregation, then combining the hash
   values with a bitwise-XOR operation.

   Then, for each aggregator endpoint [aggregator] in the parameters
   associated with CollectReq.task_id (see Section 4.4) except its own,
   the leader sends a POST request to [aggregator]/aggregate_share with
   the following message:

   struct {
     TaskID task_id;
     Interval batch_interval;
     opaque agg_param<0..2^16-1>;
     uint64 report_count;
     opaque checksum[32];
   } AggregateShareReq;



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   *  task_id is the task ID associated with the DAP parameters.

   *  batch_interval is the batch interval of the request.

   *  agg_param, an aggregation parameter for the VDAF being executed.
      This is the same value as in AggregateInitializeReq (see
      Section 4.3.1.1) and in CollectReq (see Section 4.4.1).

   *  report_count is the number of reports included in the aggregation.

   *  checksum is the checksum computed over the set of client reports.

   This request MUST be authenticated as described in Section 3.2.  To
   handle the leader's request, the helper first ensures that the
   request meets the requirements for batch parameters following the
   procedure in Section 4.4.5.

   Next, it computes a checksum based on its view of the output shares
   included in the batch window, and checks that the report_count and
   checksum included in the request match its computed values.  If not,
   then it MUST abort with error "batchMismatch".

   Next, it computes the aggregate share agg_share corresponding to the
   set of output shares, denoted out_shares, for the batch interval, as
   follows:

   agg_share = VDAF.out_shares_to_agg_share(agg_param, out_shares)

   Note that for most VDAFs, it is possible to aggregate output shares
   as they arrive rather than wait until the batch is collected.  To do
   so however, it is necessary to enforce the batch parameters as
   described in Section 4.4.5 so that the aggregator knows which
   aggregate share to update.

   The helper then encrypts agg_share under the collector's HPKE public
   key as described in Section 4.4.4, yielding encrypted_agg_share.
   Encryption prevents the leader from learning the actual result, as it
   only has its own aggregate share and cannot compute the helper's.

   The helper responds to the leader with HTTP status code 200 OK and a
   body consisting of an AggregateShareResp:

   struct {
       HpkeCiphertext encrypted_aggregate_share;
   } AggregateShareResp;






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   encrypted_aggregate_share.config_id is set to the collector's HPKE
   config ID. encrypted_aggregate_share.enc is set to the encapsulated
   HPKE context enc computed above and
   encrypted_aggregate_share.ciphertext is the ciphertext
   encrypted_agg_share computed above.

   After receiving the helper's response, the leader uses the
   HpkeCiphertext to respond to a collect request (see Section 4.4).

   After issuing an aggregate-share request for a given batch interval,
   it is an error for the leader to issue any more aggregate or
   aggregate-init requests for additional reports in the batch interval.
   These reports will be rejected by helpers as described Section 4.3.1.

   Before completing the collect request, the leader also computes its
   own aggregate share agg_share by aggregating all of the prepared
   output shares that fall within the batch interval.  Finally, it
   encrypts it under the collector's HPKE public key as described in
   Section 4.4.4.

4.4.3.  Collection Finalization

   Once the collector has received a successful collect response from
   the leader, it can decrypt the aggregate shares and produce an
   aggregate result.  The collector decrypts each aggregate share as
   described in Section 4.4.4.  If the collector successfully decrypts
   all aggregate shares, the collector then unshards the aggregate
   shares into an aggregate result using the VDAF's agg_shares_to_result
   algorithm.  In particular, let agg_shares denote the ordered sequence
   of aggregator shares, ordered by aggregator index, and let agg_param
   be the opaque aggregation parameter.  The final aggregate result is
   computed as follows:

   agg_result = VDAF.agg_shares_to_result(agg_param, agg_shares)

4.4.4.  Aggregate Share Encryption

   Encrypting an aggregate share agg_share for a given AggregateShareReq
   is done as follows:

enc, payload = SealBase(pk, "dap-01 aggregate share" || server_role || 0x00,
  AggregateShareReq.task_id || AggregateShareReq.batch_interval, agg_share)

   where pk is the HPKE public key encoded by the collector's HPKE key,
   server_role is 0x02 for the leader and 0x03 for a helper.






