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Fix all build warnings, remove references to closed issues
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* Add a submission type (IETF).
* Remove unused references.
* Remove square brackets, which markdown wants to interpret as a link.
  In one case we escape the brackets, e.g., `\[`, `\]`, in other cases
  we rework the text.
* Fix all instances of art being too wide.
* Remove OPEN ISSUE about the size of the HPKE config ID, as there's
  no reason to change this at this point.
* Remove OPEN ISSUE referencing #217 (relax ordering of prep inits).
* Remove TODO referencing #384 (latest discussion is #581).
* Remove OPEN ISSUE referencing #438 (Leader signals rejection).
* Remove OPEN ISSUE referencing #401 (aggregation job rewinding).
* Remove OPEN ISSUE referencing #195 (collection flexibility).
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121 changes: 41 additions & 80 deletions draft-ietf-ppm-dap.md
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Expand Up @@ -3,6 +3,7 @@ title: "Distributed Aggregation Protocol for Privacy Preserving Measurement"
abbrev: DAP-PPM
docname: draft-ietf-ppm-dap-latest
category: std
submissiontype: IETF

venue:
group: "Privacy Preserving Measurement"
Expand Down Expand Up @@ -48,36 +49,6 @@ author:

informative:

CGB17:
title: "Prio: Private, Robust, and Scalable Computation of Aggregate Statistics"
date: 2017-03-14
target: "https://crypto.stanford.edu/prio/paper.pdf"
author:
- ins: H. Corrigan-Gibbs
- ins: D. Boneh

BBCGGI19:
title: "Zero-Knowledge Proofs on Secret-Shared Data via Fully Linear PCPs"
date: 2021-01-05
target: "https://eprint.iacr.org/2019/188"
author:
- ins: D. Boneh
- ins: E. Boyle
- ins: H. Corrigan-Gibbs
- ins: N. Gilboa
- ins: Y. Ishai

BBCGGI21:
title: "Lightweight Techniques for Private Heavy Hitters"
date: 2021-01-05
target: "https://eprint.iacr.org/2021/017"
author:
- ins: D. Boneh
- ins: E. Boyle
- ins: H. Corrigan-Gibbs
- ins: N. Gilboa
- ins: Y. Ishai

Dou02:
title: "The Sybil Attack"
date: 2022-10-10
Expand Down Expand Up @@ -399,7 +370,6 @@ Report:
Report Share:
: An encrypted input share comprising a piece of a report.


{:br}

# Overview {#overview}
Expand Down Expand Up @@ -433,7 +403,7 @@ provides two important properties:

The overall system architecture is shown in {{dap-topology}}.

~~~~
~~~
+--------+
| |
| Client +----+
Expand All @@ -453,7 +423,7 @@ The overall system architecture is shown in {{dap-topology}}.
+--------+ | Helper |
| |
+------------+
~~~~
~~~
{: #dap-topology title="System Architecture"}

The main participants in the protocol are as follows:
Expand Down Expand Up @@ -483,6 +453,7 @@ Helper:
designed so that the Helper is relatively lightweight, with most of the
operational burden borne by the Leader.


{:br}

The basic unit of DAP is the "task" which represents a single measurement
Expand Down Expand Up @@ -632,7 +603,7 @@ refer to error types, rather than the full URNs. For example, an "error of type
'invalidMessage'" refers to an error document with "type" value
"urn:ietf:params:ppm:dap:error:invalidMessage".

This document uses the verbs "abort" and "alert with [some error message]" to
This document uses the verbs "abort" and "alert with \[some error message\]" to
describe how protocol participants react to various error conditions. This
implies HTTP status code 400 Bad Request unless explicitly specified otherwise.

Expand Down Expand Up @@ -686,7 +657,8 @@ variant {
/* Field exists regardless of variant */
uint32 always_present;
ExampleEnum type = number;
/* Only fields included in the type == number variant is described */
/* Only fields included in the `type == number`
variant is described */
uint32 a_number;
} ExampleStruct;
~~~
Expand Down Expand Up @@ -735,14 +707,15 @@ uint64 Duration; /* Number of seconds elapsed between two instants */

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

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

/* An ID used to uniquely identify a report in the context of a DAP task. */
/* An ID used to uniquely identify a report in the context of a
DAP task. */
opaque ReportID[16];

/* The various roles in the DAP protocol. */
Expand Down Expand Up @@ -991,8 +964,6 @@ uint16 HpkeKemId; /* Defined in [HPKE] */
uint16 HpkeKdfId; /* Defined in [HPKE] */
~~~

[OPEN ISSUE: Decide whether to expand the width of the id.]

Aggregators MUST allocate distinct id values for each `HpkeConfig` in an
`HpkeConfigList`.

Expand Down Expand Up @@ -1451,8 +1422,6 @@ The Helper's response will be an `AggregationJobResp` message (see
the same report IDs in the same order as the Leader's `AggregationJobInitReq`.
Otherwise, the Leader MUST abort the aggregation job.

