InternetDraft  VDAF  May 2022 
Barnes, et al.  Expires 27 November 2022  [Page] 
This document describes Verifiable Distributed Aggregation Functions (VDAFs), a family of multiparty 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 note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on the Crypto Forum Research Group mailing list (cfrg@ietf.org), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=cfrg.¶
Source for this draft and an issue tracker can be found at https://github.com/cjpatton/vdaf.¶
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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/licenseinfo) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
The ubiquity of the Internet makes it an ideal platform for measurement of largescale phenomena, whether public health trends or the behavior of computer systems at scale. There is substantial overlap, however, between information that is valuable to measure and information that users consider private.¶
For example, consider an application that provides health information to users. The operator of an application might want to know which parts of their application are used most often, as a way to guide future development of the application. Specific users' patterns of usage, though, could reveal sensitive things about them, such as which users are researching a given health condition.¶
In many situations, the measurement collector is only interested in aggregate statistics, e.g., which portions of an application are most used or what fraction of people have experienced a given disease. Thus systems that provide aggregate statistics while protecting individual measurements can deliver the value of the measurements while protecting users' privacy.¶
Most prior approaches to this problem fall under the rubric of "differential privacy (DP)" [Dwo06]. Roughly speaking, a data aggregation system that is differentially private ensures that the degree to which any individual measurement influences the value of the aggregate result can be precisely controlled. For example, in systems like RAPPOR [EPK14], each user samples noise from a wellknown distribution and adds it to their input before submitting to the aggregation server. The aggregation server then adds up the noisy inputs, and because it knows the distribution from whence the noise was sampled, it can estimate the true sum with reasonable precision.¶
Differentially private systems like RAPPOR are easy to deploy and provide a useful guarantee. On its own, however, DP falls short of the strongest privacy property one could hope for. Specifically, depending on the "amount" of noise a client adds to its input, it may be possible for a curious aggregator to make a reasonable guess of the input's true value. Indeed, the more noise the clients add, the less reliable will be the server's estimate of the output. Thus systems employing DP techniques alone must strike a delicate balance between privacy and utility.¶
The ideal goal for a privacypreserving measurement system is that of secure multiparty computation: No participant in the protocol should learn anything about an individual input beyond what it can deduce from the aggregate. In this document, we describe Verifiable Distributed Aggregation Functions (VDAFs) as a general class of protocols that achieve this goal.¶
VDAF schemes achieve their privacy goal by distributing the computation of the aggregate among a number of noncolluding aggregation servers. As long as a subset of the servers executes the protocol honestly, VDAFs guarantee that no input is ever accessible to any party besides the client that submitted it. At the same time, VDAFs are "verifiable" in the sense that malformed inputs that would otherwise garble the output of the computation can be detected and removed from the set of inputs.¶
The cost of achieving these security properties is the need for multiple servers to participate in the protocol, and the need to ensure they do not collude to undermine the VDAF's privacy guarantees. Recent implementation experience has shown that practical challenges of coordinating multiple servers can be overcome. The Prio system [CGB17] (essentially a VDAF) has been deployed in systems supporting hundreds of millions of users: The Mozilla Origin Telemetry project [OriginTelemetry] and the Exposure Notification Private Analytics collaboration among the Internet Security Research Group (ISRG), Google, Apple, and others [ENPA].¶