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   The collector decrypts these aggregate shares using the opposite
   process.  Specifically, given an encrypted input share, denoted
   enc_share, for a given batch interval, denoted batch_interval,
   decryption works as follows:

  agg_share = OpenBase(enc_share.enc, sk, "dap-01 aggregate share" ||
      server_role || 0x00, task_id || batch_interval, enc_share.payload)

   where sk is the HPKE secret key, task_id is the task ID for the
   collect request, and server_role is the role of the server that sent
   the aggregate share (0x02 for the leader and 0x03 for the helper).

4.4.5.  Validating Batch Parameters

   Before an aggregator responds to a collect request or aggregate-share
   request, it must first check that the request does not violate the
   parameters associated with the DAP task.  It does so as described
   here.

   First the aggregator checks that the request's batch interval
   respects the boundaries defined by the DAP task's parameters.
   Namely, it checks that both batch_interval.start and
   batch_interval.duration are divisible by min_batch_duration and that
   batch_interval.duration >= min_batch_duration.  Unless both these
   conditions are true, the aggregator MUST abort and alert its peer
   with error "batchInvalid".

   Next, the aggregator checks that the request respects the generic
   privacy parameters of the DAP task.  Let X denote the set of reports
   for which the aggregator has recovered a valid output share and which
   fall in the batch interval of the request.

   *  If len(X) < min_batch_size, then the aggregator MUST abort and
      alert its peer with "insufficientBatchSize".

   *  The aggregator keeps track of the number of times each report was
      added to the batch of an AggregateShareReq.  If any report in X
      was added to at least max_batch_lifetime previous batches, then
      the aggregator MUST abort and alert the peer with
      "batchLifetimeExceeded".

   Finally, the aggregator checks that the batch interval defined by the
   collect request satisfies one of the conditions:

   1.  The batch interval does not overlap with the batch interval of
       any prior completed collect requests.





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   2.  The batch interval, including its start and duration values,
       match a prior completed collect request.

   [[OPEN ISSUE: #195 tracks how we might relax this constraint to allow
   for more collect query flexibility.  As of now, this is quite rigid
   and doesn't give the collector much room for mistakes.]]

4.4.6.  Anti-replay

   Using a client-provided report multiple times within a single batch,
   or using the same report in multiple batches, may allow a server to
   learn information about the client's measurement, violating the
   privacy goal of DAP.  To prevent such replay attacks, this
   specification requires the aggregators to detect and filter out
   replayed reports.

   To detect replay attacks, each aggregator keeps track of the set of
   nonces pertaining to reports that were previously aggregated for a
   given task.  If the leader receives a report from a client whose
   nonce is in this set, it simply ignores it.  A helper who receives an
   encrypted input share whose nonce is in this set replies to the
   leader with an error as described in Section 4.3.1.4.

   [OPEN ISSUE: This has the potential to require aggreagtors to store
   nonce sests indefinitely.  See issue#180.]

   A malicious aggregator may attempt to force a replay by replacing the
   nonce generated by the client with a nonce its peer has not yet seen.
   To prevent this, clients incorporate the nonce into the AAD for HPKE
   encryption, ensuring that the output share is only recovered if the
   aggregator is given the correct nonce.  (See Section 4.2.2.)

   Aggregators prevent the same report from being used in multiple
   batches (except as required by the protocol) by only responding to
   valid collect requests, as described in Section 4.4.5.

5.  Operational Considerations

   The DAP protocol has inherent constraints derived from the tradeoff
   between privacy guarantees and computational complexity.  These
   tradeoffs influence how applications may choose to utilize services
   implementing the specification.









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5.1.  Protocol participant capabilities

   The design in this document has different assumptions and
   requirements for different protocol participants, including clients,
   aggregators, and collectors.  This section describes these
   capabilities in more detail.

5.1.1.  Client capabilities

   Clients have limited capabilities and requirements.  Their only
   inputs to the protocol are (1) the parameters configured out of band
   and (2) a measurement.  Clients are not expected to store any state
   across any upload flows, nor are they required to implement any sort
   of report upload retry mechanism.  By design, the protocol in this
   document is robust against individual client upload failures since
   the protocol output is an aggregate over all inputs.

