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Security Privacy Preserving Measurement next generation unicorn sparkling distributed ledger An extension for the Distributed Aggregation Protocol (DAP) is specified that allows the task configuration to be provisioned in-band. 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 The DAP protocol 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 ), 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, 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. It relies solely on the upload channel and the metadata carried by reports themselves. 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 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 , 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.

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. This document uses the same conventions for error handling as . 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

The terms used follow those described in . 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.

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 ), 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 aggregate protocol (). In particular, it includes the task configuration in an HTTP header of its first aggregate 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. 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 and of . ; /* A list of URLs relative to which an Aggregator's API endpoints can be found. Defined in I-D.draft-ietf-ppm-dap-04. */ Url aggregator_endpoints<1..2^16-1>; /* This determines the query type for batch selection and the properties that all batches for this task must have. */ QueryConfig query_config; /* Time up to which Clients are allowed to upload to this task. Defined in I-D.draft-ietf-ppm-dap-04. */ Time task_expiration; /* Determines the VDAF type and its config parameters. */ VdafConfig vdaf_config; } 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 query_config field defines the DAP query configuration used to guide batch selection. It is defined as follows: The vdaf_config defines the configuration of the VDAF in use for this task. It is structured as follows (codepoints are as defined in ): ; /* buckets */ case poplar1: uint16; /* bit length of input string */ } } VdafConfig; ]]> Apart from the VDAF-specific parameters, this structure includes a mechanism for differential privacy (DP). This field, dp_config, is structured as follows:

• OPEN ISSUE: Should spell out definition of DpConfig for various differential privacy mechanisms and parameters. See issue #94 for discussion.

The definition of Time, Duration, Url, and QueryType follow those in .

Deriving the Task ID When using the Taskprov extension, the task ID is computed as follows: where task_config is the TaskConfig structure disseminated by the Author. Function SHA-256() is as defined in .

Deriving the VDAF Verification Key When a Leader and Helper implement the task_prov extension in the context of a particular DAP deployment, they SHOULD compute the shared VDAF verification key 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 .) The VDAF verification key used for the task is computed as follows: where task_prov_salt is defined to be the SHA-256 hash of the octet string "dap-taskprov" and task_id is as defined in . Functions HKDF-Extract() and HKDF-Expand() are as defined in . Both functions are instantiated with SHA-256.

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 ) 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. The behavior of each protocol participant is determined by whether or not they opt in to a task.

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 .) 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 , 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.

Client Behavior Upon receiving a TaskConfig from the Author, the Client decides whether to opt in to the task as described in . 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 in to a task, it MAY begin uploading reports for the task. Each upload request for that task MUST advertise the task configuration. In addition, each report's task ID MUST be computed as described in .

Leader Behavior

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 "unrecognizedMessage". Next, it checks that the task ID indicated by the upload request matches the task ID derived from the extension payload as specified in . 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 . 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.

Aggregate Protocol When the Leader opts in to a task, it SHOULD derive the VDAF verification key for that task as described in . The Leader MUST advertise the task to the Helper in every request incident to the task as described in .

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 SHOULD also include a "Retry-After" header in its HTTP response indicating the time after which the Collector should retry its request.

• TODO: Find RFC reference for "Retry-After".

• 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.

The Leader MUST advertise the task in every aggregate share request issued to the Helper as described in .

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 "unrecognizedMessage". Next, the Helper checks that the task ID indicated in the upload request matches the task ID derived from the TaskConfig as defined in . If not, the Helper MUST abort with "unrecognizedTask". Next, the Helper decides whether to opt in to the task as described in . If it opts out, it MUST abort the aggregate 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 aggregate initialize request as usual, deriving the VDAF verification key for the task as described in .

Collector Behavior Upon receiving a TaskConfig from the Author, the Collector first decides whether to opt in to the task as described in . 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 . The Collector MUST advertise the task as described in . If the Leader responds to a collect request with an "unrecognizedTask" error, but the HTTP response includes a "Retry-After" header, the Collector SHOULD retry its collect request after waiting for the duration indicated by the header.

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.

• 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?

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. 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.

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.

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 ISRG 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. This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. 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 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] This document specifies a simple Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF), which can be used as a building block in various protocols and applications. The key derivation function (KDF) is intended to support a wide range of applications and requirements, and is conservative in its use of cryptographic hash functions. This document is not an Internet Standards Track specification; it is published for informational purposes.

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