The VDAF abstraction laid out in Section 5 represents a class of multiparty protocols for privacypreserving measurement proposed in the literature. These protocols vary in their operational and security considerations, sometimes in subtle but consequential ways. This document therefore has two important goals:¶
Providing higherlevel protocols like [DAP] with a simple, uniform interface for accessing privacypreserving measurement schemes, and documenting relevant operational and security bounds for that interface:¶
This document also specifies two concrete VDAF schemes, each based on a protocol from the literature.¶
The aforementioned Prio system [CGB17] allows for the privacypreserving computation of a variety aggregate statistics. The basic idea underlying Prio is fairly simple:¶
The difficult part of this system is ensuring that the servers hold shares of a valid input, e.g., the input is an integer in a specific range. Thus Prio specifies a multiparty protocol for accomplishing this task.¶
In Section 7 we describe Prio3, a VDAF that follows the same overall framework as the original Prio protocol, but incorporates techniques introduced in [BBCGGI19] that result in significant performance gains.¶
More recently, Boneh et al. [BBCGGI21] described a protocol called Poplar
for solving the t
heavyhitters problem in a privacypreserving manner. Here
each client holds a bitstring of length n
, and the goal of the aggregation
servers is to compute the set of inputs that occur at least t
times. The
core primitive used in their protocol is a specialized Distributed Point
Function (DPF) [GI14] that allows the servers to "query" their DPF shares on
any bitstring of length shorter than or equal to n
. As a result of this
query, each of the servers has an additive share of a bit indicating whether
the string is a prefix of the client's input. The protocol also specifies a
multiparty computation for verifying that at most one string among a set of
candidates is a prefix of the client's input.¶
In Section 8 we describe a VDAF called Poplar1 that implements this functionality.¶
Finally, perhaps the most complex aspect of schemes like Prio3 and Poplar1 is the process by which the clientgenerated measurements are prepared for aggregation. Because these constructions are based on secret sharing, the servers will be required to exchange some amount of information in order to verify the measurement is valid and can be aggregated. Depending on the construction, this process may require multiple round trips over the network.¶
There are applications in which this verification step may not be necessary, e.g., when the client's software is run a trusted execution environment. To support these applications, this document also defines Distributed Aggregation Functions (DAFs) as a simpler class of protocols that aim to provide the same privacy guarantee as VDAFs but fall short of being verifiable.¶
OPEN ISSUE Decide if we should give one or two example DAFs. There are natural variants of Prio3 and Poplar1 that might be worth describing.¶
The remainder of this document is organized as follows: Section 3 gives a brief overview of DAFs and VDAFs; Section 4 defines the syntax for DAFs; Section 5 defines the syntax for VDAFs; Section 6 defines various functionalities that are common to our constructions; Section 7 describes the Prio3 construction; Section 8 describes the Poplar1 construction; and Section 9 enumerates the security considerations for VDAFs.¶
(*) Indicates a change that breaks compatibility with the previous draft.¶
01:¶
prep_next()
to
prep_shares_to_prep()
. (*)¶
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.¶
Algorithms in this document are written in Python 3. Type hints are used to define input and output types. A fatal error in a program (e.g., failure to parse one of the function parameters) is usually handled by raising an exception.¶
A variable with type Bytes
is a byte string. This document defines several
bytestring constants. When comprised of printable ASCII characters, they are
written as Python 3 bytestring literals (e.g., b'some constant string'
).¶
A global constant VERSION
