Internet DRAFT - draft-wang-ppm-dap-taskprov
draft-wang-ppm-dap-taskprov
Privacy Preserving Measurement S. Wang
Internet-Draft Apple Inc.
Intended status: Informational C. Patton
Expires: 5 August 2024 Cloudflare
2 February 2024
In-band Task Provisioning for DAP
draft-wang-ppm-dap-taskprov-06
Abstract
An extension for the Distributed Aggregation Protocol (DAP) is
specified that allows the task configuration to be provisioned in-
band.
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://wangshan.github.io/draft-wang-ppm-dap-taskprov/draft-wang-
ppm-dap-taskprov.html. Status information for this document may be
found at https://datatracker.ietf.org/doc/draft-wang-ppm-dap-
taskprov/.
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/.
Subscribe at https://www.ietf.org/mailman/listinfo/ppm/.
Source for this draft and an issue tracker can be found at
https://github.com/wangshan/draft-wang-ppm-dap-taskprov.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 5 August 2024.
Copyright Notice
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document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. The Taskprov Extension . . . . . . . . . . . . . . . . . . . 4
3.1. Deriving the Task ID . . . . . . . . . . . . . . . . . . 8
3.2. Deriving the VDAF Verification Key . . . . . . . . . . . 8
3.3. Configuring a Task . . . . . . . . . . . . . . . . . . . 9
3.4. Supporting HPKE Configurations Independent of Tasks . . . 10
4. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 10
5. Leader Behavior . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Upload Protocol . . . . . . . . . . . . . . . . . . . . . 11
5.2. Aggregate Protocol . . . . . . . . . . . . . . . . . . . 11
5.3. Collect Protocol . . . . . . . . . . . . . . . . . . . . 11
6. Helper Behavior . . . . . . . . . . . . . . . . . . . . . . . 12
7. Collector Behavior . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Operational Considerations . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
11. Normative References . . . . . . . . . . . . . . . . . . . . 14
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The DAP protocol [DAP] enables secure aggregation of a set of reports
submitted by Clients. This process is centered around a "task" that
determines, among other things, the cryptographic scheme to use for
the secure computation (a Verifiable Distributed Aggregation Function
[VDAF]), how reports are partitioned into batches, and privacy
parameters such as the minimum size of each batch. Before a task can
be executed, it is necessary to first provision the Clients,
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Aggregators, and Collector with the task's configuration.
The core DAP specification does not define a mechanism for
provisioning tasks. This document describes a mechanism designed to
fill this gap. Its key feature is that task configuration is
performed completely in-band, via HTTP request headers.
This method presumes the existence of a logical "task author"
(written as "Author" hereafter) who is capable of pushing
configurations to Clients. All parameters required by downstream
entities (the Aggregators and Collector) are encoded in an extension
field of the Client's report. There is no need for out-of-band task
orchestration between Leader and Helpers, therefore making adoption
of DAP easier.
The extension is designed with the same security and privacy
considerations of the core DAP protocol. The Author is not regarded
as a trusted third party: It is incumbent on all protocol
participants to verify the task configuration disseminated by the
Author and opt-out if the parameters are deemed insufficient for
privacy. In particular, adopters of this extension should presume
the Author is under the adversary's control. In fact, we expect in a
real-world deployment that the Author may be implemented by one of
the Aggregators or Collector.
Finally, the DAP protocol requires configuring the entities with a
variety of assets that are not task-specific, but are important for
establishing Client-Aggregator, Collector-Aggregator, and Aggregator-
Aggregator relationships. These include:
* The Collector's HPKE [RFC9180] configuration used by the
Aggregators to encrypt aggregate shares.
* Any assets required for authenticating HTTP requests.
This document does not specify a mechanism for provisioning these
assets; as in the core DAP protocol; these are presumed to be
configured out-of-band.
Note that we consider the VDAF verification key [VDAF], used by the
Aggregators to aggregate reports, to be a task-specific asset. This
document specifies how to derive this key for a given task from a
pre-shared secret, which in turn is presumed to be configured out-of-
band.
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2. 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.