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

Otherwise, the Leader proceeds as follows with each report:

1. If the inbound prep response has type "continue", then the Leader computes
Expand Down Expand Up @@ -1565,11 +1534,12 @@ For all other reports it initializes the VDAF prep state as follows (let
`inbound` denote the payload of the prep step sent by the Leader):

~~~
(state, outbound) = Vdaf.ping_pong_helper_init(vdaf_verify_key,
agg_param,
report_id,
public_share,
plaintext_input_share.payload)
(state, outbound) = Vdaf.ping_pong_helper_init(
vdaf_verify_key,
agg_param,
report_id,
public_share,
plaintext_input_share.payload)
~~~

where:
Expand Down Expand Up @@ -1708,11 +1678,8 @@ following checks:
responded to an `AggregateShareReq` message from the Leader. A batch is
determined by query ({{batch-mode}}) conveyed in these messages. Queries
must satisfy the criteria covered in {{batch-validation}}. These criteria
are meant to restrict queries in a way make it easy to determine wither a
report pertains to a batch that was collected.

[TODO: If a section to clarify report and batch states is added this can be
removed. See Issue #384]
are meant to restrict queries in a way that makes it easy to determine
wither a report pertains to a batch that was collected.

1. Finally, if an Aggregator cannot determine if an input share is valid, it
MUST mark the input share as invalid with error `report_dropped`. For
Expand Down Expand Up @@ -1777,8 +1744,6 @@ exactly the same report IDs in the same order as the Leader's
`AggregationJobContinueReq`. Otherwise, the Leader MUST abort the aggregation
job.

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

Otherwise, the Leader proceeds as follows with each report:

1. If the inbound prep response type is "continue" and the state is
Expand Down Expand Up @@ -1835,9 +1800,6 @@ to the previous request, then the Helper MAY abort with error `invalidMessage`.
(Note that a report may be missing, in which case the Helper should assume the
Leader rejected it.)

[OPEN ISSUE: Issue 438: It may be useful for the Leader to explicitly signal
rejection.]

Next, the Helper checks if the continuation step indicated by the request is
correct. (For the first `AggregationJobContinueReq` the value should be `1`;
for the second the value should be `2`; and so on.) If the Leader is one step
Expand Down Expand Up @@ -1922,11 +1884,6 @@ it was re-sent (i.e., the two messages must be identical). This prevents the
Leader from re-winding an aggregation job and re-running an aggregation step
with different parameters.

[[OPEN ISSUE: Allowing the Leader to "rewind" aggregation job state of the
Helper may allow an attack on privacy. For instance, if the VDAF verification
key changes, the prep shares in the Helper's response would change even if the
consistency check is made. Security analysis is required. See #401.]]

One way the Helper could address this would be to store a digest of the Leader's
request, indexed by aggregation job ID and step, and refuse to service a request
for a given aggregation step unless it matches the previously seen request (if
Expand Down Expand Up @@ -2224,8 +2181,11 @@ Encrypting an aggregate share `agg_share` for a given `AggregateShareReq` is
done as follows:

~~~
enc, payload = SealBase(pk, "dap-11 aggregate share" || server_role || 0x00,
agg_share_aad, agg_share)
(enc, payload) = SealBase(
pk,
"dap-11 aggregate share" || server_role || 0x00,
agg_share_aad,
agg_share)
~~~

where `pk` is the HPKE public key encoded by the Collector's HPKE key,
Expand Down Expand Up @@ -2254,8 +2214,12 @@ Specifically, given an encrypted input share, denoted `enc_share`, for a given
batch selector, decryption works as follows:

~~~
agg_share = OpenBase(enc_share.enc, sk, "dap-11 aggregate share" ||
server_role || 0x00, agg_share_aad, enc_share.payload)
agg_share = OpenBase(
enc_share.enc,
sk,
"dap-11 aggregate share" || server_role || 0x00,
agg_share_aad,
enc_share.payload)
~~~

where `sk` is the HPKE secret key, `server_role` is the Role of the server that
Expand Down Expand Up @@ -2309,10 +2273,6 @@ does not overlap with the batch interval of any previous query. If this batch
interval check fails, then the Aggregator MAY abort with error of type
"batchOverlap".

[[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.]]

#### Time-interval Batch Mode {#time-interval-batch-validation}

##### Boundary Check
Expand Down Expand Up @@ -2436,7 +2396,7 @@ computation cost of aggregation in a few ways, such as producing aggregates over
a sample of the data or choosing a representation of the data permitting a
simpler aggregation scheme.

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

## Aggregation Utility and Soft Batch Deadlines

Expand Down Expand Up @@ -2752,11 +2712,12 @@ 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.
IANA shall update (RFC EDITOR: change "shall update" to "has update") 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.]
> TODO Solicit review of these allocations from domain experts.

### Message versioning

Expand Down Expand Up @@ -3424,14 +3385,14 @@ following sections.
This document requests creation of a new registry for Batch Modes. This registry
should contain the following columns:

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

### 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]
> TODO Define how we want to structure this registry.

### Prepare Error Registry {#prepare-error-reg}

Expand All @@ -3452,9 +3413,9 @@ with this document as the reference.

## URN Sub-namespace for DAP (urn:ietf:params:ppm:dap) {#urn-space}

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}}:
The following value will be (RFC EDITOR: change "will be" to "has been")
registered in the "IETF URN Sub-namespace for Registered Protocol
Parameter Identifiers" registry, following the template in {{!RFC3553}}:

~~~
Registry name: dap
Expand Down

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