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Security Privacy Preserving Measurement This document defines a common test interface for implementations of the Distributed Aggregation Protocol for Privacy Preserving Measurement (DAP) and describes how this test interface can be used to perform interoperation testing between the implementations. Tests are orchestrated with containers, and new test-only APIs are introduced to provision DAP tasks and initiate processing. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Privacy Preserving Measurement Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction This document defines a common test interface for implementations of the Distributed Aggregation Protocol for Privacy Preserving Measurement . This test interface facilitates interoperation tests between different participating DAP implementations. As DAP has four distinct protocol roles, (client, leader aggregator, helper aggregator, and collector) manual interoperation testing between all combinations of even a small number of DAP implementations could be taxing. The goal of this document's common test interface is to enable automation of these interoperation tests, so that different participating implementations can be exchanged for each other, and the same test suite can be re-run on different combinations of implementations. Simplifying interoperation testing will aid in identifying errors in implementations, identifying ambiguities in the protocol specification, and reducing regressions in implementations. Taking inspiration from QuicInteropRunner , each participating implementation provides one or more container images adhering to a common interface. A test runner will start one container for each protocol participant, configure networking between the containers, and send various HTTP API requests. As part of this common testing interface, the HTTP servers in the containers will support some new test-only HTTP APIs, which will allow the test runner to provision shared task parameters and secrets, as well as trigger the start of different sub-protocols.

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 when, and only when, they appear in all capitals, as shown here.

Container Interface Each participating DAP implementation may provide one or more container images, one for each protocol role it implements. (client, leader, helper, and collector) A list of available container images will be maintained for each role. Implementations may want to submit a single aggregator image in both the leader list and helper list. The test runner will fetch each container using the given repository, image name, and tag. When the container's entry point executable is run, it SHALL start up an HTTP server listening on port 8080. In all cases, the container will serve the endpoints described in (particularly, the subsection or subsections appropriate to its protocol role). In the case of a helper or leader container, it SHALL also serve the endpoints specified by on a port (which MAY be the same port 8080 as used to serve the interoperation test API) at some relative path. The container should run indefinitely, and the test runner will terminate the container on completion of the test case. (While DAP requires HTTPS connections, only using HTTP between containers simplifies test setup. Putting TLS client/server interop out-of-scope for these tests is acceptable, as it's not of interest.) Log output SHOULD be captured into the directory "/logs" inside the container. This will be copied out to the host for inspection on completion of the test case. No environment variables or volume mounts will be provided to the containers.

Interoperation Test API Each container will have an HTTP server listening on port 8080 for commands from the test runner. All requests MUST use the HTTP method POST. Requests and responses for each endpoint listed below SHALL be encoded JSON objects , with media type application/json. All binary blobs (i.e. task IDs, batch IDs, HPKE configurations, and verification keys) SHALL be encoded as strings with base64url , inside the JSON objects. Any integer values in the parameters, measurement, or aggregate result of a will be encoded as strings in base 10 instead of as numbers. This avoids incompatibilities due to limitations on the range of JSON numbers that different implementations can process. Each of these test APIs should return a status code of 200 OK if the command was received, recognized, and parsed successfully, regardless of whether any underlying DAP request succeeded or failed. The DAP-level success or failure will be included in the test API response body. If a request is made to an endpoint starting with "/internal/test/", but not listed here, a status code of 404 Not Found SHOULD be returned, to simplify the introduction of new test APIs.

Common Structures

VDAF In multiple APIs defined below, the test runner will send the name of a , along with the parameters necessary to fully specify the VDAF. These will be stored in a nested object, with the following attributes (new type values and new keys will be added as new VDAFs are defined). VDAF JSON object structure

Key	Value
type	One of "Prio3Aes128Count", "Prio3Aes128CountVec", "Prio3Aes128Sum", or "Prio3Aes128Histogram"
length (only present if type is "Prio3Aes128CountVec")	The length of the vectors being summed, (encoded in base 10 as a string) used to parameterize the Prio3Aes128CountVec VDAF.
bits (only present if type is "Prio3Aes128Sum")	The bit width of the integers being summed, (encoded in base 10 as a string) used to parameterize the Prio3Aes128Sum VDAF.
buckets (only present if type is "Prio3Aes128Histogram")	An array of histogram bucket boundaries, (encoded in base 10 as strings) used to parameterize the Prio3Aes128Histogram VDAF.