5.1.2.  Aggregator capabilities

   Helpers and leaders have different operational requirements.  The
   design in this document assumes an operationally competent leader,
   i.e., one that has no storage or computation limitations or
   constraints, but only a modestly provisioned helper, i.e., one that
   has computation, bandwidth, and storage constraints.  By design,
   leaders must be at least as capable as helpers, where helpers are
   generally required to:

   *  Support the collect protocol, which includes validating and
      aggregating reports; and

   *  Publish and manage an HPKE configuration that can be used for the
      upload protocol.

   In addition, for each DAP task, helpers are required to:

   *  Implement some form of batch-to-report index, as well as inter-
      and intra-batch replay mitigation storage, which includes some way
      of tracking batch report size.  Some of this state may be used for
      replay attack mitigation.  The replay mitigation strategy is
      described in Section 4.4.6.

   Beyond the minimal capabilities required of helpers, leaders are
   generally required to:

   *  Support the upload protocol and store reports; and

   *  Track batch report size during each collect flow and request
      encrypted output shares from helpers.



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   In addition, for each DAP task, leaders are required to:

   *  Implement and store state for the form of inter- and intra-batch
      replay mitigation in Section 4.4.6; and

5.1.3.  Collector capabilities

   Collectors statefully interact with aggregators to produce an
   aggregate output.  Their input to the protocol is the task
   parameters, configured out of band, which include the corresponding
   batch window and size.  For each collect invocation, collectors are
   required to keep state from the start of the protocol to the end as
   needed to produce the final aggregate output.

   Collectors must also maintain state for the lifetime of each task,
   which includes key material associated with the HPKE key
   configuration.

5.2.  Data resolution limitations

   Privacy comes at the cost of computational complexity.  While affine-
   aggregatable encodings (AFEs) can compute many useful statistics,
   they require more bandwidth and CPU cycles to account for finite-
   field arithmetic during input-validation.  The increased work from
   verifying inputs decreases the throughput of the system or the inputs
   processed per unit time.  Throughput is related to the verification
   circuit's complexity and the available compute-time to each
   aggregator.

   Applications that utilize proofs with a large number of
   multiplication gates or a high frequency of inputs may need to limit
   inputs into the system to meet bandwidth or compute constraints.
   Some methods of overcoming these limitations include choosing a
   better representation for the data or introducing sampling into the
   data collection methodology.

   [[TODO: Discuss explicit key performance indicators, here or
   elsewhere.]]

5.3.  Aggregation utility and soft batch deadlines

   A soft real-time system should produce a response within a deadline
   to be useful.  This constraint may be relevant when the value of an
   aggregate decreases over time.  A missed deadline can reduce an
   aggregate's utility but not necessarily cause failure in the system.






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   An example of a soft real-time constraint is the expectation that
   input data can be verified and aggregated in a period equal to data
   collection, given some computational budget.  Meeting these deadlines
   will require efficient implementations of the input-validation
   protocol.  Applications might batch requests or utilize more
   efficient serialization to improve throughput.

   Some applications may be constrained by the time that it takes to
   reach a privacy threshold defined by a minimum number of reports.
   One possible solution is to increase the reporting period so more
   samples can be collected, balanced against the urgency of responding
   to a soft deadline.

5.4.  Protocol-specific optimizations

   Not all DAP tasks have the same operational requirements, so the
   protocol is designed to allow implementations to reduce operational
   costs in certain cases.

5.4.1.  Reducing storage requirements

   In general, the aggregators are required to keep state for all valid
   reports for as long as collect requests can be made for them.  In
   particular, the aggregators must store a batch as long as the batch
   has not been queried more than max_batch_lifetime times.  However, it
   is not always necessary to store the reports themselves.  For schemes
   like Prio in which the input-validation protocol is only run once per
   report, each aggregator only needs to store its aggregate share for
   each possible batch interval, along with the number of times the
   aggregate share was used in a batch.  This is due to the requirement
   that the batch interval respect the boundaries defined by the DAP
   parameters.  (See Section 4.4.5.)