This document uses the same conventions for error handling as [DAP].
In addition, this document extends the core specification by adding
the following error types:
+=============+============================================+
| Type | Description |
+=============+============================================+
| invalidTask | An Aggregator has opted out of the |
| | indicated task as described in Section 3.3 |
+-------------+--------------------------------------------+
Table 1
The terms used follow those described in [DAP]. The following new
terms are used:
Task configuration: The non-secret parameters of a task.
Task author: The entity that defines a task's configuration.
3. The Taskprov Extension
The process of provisioning a task begins when the Author
disseminates the task configuration to the Collector and each of the
Clients. When a Client issues an upload request to the Leader (as
described in Section 4.3 of [DAP]), it includes in an HTTP header the
task configuration it used to generate the report. We refer to this
process as "task advertisement". Before consuming the report, the
Leader parses the configuration and decides whether to opt-in; if
not, the task's execution halts.
Otherwise, if the Leader does opt-in, it advertises the task to the
Helpers during the aggregation protocol (Section 4.4 of [DAP]). In
particular, it includes the task configuration in an HTTP header of
each aggregation job request for that task. Before proceeding, the
Helper must first parse the configuration and decide whether to opt-
in; if not, the task's execution halts.
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To advertise a task to its peer, a Taskprov participant includes a
header "dap-taskprov" with a request incident to the task execution.
The value is the TaskConfig structure defined below, expanded into
its URL-safe, unpadded Base 64 representation as specified in
Sections 5 and 3.2 of [RFC4648].
struct {
/* Info specific for a task. */
opaque task_info<1..2^8-1>;
/* Leader API endpoint as defined in I-D.draft-ietf-ppm-dap-09. */
Url leader_aggregator_endpoint;
/* Helper API endpoint as defined in I-D.draft-ietf-ppm-dap-09. */
Url helper_aggregator_endpoint;
/* This determines the query type for batch selection and the
properties that all batches for this task must have. */
opaque query_config<1..2^16-1>;
/* Time up to which Clients are allowed to upload to this task.
Defined in I-D.draft-ietf-ppm-dap-09. */
Time task_expiration;
/* Determines the VDAF type and its config parameters. */
opaque vdaf_config<1..2^16-1>;
} TaskConfig;
The purpose of TaskConfig is to define all parameters that are
necessary for configuring an Aggregator. It includes all the fields
to be associated with a task. In addition to the Aggregator
endpoints, maximum batch query count, and task expiration, the
structure includes an opaque task_info field that is specific to a
deployment. For example, this can be a string describing the purpose
of this task.
The opaque query_config field defines the DAP query configuration
used to guide batch selection. Its content is structured as follows:
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struct {
Duration time_precision;
uint16 max_batch_query_count;
uint32 min_batch_size;
QueryType query_type;
select (QueryConfig.query_type) {
case time_interval: Empty;
case fixed_size: uint32 max_batch_size;
};
} QueryConfig;
The length prefix of the query_config ensures that the QueryConfig
structure can be decoded even if an unrecognized variant is
encountered (i.e., an unimplemented query type).
The maximum batch size for fixed_size query is optional. If
query_type is fixed_size and max_batch_size is 0, Aggregator should
provision the task without maximum batch size limit. Which means
during batch validation (Section 4.6.5.2.2 of [DAP]), Aggregator does
not check len(X) <= max_batch_size, where X is the set of reports
successfully aggregated into the batch.
The vdaf_config defines the configuration of the VDAF in use for this
task. Its content is as follows (codepoints are as defined in
[VDAF]):
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enum {
prio3_count(0x00000000),
prio3_sum(0x00000001),
prio3_sum_vec(0x00000002),
prio3_histogram(0x00000003),
poplar1(0x00001000),
(2^32-1)
} VdafType;
struct {
opaque dp_config<1..2^16-1>; /* Encoded differential privacy parameters */
VdafType vdaf_type;
select (VdafConfig.vdaf_type) {
case prio3_count:
Empty;
case prio3_sum:
uint8; /* bit length of the summand */
case prio3_sum_vec:
uint32; /* length of the vector */
uint8; /* bit length of each summand */
uint32; /* size of each proof chunk */
case prio3_histogram:
uint32; /* number of buckets */
uint32; /* size of each proof chunk */
case poplar1:
uint16; /* bit length of input string */
};
} VdafConfig;
The length prefix of the vdaf_config ensures that the VdafConfig
structure can be decoded even if an unrecognized variant is
encountered (i.e., an unimplemented VDAF).