Query In multiple APIs defined below, the test runner will need to send a query type, and in one API, it will need to send a query type along with the associated query parameters. Query types are represented in API requests as numbers, following the values of the QueryType enum in . Queries are represented in API requests as a nested object, with the following attributes (new keys will be added as new query types are defined). Query JSON object structure

Key	Value
type	A number, representing a query type, as described above.
batch_interval_start (only present if type is 1, for time interval queries)	The start of the batch interval, represented as a number equal to the number of seconds since the UNIX epoch.
batch_interval_duration (only present if type is 1, for time interval queries)	The duration of the batch interval in seconds, as a number.
batch_id (only present if type is 2, for fixed size queries)	A base64url-encoded DAP BatchID.

Client

/internal/test/ready The test runner will POST an empty object (i.e. {}) to this endpoint to check if the client container is ready to serve requests. If it is ready, it MUST return a status code of 200 OK.

/internal/test/upload Upon receipt of this command, the client container will construct a DAP report with the given configuration and measurement, and submit it. The client container will send its response to the test runner once report submission has either succeeded or permanently failed. Request JSON object structure

Key	Value
task_id	A base64url-encoded DAP TaskId.
leader	The leader's endpoint URL.
helper	The helper's endpoint URL.
vdaf	An object, with the layout given in . This determines the VDAF to be used when constructing a report.
measurement	If the VDAF's type is "Prio3Aes128Count": "0" or "1". If the VDAF's type is "Prio3Aes128CountVec": an array of strings, each of which is "0" or "1". If the VDAF's type is "Prio3Aes128Sum": a string (representing an integer in base 10). If the VDAF's type is "Prio3Aes128Histogram": a string (representing an integer in base 10).
time (optional)	If present, this provides a substitute time value that should be used when constructing the report. If not present, the current system time should be used, as per normal. The time is represented as a number, with a value of the number of seconds since the UNIX epoch.
time_precision	A number, providing the precision in seconds of report timestamps.

Response JSON object structure

Key	Value
status	"success" if the report was submitted to the leader successfully, or "error" otherwise.
error (optional)	An optional error message, to assist in troubleshooting. This will be included in the test runner logs.

Aggregator (Leader or Helper)

/internal/test/ready The test runner will POST an empty object (i.e. {}) to this endpoint to check if the aggregator container is ready to serve requests. If it is ready, it MUST return a status code of 200 OK.

/internal/test/endpoint_for_task Request the base URL for DAP endpoints for a new task. This API will be invoked immediately before /internal/test/add_task (see ), to determine the endpoint URLs of the aggregators. If the aggregator uses a common set of DAP endpoints for all tasks, it could always return the same value, such as the relative URL /. Alternately, implementations may wish to generate new endpoints for each task, derive the endpoint based on the TaskId, etc. The test runner will provide the hostname at which the aggregator is externally reachable. If the aggregator returns a relative URL, the test runner will combine it with the hostname into an absolute URL, assuming that the port is 8080. Otherwise, the aggregator can incorporate the hostname into an absolute URL and return that. Request JSON object structure

Key	Value
task_id	A base64url-encoded DAP TaskId.
role	Either "leader" or "helper".
hostname	This aggregator's hostname in the interoperation test environment. This may optionally be used in constructing the endpoint URL as an absolute URL.

Response JSON object structure

Key	Value
status	"success" if the endpoint was successfully selected or set up, or "error" otherwise.
error (optional)	An optional error message, to assist in troubleshooting. This will be included in the test runner logs.
endpoint	A relative or absolute URL, specifying the DAP aggregator endpoint that should be used for this task. If the test runner receives a relative URL, it will transform it into an absolute URL before performing the next phase of task setup.