6.  Compliance Requirements

   In the absence of an application or deployment-specific profile
   specifying otherwise, a compliant DAP application MUST implement the
   following HPKE cipher suite:

   *  KEM: DHKEM(X25519, HKDF-SHA256) (see [HPKE], Section 7.1)

   *  KDF: HKDF-SHA256 (see [HPKE], Section 7.2)

   *  AEAD: AES-128-GCM (see [HPKE], Section 7.3)







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7.  Security Considerations

   DAP assumes an active attacker that controls the network and has the
   ability to statically corrupt any number of clients, aggregators, and
   collectors.  That is, the attacker can learn the secret state of any
   party prior to the start of its attack.  For example, it may coerce a
   client into providing malicious input shares for aggregation or
   coerce an aggregator into diverting from the protocol specified
   (e.g., by divulging its input shares to the attacker).

   In the presence of this adversary, DAP aims to achieve the following
   high-level secure aggregation goals:

   1.  Privacy.  Clients trust that some aggregator is honest.  That is,
       as long as at least one aggregator executes the protocol
       faithfully, the parties learn nothing beyond the aggregate result
       (i.e., the output of the aggregation function computed over the
       honest measurements).

   2.  Correctness.  The collector trusts that the aggregators execute
       the protocol correctly.  That is, as long as the aggregators
       execute the protocol faithfully, a malicious client can skew the
       aggregate result only by reporting a false (untruthful)
       measurement.  The result cannot be influenced in any other way.

   Currently, the specification does not achieve these goals.  In
   particular, there are several open issues that need to be addressed
   before these goals are met.  Details for each issue are below.

   1.  When crafted maliciously, collect requests may leak more
       information about the measurements than the system intends.  For
       example, the spec currently allows sequences of collect requests
       to reveal an aggregate result for a batch smaller than the
       minimum batch size.  [OPEN ISSUE: See issue#195.  This also has
       implications for how we solve issue#183.]

   2.  Even benign collect requests may leak information beyond what one
       might expect intuitively.  For example, the Poplar1 VDAF [VDAF]
       can be used to compute the set of heavy hitters among a set of
       arbitrary bit strings uploaded by clients.  This requires
       multiple evaluations of the VDAF, the results of which reveal
       information to the aggregators and collector beyond what follows
       from the heavy hitters themselves.  Note that this leakage can be
       mitigated using differential privacy.  [OPEN ISSUE: We have yet
       not specified how to add DP.]






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   3.  The core DAP spec does not defend against Sybil attacks.  In this
       type of attack, the adversary adds to a batch a number of reports
       that skew the aggregate result in its favor.  For example: The
       result may reveal additional information about the honest
       measurements, leading to a privacy violation; or the result may
       have some property that is desirable to the adversary ("stats
       poisoning").  The upload sub-protocol includes an extensions
       mechanism that can be used to prevent --- or at least mitigate
       --- these types of attacks.  See Section 4.2.3.  [OPEN ISSUE: No
       such extension has been implemented, so we're not yet sure if the
       current mechanism is sufficient.]

   4.  Attacks may also come from the network.  Thus, it is required
       that the aggregators and collector communicate with one another
       over mutually authenticated and confidential channels.

7.1.  Threat model

   [OPEN ISSUE: This subsection is a bit out-of-date.]

   In this section, we enumerate the actors participating in the Prio
   system and enumerate their assets (secrets that are either inherently
   valuable or which confer some capability that enables further attack
   on the system), the capabilities that a malicious or compromised
   actor has, and potential mitigations for attacks enabled by those
   capabilities.

   This model assumes that all participants have previously agreed upon
   and exchanged all shared parameters over some unspecified secure
   channel.

7.1.1.  Client/user

7.1.1.1.  Assets

   1.  Unshared inputs.  Clients are the only actor that can ever see
       the original inputs.

   2.  Unencrypted input shares.

7.1.1.2.  Capabilities

   1.  Individual users can reveal their own input and compromise their
       own privacy.

   2.  Clients (that is, software which might be used by many users of
       the system) can defeat privacy by leaking input outside of the
       Prio system.



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   3.  Clients may affect the quality of aggregations by reporting false
       input.

       *  Prio can only prove that submitted input is valid, not that it
          is true.  False input can be mitigated orthogonally to the
          Prio protocol (e.g., by requiring that aggregations include a
          minimum number of contributions) and so these attacks are
          considered to be outside of the threat model.