Apart from the VDAF-specific parameters, this structure includes a
mechanism for differential privacy (DP). The opaque dp_config
contains the following structure:
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enum {
reserved(0), /* Reserved for testing purposes */
none(1),
(255)
} DpMechanism;
struct {
DpMechanism dp_mechanism;
select (DpConfig.dp_mechanism) {
case none: Empty;
};
} DpConfig;
OPEN ISSUE: Should spell out definition of DpConfig for various
differential privacy mechanisms and parameters. See draft draft
(https://github.com/wangshan/draft-wang-ppm-differential-privacy)
for discussion.
The length prefix of the dp_config ensures that the DpConfig
structure can be decoded even if an unrecognized variant is
encountered (i.e., an unimplemented DP mechanism).
The definition of Time, Duration, Url, and QueryType follow those in
[DAP].
3.1. Deriving the Task ID
When using the Taskprov extension, the task ID is computed as
follows:
task_id = SHA-256(task_config)
where task_config is the TaskConfig structure disseminated by the
Author. Function SHA-256() is as defined in [SHS].
3.2. Deriving the VDAF Verification Key
When a Leader and Helper implement the taskprov extension in the
context of a particular DAP deployment, they SHOULD compute the
shared VDAF verification key [VDAF] as described in this section.
The Aggregators are presumed to have securely exchanged a pre-shared
secret out-of-band. The length of this secret MUST be 32 bytes. Let
us denote this secret by verify_key_init.
Let VERIFY_KEY_SIZE denote the length of the verification key for the
VDAF indicated by the task configuration. (See [VDAF], Section 5.)
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The VDAF verification key used for the task is computed as follows:
verify_key = HKDF-Expand(
HKDF-Extract(
taskprov_salt, # salt
verify_key_init, # IKM
),
task_id, # info
VERIFY_KEY_SIZE, # L
)
where taskprov_salt is defined to be the SHA-256 hash of the octet
string "dap-taskprov" and task_id is as defined in Section 3.1.
Functions HKDF-Extract() and HKDF-Expand() are as defined in
[RFC5869]. Both functions are instantiated with SHA-256.
3.3. Configuring a Task
Prior to participating in a task, each protocol participant must
determine if the TaskConfig disseminated by the Author can be
configured. The participant is said to "opt in" to the task if the
derived task ID (see Section 3.1) corresponds to an already
configured task or the task ID is unrecognized and therefore
corresponds to a new task.
A protocol participant MAY "opt out" of a task if:
1. The derived task ID corresponds to an already configured task,
but the task configuration disseminated by the Author does not
match the existing configuration.
2. The VDAF, DP, or query configuration is deemed insufficient for
privacy.
3. A secure connection to one or both of the Aggregator endpoints
could not be established.
4. The task lifetime is too long.
A protocol participant MUST opt out if the task has expired or if it
does not support an indicated task parameter (e.g., VDAF, DP
mechanism, or query type).
The behavior of each protocol participant is determined by whether or
not they opt in to a task.
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3.4. Supporting HPKE Configurations Independent of Tasks
In DAP, Clients need to know the HPKE configuration of each
Aggregator before sending reports. (See HPKE Configuration Request
in [DAP].) However, in a DAP deployment that supports the Taskprov
extension, if a Client requests the Aggregator's HPKE configuration
with the task ID computed as described in Section 3.1, the task ID
may not be configured in the Aggregator yet, because the Aggregator
is still waiting for the task to be advertised by a Client.
To mitigate this issue, if an Aggregator wants to support the
Taskprov extension, it SHOULD choose which HPKE configuration to
advertise to Clients independent of the task ID. It MAY continue to
support per-task HPKE configurations for other tasks that are
configured out-of-band.