is defined, which algorithms are free to use as
desired. Its value SHALL be b'vdaf01'
.¶
This document describes algorithms for multiparty computations in which the parties typically communicate over a network. Wherever a quantity is defined that must be be transmitted from one party to another, this document prescribes a particular encoding of that quantity as a byte string.¶
OPEN ISSUE It might be better to not be prescriptive about how quantities are encoded on the wire. See issue #58.¶
Some common functionalities:¶
zeros(len: Unsigned) > Bytes
returns an array of zero bytes. The length of
output
MUST be len
.¶
gen_rand(len: Unsigned) > Bytes
returns an array of random bytes. The
length of output
MUST be len
.¶
byte(int: Unsigned) > Bytes
returns the representation of int
as a byte
string. The value of int
MUST be in [0,256)
.¶
xor(left: Bytes, right: Bytes) > Bytes
returns the bitwise XOR of left
and right
. An exception is raised if the inputs are not the same length.¶
I2OSP
and OS2IP
from [RFC8017], which are used, respectively, to
convert a nonnegative integer to a byte string and convert a byte string to a
nonnegative integer.¶
next_power_of_2(n: Unsigned) > Unsigned
returns the smallest integer
greater than or equal to n
that is also a power of two.¶
In a DAF or VDAFbased private measurement system, we distinguish three types of actors: Clients, Aggregators, and Collectors. The overall flow of the measurement process is as follows:¶
The Aggregators convert their input shares into "output shares".¶
Aggregators are a new class of actor relative to traditional measurement systems where clients submit measurements to a single server. They are critical for both the privacy properties of the system and, in the case of VDAFs, the correctness of the measurements obtained. The privacy properties of the system are assured by noncollusion among Aggregators, and Aggregators are the entities that perform validation of Client measurements. Thus clients trust Aggregators not to collude (typically it is required that at least one Aggregator is honest), and Collectors trust Aggregators to correctly run the protocol.¶
Within the bounds of the noncollusion requirements of a given (V)DAF instance, it is possible for the same entity to play more than one role. For example, the Collector could also act as an Aggregator, effectively using the other Aggregator(s) to augment a basic clientserver protocol.¶
In this document, we describe the computations performed by the actors in this system. It is up to the higherlevel protocol making use of the (V)DAF to arrange for the required information to be delivered to the proper actors in the proper sequence. In general, we assume that all communications are confidential and mutually authenticated, with the exception that Clients submitting measurements may be anonymous.¶
By way of a gentle introduction to VDAFs, this section describes a simpler class of schemes called Distributed Aggregation Functions (DAFs). Unlike VDAFs, DAFs do not provide verifiability of the computation. Clients must therefore be trusted to compute their input shares correctly. Because of this fact, the use of a DAF is NOT RECOMMENDED for most applications. See Section 9 for additional discussion.¶
A DAF scheme is used to compute a particular "aggregation function" over a set of measurements generated by Clients. Depending on the aggregation function, the Collector might select an "aggregation parameter" and disseminates it to the Aggregators. The semantics of this parameter is specific to the aggregation function, but in general it is used to represent the set of "queries" that can be made on the measurement set. For example, the aggregation parameter is used to represent the candidate prefixes in Poplar1 Section 8.¶
Execution of a DAF has four distinct stages:¶
Sharding and Preparation are done once per measurement. Aggregation and Unsharding are done over a batch of measurements (more precisely, over the recovered output shares).¶
A concrete DAF specifies an algorithm for the computation needed in each of these stages. The interface of each algorithm is defined in the remainder of this section. In addition, a concrete DAF defines the associated constants and types enumerated in the following table.¶
Parameter  Description 