   4.  Clients can send invalid encodings of input.

7.1.1.3.  Mitigations

   1.  The input validation protocol executed by the aggregators
       prevents either individual clients or coalitions of clients from
       compromising the robustness property.

   2.  If aggregator output satisifes differential privacy Section 7.5,
       then all records not leaked by malicious clients are still
       protected.

7.1.2.  Aggregator

7.1.2.1.  Assets

   1.  Unencrypted input shares.

   2.  Input share decryption keys.

   3.  Client identifying information.

   4.  Aggregate shares.

   5.  Aggregator identity.

7.1.2.2.  Capabilities

   1.  Aggregators may defeat the robustness of the system by emitting
       bogus output shares.

   2.  If clients reveal identifying information to aggregators (such as
       a trusted identity during client authentication), aggregators can
       learn which clients are contributing input.

       1.  Aggregators may reveal that a particular client contributed
           input.





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       2.  Aggregators may attack robustness by selectively omitting
           inputs from certain clients.

           *  For example, omitting submissions from a particular
              geographic region to falsely suggest that a particular
              localization is not being used.

   3.  Individual aggregators may compromise availability of the system
       by refusing to emit aggregate shares.

   4.  Input validity proof forging.  Any aggregator can collude with a
       malicious client to craft a proof that will fool honest
       aggregators into accepting invalid input.

   5.  Aggregators can count the total number of input shares, which
       could compromise user privacy (and differential privacy
       Section 7.5) if the presence or absence of a share for a given
       user is sensitive.

7.1.2.3.  Mitigations

   1.  The linear secret sharing scheme employed by the client ensures
       that privacy is preserved as long as at least one aggregator does
       not reveal its input shares.

   2.  If computed over a sufficient number of reports, aggregate shares
       reveal nothing about either the inputs or the participating
       clients.

   3.  Clients can ensure that aggregate counts are non-sensitive by
       generating input independently of user behavior.  For example, a
       client should periodically upload a report even if the event that
       the task is tracking has not occurred, so that the absence of
       reports cannot be distinguished from their presence.

   4.  Bogus inputs can be generated that encode "null" shares that do
       not affect the aggregate output, but mask the total number of
       true inputs.

       *  Either leaders or clients can generate these inputs to mask
          the total number from non-leader aggregators or all the
          aggregators, respectively.

       *  In either case, care must be taken to ensure that bogus inputs
          are indistinguishable from true inputs (metadata, etc),
          especially when constructing timestamps on reports.





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   [OPEN ISSUE: Define what "null" shares are.  They should be defined
   such that inserting null shares into an aggregation is effectively a
   no-op.  See issue#98.]

7.1.3.  Leader

   The leader is also an aggregator, and so all the assets, capabilities
   and mitigations available to aggregators also apply to the leader.

7.1.3.1.  Capabilities

   1.  Input validity proof verification.  The leader can forge proofs
       and collude with a malicious client to trick aggregators into
       aggregating invalid inputs.

       *  This capability is no stronger than any aggregator's ability
          to forge validity proof in collusion with a malicious client.

   2.  Relaying messages between aggregators.  The leader can compromise
       availability by dropping messages.

       *  This capability is no stronger than any aggregator's ability
          to refuse to emit aggregate shares.

   3.  Shrinking the anonymity set.  The leader instructs aggregators to
       construct output parts and so could request aggregations over few
       inputs.

7.1.3.2.  Mitigations

   1.  Aggregators enforce agreed upon minimum aggregation thresholds to
       prevent deanonymizing.

   2.  If aggregator output satisfies differential privacy Section 7.5,
       then genuine records are protected regardless of the size of the
       anonymity set.

7.1.4.  Collector

7.1.4.1.  Capabilities

   1.  Advertising shared configuration parameters (e.g., minimum
       thresholds for aggregations, joint randomness, arithmetic
       circuits).

   2.  Collectors may trivially defeat availability by discarding
       aggregate shares submitted by aggregators.




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   3.  Known input injection.  Collectors may collude with clients to
       send known input to the aggregators, allowing collectors to
       shrink the effective anonymity set by subtracting the known
       inputs from the final output.  Sybil attacks [Dou02] could be
       used to amplify this capability.