In addition, if a Client intends to advertise a task via the Taskprov
extension, it SHOULD NOT specify the task_id parameter when
requesting the HPKE configuration from an Aggregator.
4. Client Behavior
Upon receiving a TaskConfig from the Author, the Client decides
whether to opt in to the task as described in Section 3.3. If the
Client opts out, it MUST not attempt to upload reports for the task.
OPEN ISSUE: In case of opt-out, would it be useful to specify how
to report this to the Author?
Once the client opts into a task, it may begin uploading reports for
the task. Each upload request for that task MUST advertise the task
configuration. The extension codepoint taskprov MUST be offered in
the extensions field of both Leader and Helper's PlaintextInputShare.
In addition, each report's task ID MUST be computed as described in
Section 3.1.
The taskprov extension type is defined as follows:
enum {
taskprov(0xff00),
(65535)
} ExtensionType;
The extension data in report share for taskprov MUST be zero length.
The task config is transported by the "dap-taskprov" header.
5. Leader Behavior
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5.1. Upload Protocol
Upon receiving a task advertisement from a Client, if the Leader does
not support the extension, it will ignore the HTTP header. In
particular, if the task ID is not recognized, then it MUST abort the
upload request with "unrecognizedTask".
Otherwise, if the Leader does support the extension, it first
attempts to parse the "dap-taskprov" HTTP header payload. If parsing
fails, it MUST abort with "invalidMessage".
Next, it checks that the task ID indicated by the upload request
matches the task ID derived from the extension payload as specified
in Section 3.1. If the task ID does not match, then the Leader MUST
abort with "unrecognizedTask".
The Leader then decides whether to opt in to the task as described in
Section 3.3. If it opts out, it MUST abort the upload request with
"invalidTask".
OPEN ISSUE: In case of opt-out, would it be useful to specify how
to report this to the Author?
Finally, once the Leader has opted in to the task, it completes the
upload request as usual.
During the upload flow, if the Leader's report share does not present
a taskprov extension type, Leader MUST abort the upload request with
"invalidMessage".
5.2. Aggregate Protocol
When the Leader opts in to a task, it SHOULD derive the VDAF
verification key for that task as described in Section 3.2. The
Leader MUST advertise the task to the Helper in every request
incident to the task as described in Section 3.
5.3. Collect Protocol
The Collector might issue a collect request for a task provisioned by
the Taskprov extension prior to opting in to the task. In this case,
the Leader would need to abort the collect request with
"unrecognizedTask". When it does so, it is up to the Collector to
retry its request.
OPEN ISSUE: This semantics is awkward, as there's no way for the
Leader to distinguish between Collectors who support the extension
and those that don't.
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The Leader MUST advertise the task in every aggregate share request
issued to the Helper as described in Section 3.
6. Helper Behavior
Upon receiving a task advertisement from the Leader, If the Helper
does not support the Taskprov extension, it will ignore the "dap-
taskprov" HTTP header and process the aggregate request as usual. In
particular, if the Helper does not recognize the task ID, it MUST
abort the aggregate request with error "unrecognizedTask".
Otherwise, if the Helper supports the extension, it proceeds as
follows.
First, the Helper attempts to parse payload of the "dap-taskprov"
HTTP header. If this step fails, the Helper MUST abort with
"invalidMessage".
Next, the Helper checks that the task ID indicated in the upload
request matches the task ID derived from the TaskConfig as defined in
Section 3.1. If not, the Helper MUST abort with "unrecognizedTask".
Next, the Helper decides whether to opt in to the task as described
in Section 3.3. If it opts out, it MUST abort the aggregation job
request with "invalidTask".
OPEN ISSUE: In case of opt-out, would it be useful to specify how
to report this to the Author?
Finally, the Helper completes the request as usual, deriving the VDAF
verification key for the task as described in Section 3.2. For any
report share that does not include the taskprov extension with an
empty payload, the Helper MUST mark the report as invalid with error
"invalid_message" and reject it.
7. Collector Behavior
Upon receiving a TaskConfig from the Author, the Collector first
decides whether to opt in to the task as described in Section 3.3.