SHARES

Number of input shares into which each measurement is sharded 
Measurement

Type of each measurement 
AggParam

Type of aggregation parameter 
OutShare

Type of each output share 
AggResult

Type of the aggregate result 
These types define some of the inputs and outputs of DAF methods at various
stages of the computation. Observe that only the measurements, output shares,
the aggregate result, and the aggregation parameter have an explicit type. All
other values  in particular, the input shares and the aggregate shares 
have type Bytes
and are treated as opaque byte strings. This is because these
values must be transmitted between parties over a network.¶
OPEN ISSUE It might be cleaner to define a type for each value, then have that type implement an encoding where necessary. This way each method parameter has a meaningful type hint. See issue#58.¶
In order to protect the privacy of its measurements, a DAF Client shards its
measurements into a sequence of input shares. The measurement_to_input_shares
method is used for this purpose.¶
Daf.measurement_to_input_shares(input: Measurement) > Vec[Bytes]
is the
randomized inputdistribution algorithm run by each Client. It consumes the
measurement and produces a sequence of input shares, one for each Aggregator.
The length of the output vector MUST be SHARES
.¶
Once an Aggregator has received an input share form a Client, the next step is to prepare the input share for aggregation. This is accomplished using the following algorithm:¶
Daf.prep(agg_id: Unsigned, agg_param: AggParam, input_share: Bytes) >
OutShare
is the deterministic preparation algorithm. It takes as input an
input share generated by a Client, the Aggregator's unique identifier, and the
aggregation parameter selected by the Collector and returns an output share.¶
The protocol in which the DAF is used MUST ensure that the Aggregator's
identifier is equal to the integer in range [0, SHARES)
that matches the index
of input_share
in the sequence of input shares output by the Client.¶
Once an Aggregator holds output shares for a batch of measurements (where batches are defined by the application), it combines them into a share of the desired aggregate result:¶
Daf.out_shares_to_agg_share(agg_param: AggParam, out_shares: Vec[OutShare])
> agg_share: Bytes
is the deterministic aggregation algorithm. It is run by
each Aggregator a set of recovered output shares.¶
For simplicity, we have written this algorithm in a "oneshot" form, where all output shares for a batch are provided at the same time. Many DAFs may also support a "streaming" form, where shares are processed one at a time.¶
OPEN ISSUE It may be worthwhile to explicitly define the "streaming" API. See issue#47.¶
After the Aggregators have aggregated a sufficient number of output shares, each sends its aggregate share to the Collector, who runs the following algorithm to recover the following output:¶
Daf.agg_shares_to_result(agg_param: AggParam, agg_shares: Vec[Bytes]) >
AggResult
is run by the Collector in order to compute the aggregate result
from the Aggregators' shares. The length of agg_shares
MUST be SHARES
.
This algorithm is deterministic.¶
QUESTION Maybe the aggregation algorithms should be randomized in order to allow the Aggregators (or the Collector) to add noise for differential privacy. (See the security considerations of [DAP].) Or is this outofscope of this document? See https://github.com/ietfwgppm/ppmspecification/issues/19.¶
Securely executing a DAF involves emulating the following procedure.¶
The inputs to this procedure are the same as the aggregation function computed by the DAF: An aggregation parameter and a sequence of measurements. The procedure prescribes how a DAF is executed in a "benign" environment in which there is no adversary and the messages are passed among the protocol participants over secure pointtopoint channels. In reality, these channels need to be instantiated by some "wrapper protocol", such as [DAP], that realizes these channels using suitable cryptographic mechanisms. Moreover, some fraction of the Aggregators (or Clients) may be malicious and diverge from their prescribed behaviors. Section 9 describes the execution of the DAF in various adversarial environments and what properties the wrapper protocol needs to provide in each.¶
Like DAFs described in the previous section, a VDAF scheme is used to compute a particular aggregation function over a set of Clientgenerated measurements. Evaluation of a VDAF involves the same four stages as for DAFs: Sharding, Preparation, Aggregation, and Unsharding. However, the Preparation stage will require interaction among the Aggregators in order to facilitate verifiability of the computation's correctness. Accommodating this interaction will require syntactic changes.¶
Overall execution of a VDAF comprises the following stages:¶
In contrast to DAFs, the Preparation stage for VDAFs now performs an additional task: Verification of the validity of the recovered output shares. This process ensures that aggregating the output shares will not lead to a garbled aggregate result.¶
The remainder of this section defines the VDAF interface. The attributes are listed in Table 2 are defined by each concrete VDAF.¶
Parameter  Description 

VERIFY_KEY_SIZE

Size (in bytes) of the verification key (Section 5.2) 
ROUNDS

Number of rounds of communication during the Preparation stage (Section 5.2) 
SHARES

Number of input shares into which each measurement is sharded (Section 5.1) 
Measurement