/internal/test/add_task Register a task with the aggregator, with the given configuration and secrets. The HPKE keypair generated for this task should use the mandatory-to-implement algorithms in section 6 of , for broad compatibility. Request JSON object structure

Key	Value
task_id	A base64url-encoded DAP TaskId.
leader	The leader's endpoint URL. The test runner will ensure this is an absolute URL.
helper	The helper's endpoint URL. The test runner will ensure this is an absolute URL.
vdaf	An object, with the layout given in . This determines the task's VDAF.
leader_authentication_token	The authentication token that is shared with the other aggregator, as a string. This string MUST be safe for use as an HTTP header value. When the leader sends HTTP requests to the helper, it MUST include this value in a header named DAP-Auth-Token.
collector_authentication_token (only present if role is "leader")	The authentication token that is shared between the leader and collector, as a string. This string MUST be safe for use as an HTTP header value. When the collector sends HTTP requests to the leader, it MUST include this value in a header named DAP-Auth-Token.
role	Either "leader" or "helper".
verify_key	The verification key shared by the two aggregators, encoded with base64url.
max_batch_query_count	A number, providing the maximum number of times any report can be included in a collect request.
query_type	A number, representing the task's query type, as described in .
min_batch_size	A number, providing the minimum number of reports that must be in a batch for it to be collected.
max_batch_size (only present if query_type is 2, for fixed size queries)	A number, providing the maximum number of reports that may be in a batch for it to be collected.
time_precision	A number, providing the precision in seconds of report timestamps. For tasks using the time interval query type, the batch interval's duration will always be a multiple of this value.
collector_hpke_config	The collector's HPKE configuration, encoded in base64url, for encryption of aggregate shares.
task_expiration	A number, providing the time when clients are no longer expected to upload to this task. This is represented as a number of seconds since the UNIX epoch.

Response JSON object structure

Key	Value
status	"success" if the task was successfully set up, or "error" otherwise. (for example, if the VDAF was not supported)
error (optional)	An optional error message, to assist in troubleshooting. This will be included in the test runner logs.

Leader

/internal/test/fetch_batch_ids Retrieve the identifiers of every batch generated by the leader from a task's reports. This is only applicable to tasks with a fixed size query type. Request JSON object structure

Key	Value
task_id	A base64url-encoded DAP TaskId.

Response JSON object structure

Key	Value
status	"success" if the task and its batch IDs were successfully looked up, or "error" otherwise.
error (optional)	An optional error message, to assist in troubleshooting. This will be included in the test runner logs.
batch_ids (if successful)	An array of strings, where each string is a base64url-encoded DAP BatchID.

Collector

/internal/test/ready The test runner will POST an empty object (i.e. {}) to this endpoint to check if the collector container is ready to serve requests. If it is ready, it MUST return a status code of 200 OK.

/internal/test/add_task Register a task with the collector, with the given configuration. Returns the collector's HPKE configuration for this task. The HPKE keypair generated for this task should use the mandatory-to-implement algorithms in section 6 of , for broad compatibility. Request JSON object structure

Key	Value
task_id	A base64url-encoded DAP TaskId.
leader	The leader's endpoint URL.
vdaf	An object, with the layout given in . This determines the task's VDAF.
collector_authentication_token	The authentication token that is shared between the leader and collector, as a string. This string MUST be safe for use as an HTTP header value. When the collector sends HTTP requests to the leader, it MUST include this value in a header named DAP-Auth-Token.
query_type	A number, representing the task's query type, as described in .

Response JSON object structure

Key	Value
status	"success" if the task was successfully set up, or "error" otherwise. (for example, if the VDAF was not supported)
error (optional)	An optional error message, to assist in troubleshooting. This will be included in the test runner logs.
collector_hpke_config (if successful)	The collector's HPKE configuration, encoded in base64url, for encryption of aggregate shares.

/internal/test/collect_start Send a collect request to the leader with the provided parameters, and return a handle to the test runner identifying this collect request. The test runner will provide this handle to the collector in subsequent /internal/test/collect_poll requests (see ). Request JSON object structure

Key	Value
task_id	A base64url-encoded DAP TaskId.
agg_param	A base64url-encoded aggregation parameter.
query	An object, with the layout given in . This provides the collect request's query, and in turn determines which reports should be included.

Response JSON object structure

Key	Value
status	"success" if the collect request succeeded, or "error" otherwise.
error (optional)	An optional error message, to assist in troubleshooting. This will be included in the test runner logs.
handle (if successful)	A handle produced by the collector to refer to this collect request. This must be a string.