7.1.4.2.  Mitigations

   1.  Aggregators should refuse shared parameters that are trivially
       insecure (i.e., aggregation threshold of 1 contribution).

   2.  If aggregator output satisfies differential privacy Section 7.5,
       then genuine records are protected regardless of the size of the
       anonymity set.

7.1.5.  Aggregator collusion

   If all aggregators collude (e.g. by promiscuously sharing unencrypted
   input shares), then none of the properties of the system hold.
   Accordingly, such scenarios are outside of the threat model.

7.1.6.  Attacker on the network

   We assume the existence of attackers on the network links between
   participants.

7.1.6.1.  Capabilities

   1.  Observation of network traffic.  Attackers may observe messages
       exchanged between participants at the IP layer.

       1.  The time of transmission of input shares by clients could
           reveal information about user activity.

           *  For example, if a user opts into a new feature, and the
              client immediately reports this to aggregators, then just
              by observing network traffic, the attacker can infer what
              the user did.

       2.  Observation of message size could allow the attacker to learn
           how much input is being submitted by a client.

           *  For example, if the attacker observes an encrypted message
              of some size, they can infer the size of the plaintext,
              plus or minus the cipher block size.  From this they may
              be able to infer which aggregations the user has opted
              into or out of.




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   2.  Tampering with network traffic.  Attackers may drop messages or
       inject new messages into communications between participants.

7.1.6.2.  Mitigations

   1.  All messages exchanged between participants in the system should
       be encrypted.

   2.  All messages exchanged between aggregators, the collector and the
       leader should be mutually authenticated so that network attackers
       cannot impersonate participants.

   3.  Clients should be required to submit inputs at regular intervals
       so that the timing of individual messages does not reveal
       anything.

   4.  Clients should submit dummy inputs even for aggregations the user
       has not opted into.

   [[OPEN ISSUE: The threat model for Prio --- as it's described in the
   original paper and [BBCGGI19] --- considers *either* a malicious
   client (attacking soundness) *or* a malicious subset of aggregators
   (attacking privacy).  In particular, soundness isn't guaranteed if
   any one of the aggregators is malicious; in theory it may be possible
   for a malicious client and aggregator to collude and break soundness.
   Is this a contingency we need to address?  There are techniques in
   [BBCGGI19] that account for this; we need to figure out if they're
   practical.]]

7.2.  Client authentication or attestation

   [TODO: Solve issue#89]

7.3.  Anonymizing proxies

   Client reports can contain auxiliary information such as source IP,
   HTTP user agent or in deployments which use it, client authentication
   information, which could be used by aggregators to identify
   participating clients or permit some attacks on robustness.  This
   auxiliary information could be removed by having clients submit
   reports to an anonymizing proxy server which would then use Oblivous
   HTTP [I-D.thomson-http-oblivious] to forward inputs to the DAP
   leader, without requiring any server participating in DAP to be aware
   of whatever client authentication or attestation scheme is in use.







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7.4.  Batch parameters

   An important parameter of a DAP deployment is the minimum batch size.
   If an aggregation includes too few inputs, then the outputs can
   reveal information about individual participants.  Aggregators use
   the batch size field of the shared task parameters to enforce minimum
   batch size during the collect protocol, but server implementations
   may also opt out of participating in a DAP task if the minimum batch
   size is too small.  This document does not specify how to choose
   minimum batch sizes.

   The DAP parameters also specify the maximum number of times a report
   can be used.  Some protocols, such as Poplar [BBCGGI21], require
   reports to be used in multiple batches spanning multiple collect
   requests.

7.5.  Differential privacy

   Optionally, DAP deployments can choose to ensure their output F
   achieves differential privacy [Vad16].  A simple approach would
   require the aggregators to add two-sided noise (e.g. sampled from a
   two-sided geometric distribution) to outputs.  Since each aggregator
   is adding noise independently, privacy can be guaranteed even if all
   but one of the aggregators is malicious.  Differential privacy is a
   strong privacy definition, and protects users in extreme
   circumstances: Even if an adversary has prior knowledge of every
   input in a batch except for one, that one record is still formally
   protected.