Type of each measurement 
AggParam

Type of aggregation parameter 
Prep

State of each Aggregator during Preparation (Section 5.2) 
OutShare

Type of each output share 
AggResult

Type of the aggregate result 
Similarly to DAFs (see {[secdaf}}), any output of a VDAF method that must be transmitted from one party to another is treated as an opaque byte string. All other quantities are given a concrete type.¶
OPEN ISSUE It might be cleaner to define a type for each value, then have that type implement an encoding where necessary. See issue#58.¶
Sharding is syntactically identical to DAFs (cf. Section 4.1):¶
Vdaf.measurement_to_input_shares(measurement: Measurement) > Vec[Bytes]
is
the randomized inputdistribution algorithm run by each Client. It consumes
the measurement and produces a sequence of input shares, one for each
Aggregator. Depending on the VDAF, the input shares may encode additional
information used to verify the recovered output shares (e.g., the "proof
shares" in Prio3 Section 7). The length of the output vector MUST be SHARES
.¶
To recover and verify output shares, the Aggregators interact with one another
over ROUNDS
rounds. Prior to each round, each Aggregator constructs an
outbound message. Next, the sequence of outbound messages is combined into a
single message, called a "preparation message". (Each of the outbound messages
are called "preparationmessage shares".) Finally, the preparation message is
distributed to the Aggregators to begin the next round.¶
An Aggregator begins the first round with its input share and it begins each subsequent round with the previous preparation message. Its output in the last round is its output share and its output in each of the preceding rounds is a preparationmessage share.¶
This process involves a value called the "aggregation parameter" used to map the input shares to output shares. The Aggregators need to agree on this parameter before they can begin preparing inputs for aggregation.¶
To facilitate the preparation process, a concrete VDAF implements the following class methods:¶
Vdaf.prep_init(verify_key: Bytes, agg_id: Unsigned, agg_param: AggParam,
nonce: Bytes, input_share: Bytes) > Prep
is the deterministic
preparationstate initialization algorithm run by each Aggregator to begin
processing its input share into an output share. Its inputs are the shared
verification key (verify_key
), the Aggregator's unique identifier
(agg_id
), the aggregation parameter (agg_param
), the nonce provided by the
environment (nonce
, see Figure 7), and one of the input shares generated
by the client (input_share
). Its output is the Aggregator's initial
preparation state.¶
The length of verify_key
MUST be VERIFY_KEY_SIZE
. It is up to the high
level protocol in which the VDAF is used to arrange for the distribution of
the verification key among the Aggregators prior to the start of this phase of
VDAF evaluation.¶
OPEN ISSUE What security properties do we need for this key exchange? See issue#18.¶
Protocols using the VDAF MUST ensure that the Aggregator's identifier is equal
to the integer in range [0, SHARES)
that matches the index of input_share
in the sequence of input shares output by the Client.¶
Vdaf.prep_next(prep: Prep, inbound: Optional[Bytes]) > Union[Tuple[Prep,
Bytes], OutShare]
is the deterministic preparationstate update algorithm run
by each Aggregator. It updates the Aggregator's preparation state (prep
) and
returns either its next preparation state and its message share for the
current round or, if this is the last round, its output share. An exception is
raised if a valid output share could not be recovered. The input of this
algorithm is the inbound preparation message or, if this is the first round,
None
.¶
Vdaf.prep_shares_to_prep(agg_param: AggParam, prep_shares: Vec[Bytes]) >
Bytes
is the deterministic preparationmessage preprocessing algorithm. It
combines the preparationmessage shares generated by the Aggregators in the
previous round into the preparation message consumed by each in the next
round.¶
In effect, each Aggregator moves through a linear state machine with ROUNDS+1
states. The Aggregator enters the first state on using the initialization
algorithm, and the update algorithm advances the Aggregator to the next state.
Thus, in addition to defining the number of rounds (ROUNDS
), a VDAF instance
defines the state of the Aggregator after each round.¶
TODO Consider how to bake this "linear state machine" condition into the syntax. Given that Python 3 is used as our pseudocode, it's easier to specify the preparation state using a class.¶
The preparationstate update accomplishes two tasks: recovery of output shares from the input shares and ensuring that the recovered output shares are valid. The abstraction boundary is drawn so that an Aggregator only recovers an output share if it is deemed valid (at least, based on the Aggregator's view of the protocol). Another way to draw this boundary would be to have the Aggregators recover output shares first, then verify that they are valid. However, this would allow the possibility of misusing the API by, say, aggregating an invalid output share. Moreover, in protocols like Prio+ [AGJOP21] based on oblivious transfer, it is necessary for the Aggregators to interact in order to recover aggregatable output shares at all.¶