/internal/test/collect_poll Upon receiving this command, the collector will poll the leader's collect URL for the collect job associated with the provided handle, and provide the status and result to the test runner. Request JSON object structure

Key	Value
handle	The handle for a collect request from a previous invocation of /internal/test/collect_start. (see )

Response JSON object structure

Key	Value
status	Either "complete" if the result was returned, "in progress" if the result was not yet ready, or "error" if an error occurred.
error (optional)	An optional error message, to assist in troubleshooting. This will be included in the test runner logs.
report_count (if complete)	A number, reflecting the count of client reports included in this aggregated result.
result (if complete)	The result of the aggregation. If the VDAF is of type Prio3Aes128Count or Prio3Aes128Sum, this will be a string, representing an integer in base 10. If the VDAF is of type Prio3Aes128CountVec or Prio3Aes128Histogram, this will be an array of strings, each representing an integer in base 10.

Heavy Hitters Once Poplar1 reaches a future draft of , additional test APIs for collector containers should be introduced to perform an entire Heavy Hitters computation on a given Poplar1 task and collection interval, encompassing multiple collect flows automatically initiated by the collector.

Test Cases Test cases could be written to cover the following scenarios.

• Test successful aggregations with each VDAF.
• Test an aggregation over a few hundred or thousand reports, to exercise the aggregators' division of reports into aggregation jobs.
• Test that uploading a report with a time far in the future is rejected.
• Confirm that leaders and helpers reject requests with respective authentication tokens that are incorrect.
• Test enforcement of max_batch_query_count by making overlapping collect requests.
• Perform an entire aggregation and collect flow, attempt to upload a late report that falls into the same collect interval, and test that performing the collect request a second time yields the same result.
• Attempt to upload a canned report from the test runner more than once, and confirm that anti-replay measures were effective by inspecting the aggregation result.

Other Test Considerations All test cases should automatically fail after a generous timeout. It is the responsibility of the test runner to wait for all containers to start up and respond successfully to a request to /internal/test/ready before sending any further commands. Aggregator URLs will be constructed by the test runner with hostnames that resolve to the respective containers within the container network. Once a future draft solves the issue of retries in the aggregate flow, a reverse proxy could be introduced in front of each aggregator to inject failures when sending requests or responses, to test the protocol's resilience. (It is known such a test would fail with the current protocol.)

Test Runner Operation The following sequence outlines how the test runner will use the above APIs on port 8080 of each container to perform a typical integration test, executing a successful aggregation.

1. Create and start containers.
2. Set up networking between containers.
3. Try sending /internal/test/ready requests to each container, and retry until they succeed.
4. Generate a random TaskId, random authentication tokens, and a VDAF verification key.
5. Send a /internal/test/endpoint_for_task request () to the leader.
6. Send a /internal/test/endpoint_for_task request () to the helper.
7. Construct aggregator URLs using the above responses.
8. Send a /internal/test/add_task request () to the collector. (the collector generates an HPKE key pair as a side-effect)
9. Send a /internal/test/add_task request () to the leader.
10. Send a /internal/test/add_task request () to the helper.
11. Send one or more /internal/test/upload requests () to the client.
12. If the task has a fixed size query type, send a /internal/test/fetch_batch_ids request () to the leader.
13. Send one or more /internal/test/collect_start requests () to the collector. (this provides a handle for use in the next step)
14. Send /internal/test/collect_poll requests () to the collector, polling until each collection is completed. (the collector will provide the calculated aggregate results)
15. Stop containers.
16. Copy logs out of each container.
17. Delete containers, and clean up container networking resources.

Implementation Status and currently implement a version of this test interface. is a reference implementation of a test runner using this interface. Additional DAP implementations would be warmly welcomed.

Security Considerations Any DAP implementation that adopts this testing interface should ensure that the test-only APIs described herein are only present in software used for testing purposes, and not in production systems.

IANA Considerations This document has no IANA actions.

References Normative References ISRG Cloudflare Mozilla Cloudflare 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. Cisco Cloudflare Google This document describes Verifiable Distributed Aggregation Functions (VDAFs), a family of multi-party protocols for computing aggregate statistics over user measurements. These protocols are designed to ensure that, as long as at least one aggregation server executes the protocol honestly, individual measurements are never seen by any server in the clear. At the same time, VDAFs allow the servers to detect if a malicious or misconfigured client submitted an input that would result in an incorrect aggregate result. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes the framework for an RFC Series and an RFC Editor function that incorporate the principles of organized community involvement and accountability that has become necessary as the Internet technical community has grown, thereby enabling the RFC Series to continue to fulfill its mandate. This document obsoletes RFC 4844. This document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK] Informative References

Acknowledgments Thanks to Brandon Pitman, Christopher Patton, and Tim Geoghegan for feedback and contributions.