If the Collector opts out, it MUST not attempt to upload reports for
the task.
Otherwise, once opted in, the Collector MAY begin to issue collect
requests for the task. The task ID for each request MUST be derived
from the TaskConfig as described in Section 3.3. The Collector MUST
advertise the task as described in Section 3.
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If the Leader responds to a collect request with an
"unrecognizedTask" error, the Collector MAY retry its collect request
after waiting for a duration. header.
8. Security Considerations
This document has the same security and privacy considerations as the
core DAP specification. In particular, for privacy we consider the
Author to be under control of the adversary. It is therefore
incumbent on protocol participants to verify the privacy parameters
of a task before opting in.
In addition, the Taskprov extension is designed to maintain
robustness even when the Author misbehaves, or is merely
misconfigured. In particular, if the Clients and Aggregators have an
inconsistent view of the the task configuration, then aggregation of
reports will fail. This is guaranteed by the binding of the task ID
(derived from the task configuration) to report shares provided by
HPKE encryption. Furthermore, the presence of taskprov extension
type in the report share means Aggregators that do not recognize the
Taskprov extension will abort with invalidMessage, as described in
(Section 4.4.3 of [DAP]). This prevents a malicious Author from
provisioning a modified task to each party with other means, which
can lead to compromised privacy guarantee in aggregate result.
OPEN ISSUE: What if the Collector and Aggregators don't agree on
the task configuration? Decryption should fail.
A malicious coalition of Clients might attempt to pollute an
Aggregator's long-term storage by uploading reports for many
(thousands or perhaps millions) of distinct tasks. While this does
not directly impact tasks used by honest Clients, it does present a
Denial-of-Service risk for the Aggregators themselves.
TODO: Suggest mitigations for this. Perhaps the Aggregators need
to keep track of how many tasks in total they are opted in to?
9. Operational Considerations
The taskprov extension is designed so that the Aggregators do not
need to store individual task configurations long-term. Because the
task configuration is advertised in each request in the upload,
aggregation, and colletion protocols, the process of opting-in and
deriving the task ID and VDAF verify key can be re-run on the fly for
each request. This is useful if a large number of concurrent tasks
are expected.
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Once an Aggregator has opted-in to a task, the expectation is that
the task is supported until it expires. In particular, Aggregators
that operate in this manner MUST NOT opt out once they have opted in.
10. IANA Considerations
NOTE(cjpatton) Eventually we'll have IANA considerations (at the
very least we'll need to allocate a codepoint) but we can leave
this blank for now.
11. Normative References
[DAP] Geoghegan, T., Patton, C., Pitman, B., Rescorla, E., and
C. A. Wood, "Distributed Aggregation Protocol for Privacy
Preserving Measurement", Work in Progress, Internet-Draft,
draft-ietf-ppm-dap-09, 18 December 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-ppm-dap-
09>.
[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>.
[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>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/rfc/rfc5869>.
[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>.
[RFC9180] 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>.
[SHS] "Secure Hash Standard", FIPS PUB 180-4 , 4 August 2015.
[VDAF] Barnes, R., Cook, D., Patton, C., and P. Schoppmann,
"Verifiable Distributed Aggregation Functions", Work in
Progress, Internet-Draft, draft-irtf-cfrg-vdaf-08, 20
November 2023, <https://datatracker.ietf.org/doc/html/
draft-irtf-cfrg-vdaf-08>.
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Contributors
CP: Unless the order is meaningful, consider alphabetizing these
names.
Junye Chen Apple Inc. junyec@apple.com
Suman Ganta Apple Inc. sganta2@apple.com
Gianni Parsa Apple Inc. gianni_parsa@apple.com
Michael Scaria Apple Inc. mscaria@apple.com
Kunal Talwar Apple Inc. ktalwar@apple.com
Christopher A. Wood Cloudflare caw@heapingbits.net
Authors' Addresses
Shan Wang
Apple Inc.
Email: shan_wang@apple.com
Christopher Patton
Cloudflare
Email: chrispatton+ietf@gmail.com
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