   [OPEN ISSUE: While parameters configuring the differential privacy
   noise (like specific distributions / variance) can be agreed upon out
   of band by the aggregators and collector, there may be benefits to
   adding explicit protocol support by encoding them into task
   parameters.]

7.6.  Robustness in the presence of malicious servers

   Most DAP protocols, including Prio and Poplar, are robust against
   malicious clients, but are not robust against malicious servers.  Any
   aggregator can simply emit bogus aggregate shares and undetectably
   spoil aggregates.  If enough aggregators were available, this could
   be mitigated by running the protocol multiple times with distinct
   subsets of aggregators chosen so that no aggregator appears in all
   subsets and checking all the outputs against each other.  If all the
   protocol runs do not agree, then participants know that at least one
   aggregator is defective, and it may be possible to identify the
   defector (i.e., if a majority of runs agree, and a single aggregator
   appears in every run that disagrees).  See #22 (https://github.com/



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   ietf-wg-ppm/draft-ietf-ppm-dap/issues/22) for discussion.

7.7.  Infrastructure diversity

   Prio deployments should ensure that aggregators do not have common
   dependencies that would enable a single vendor to reassemble inputs.
   For example, if all participating aggregators stored unencrypted
   input shares on the same cloud object storage service, then that
   cloud vendor would be able to reassemble all the input shares and
   defeat privacy.

7.8.  System requirements

7.8.1.  Data types

8.  IANA Considerations

8.1.  Protocol Message Media Types

   This specification defines the following protocol messages, along
   with their corresponding media types types:

   *  HpkeConfig Section 4.2.1: "application/dap-hpke-config"

   *  Report Section 4.2.2: "application/dap-report"

   *  AggregateInitializeReq Section 4.4: "application/dap-aggregate-
      initialize-req"

   *  AggregateInitializeResp Section 4.4: "application/dap-aggregate-
      initialize-resp"

   *  AggregateContinueReq Section 4.4: "application/dap-aggregate-
      continue-req"

   *  AggregateContinueResp Section 4.4: "application/dap-aggregate-
      continue-resp"

   *  AggregateShareReq Section 4.4: "application/dap-aggregate-share-
      req"

   *  AggregateShareResp Section 4.4: "application/dap-aggregate-share-
      resp"

   *  CollectReq Section 4.4: "application/dap-collect-req"

   *  CollectResp Section 4.4: "application/dap-collect-resp"




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   The definition for each media type is in the following subsections.

   Protocol message format evolution is supported through the definition
   of new formats that are identified by new media types.

   IANA [shall update / has updated] the "Media Types" registry at
   https://www.iana.org/assignments/media-types with the registration
   information in this section for all media types listed above.

   [OPEN ISSUE: Solicit review of these allocations from domain
   experts.]

8.1.1.  "application/dap-hpke-config" media type

   Type name:  application

   Subtype name:  dap-hpke-config

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.1

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A



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   Author:  see Authors' Addresses section

   Change controller:  IESG

8.1.2.  "application/dap-report" media type

   Type name:  application

   Subtype name:  dap-report

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.2.2

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG







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8.1.3.  "application/dap-aggregate-continue-req" media type

   Type name:  application

   Subtype name:  dap-aggregate-initialize-req

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.4

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

8.1.4.  "application/dap-aggregate-initialize-resp" media type

   Type name:  application

   Subtype name:  dap-aggregate-initialize-resp

   Required parameters:  N/A



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   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.4

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

8.1.5.  "application/dap-aggregate-continue-req" media type

   Type name:  application

   Subtype name:  dap-aggregate-continue-req

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.4

   Interoperability considerations:  N/A



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   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

8.1.6.  "application/dap-aggregate-continue-resp" media type

   Type name:  application

   Subtype name:  dap-aggregate-continue-resp

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.4

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A



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                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

8.1.7.  "application/dap-aggregate-share-req" media type

   Type name:  application

   Subtype name:  dap-aggregate-share-req

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.4

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut



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      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

8.1.8.  "application/dap-aggregate-share-resp" media type

   Type name:  application

   Subtype name:  dap-aggregate-share-resp

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.4

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section



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   Change controller:  IESG

8.1.9.  "application/dap-collect-req" media type

   Type name:  application

   Subtype name:  dap-collect-req

   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.4

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

8.1.10.  "application/dap-collect-req" media type

   Type name:  application

   Subtype name:  dap-collect-req



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   Required parameters:  N/A

   Optional parameters:  None

   Encoding considerations:  only "8bit" or "binary" is permitted

   Security considerations:  see Section 4.4

   Interoperability considerations:  N/A

   Published specification:  this specification

   Applications that use this media type:  N/A

   Fragment identifier considerations:  N/A

   Additional information:  Magic number(s):  N/A

                            Deprecated alias names for this type:  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  see Aut
      hors' Addresses section

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  see Authors' Addresses section

   Change controller:  IESG

8.2.  Upload Extension Registry

   This document requests creation of a new registry for extensions to
   the Upload protocol.  This registry should contain the following
   columns:

   [TODO: define how we want to structure this registry when the time
   comes]








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8.3.  URN Sub-namespace for DAP (urn:ietf:params:ppm:dap)

   The following value [will be/has been] registered in the "IETF URN
   Sub-namespace for Registered Protocol Parameter Identifiers"
   registry, following the template in [RFC3553]:

   Registry name:  dap

   Specification:  [[THIS DOCUMENT]]

   Repository:  http://www.iana.org/assignments/dap

   Index value:  No transformation needed.

   Initial contents: The types and descriptions in the table in
   Section 3.1 above, with the Reference field set to point to this
   specification.

9.  Acknowledgements

   The text in Section 3 is based extensively on [RFC8555]

10.  References

10.1.  Normative References

   [HPKE]     Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.

   [I-D.thomson-http-oblivious]
              Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
              Progress, Internet-Draft, draft-thomson-http-oblivious-02,
              24 August 2021, <https://datatracker.ietf.org/doc/html/
              draft-thomson-http-oblivious-02>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/rfc/rfc2818>.







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   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
              2003, <https://www.rfc-editor.org/rfc/rfc3553>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [RFC5861]  Nottingham, M., "HTTP Cache-Control Extensions for Stale
              Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,
              <https://www.rfc-editor.org/rfc/rfc5861>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7234>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <https://www.rfc-editor.org/rfc/rfc7807>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

10.2.  Informative References

   [BBCGGI19] Boneh, D., Boyle, E., Corrigan-Gibbs, H., Gilboa, N., and
              Y. Ishai, "Zero-Knowledge Proofs on Secret-Shared Data via
              Fully Linear PCPs", 5 January 2021,
              <https://eprint.iacr.org/2019/188>.

   [BBCGGI21] Boneh, D., Boyle, E., Corrigan-Gibbs, H., Gilboa, N., and
              Y. Ishai, "Lightweight Techniques for Private Heavy
              Hitters", 5 January 2021,
              <https://eprint.iacr.org/2021/017>.

   [CGB17]    Corrigan-Gibbs, H. and D. Boneh, "Prio: Private, Robust,
              and Scalable Computation of Aggregate Statistics", 14
              March 2017, <https://crypto.stanford.edu/prio/paper.pdf>.






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   [Dou02]    Douceur, J., "The Sybil Attack", 10 October 2022,
              <https://link.springer.com/
              chapter/10.1007/3-540-45748-8_24>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/rfc/rfc8555>.

   [Vad16]    Vadhan, S., "The Complexity of Differential Privacy", 9
              August 2016,
              <https://privacytools.seas.harvard.edu/files/privacytools/
              files/complexityprivacy_1.pdf>.

   [VDAF]     Barnes, R., Patton, C., and P. Schoppmann, "Verifiable
              Distributed Aggregation Functions", Work in Progress,
              Internet-Draft, draft-irtf-cfrg-vdaf-01, 26 May 2022,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              vdaf-01>.

Authors' Addresses

   Tim Geoghegan
   ISRG
   Email: timgeog+ietf@gmail.com


   Christopher Patton
   Cloudflare
   Email: chrispatton+ietf@gmail.com


   Eric Rescorla
   Mozilla
   Email: ekr@rtfm.com


   Christopher A. Wood
   Cloudflare
   Email: caw@heapingbits.net











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