Note that it is possible for a VDAF to specify ROUNDS == 0
, in which case each
Aggregator runs the preparationstate update algorithm once and immediately
recovers its output share without interacting with the other Aggregators.
However, most, if not all, constructions will require some amount of interaction
in order to ensure validity of the output shares (while also maintaining
privacy).¶
OPEN ISSUE accommodating 0round VDAFs may require syntax changes if, for example, public keys are required. On the other hand, we could consider defining this class of schemes as a different primitive. See issue#77.¶
VDAF Aggregation is identical to DAF Aggregation (cf. Section 4.3):¶
Vdaf.out_shares_to_agg_share(agg_param: AggParam, out_shares: Vec[OutShare])
> agg_share: Bytes
is the deterministic aggregation algorithm. It is run by
each Aggregator over the output shares it has computed over a batch of
measurement inputs.¶
The data flow for this stage is illustrated in Figure 3. Here again, we have the aggregation algorithm in a "oneshot" form, where all shares for a batch are provided at the same time. VDAFs typically also support a "streaming" form, where shares are processed one at a time.¶
VDAF Unsharding is identical to DAF Unsharding (cf. Section 4.4):¶
Vdaf.agg_shares_to_result(agg_param: AggParam, agg_shares: Vec[Bytes]) >
AggResult
is run by the Collector in order to compute the aggregate result
from the Aggregators' shares. The length of agg_shares
MUST be SHARES
.
This algorithm is deterministic.¶
Secure execution of a VDAF involves simulating the following procedure.¶
The inputs to this algorithm are the aggregation parameter, a list of measurements, and a nonce for each measurement. This document does not specify how the nonces are chosen, but security requires that the nonces be unique. See Section 9 for details. As explained in Section 4.5, the secure execution of a VDAF requires the application to instantiate secure channels between each of the protocol participants.¶
This section describes the primitives that are common to the VDAFs specified in this document.¶
Both Prio3 and Poplar1 use finite fields of prime order. Finite field
elements are represented by a class Field
with the following associated
parameters:¶
MODULUS: Unsigned
is the prime modulus that defines the field.¶
ENCODED_SIZE: Unsigned
is the number of bytes used to encode a field element
as a byte string.¶
A concrete Field
also implements the following class methods:¶
Field.zeros(length: Unsigned) > output: Vec[Field]
returns a vector of
zeros. The length of output
MUST be length
.¶
Field.rand_vec(length: Unsigned) > output: Vec[Field]
returns a vector of
random field elements. The length of output
MUST be length
.¶
A field element is an instance of a concrete Field
. The concrete class defines
the usual arithmetic operations on field elements. In addition, it defines the
following instance method for converting a field element to an unsigned integer:¶
elem.as_unsigned() > Unsigned
returns the integer representation of
field element elem
.¶
Likewise, each concrete Field
implements a constructor for converting an
unsigned integer into a field element:¶
Field(integer: Unsigned)
returns integer
represented as a field element.
If integer >= Field.MODULUS
, then integer
is first reduced modulo
Field.MODULUS
.¶
Finally, each concrete Field
has two derived class methods, one for encoding
a vector of field elements as a byte string and another for decoding a vector of
field elements.¶
The following auxiliary functions on vectors of field elements are used in the remainder of this document. Note that an exception is raised by each function if the operands are not the same length.¶
Some VDAFs require fields that are suitable for efficient computation of the discrete Fourier transform. (One example is Prio3 (Section 7) when instantiated with the generic FLP of Section 7.3.3.) Specifically, a field is said to be "FFTfriendly" if, in addition to satisfying the interface described in Section 6.1, it implements the following method:¶
Field.gen() > Field
returns the generator of a large subgroup of the
multiplicative group.¶
FFTfriendly fields also define the following parameter:¶
GEN_ORDER: Unsigned
is the order of a multiplicative subgroup generated by
Field.gen()
. This value MUST be a power of 2.¶
The tables below define finite fields used in the remainder of this document.¶
Parameter  Value 

MODULUS  2^32 * 4294967295 + 1 
ENCODED_SIZE  8 
Generator  7^4294967295 
GEN_ORDER  2^32 
Parameter  Value 

MODULUS  2^66 * 4611686018427387897 + 1 
ENCODED_SIZE  16 
Generator  7^4611686018427387897 
GEN_ORDER  2^66 
A pseudorandom generator (PRG) is used to expand a short, (pseudo)random seed into a long string of pseudorandom bits. A PRG suitable for this document implements the interface specified in this section. Concrete constructions are described in the subsections that follow.¶
PRGs are defined by a class Prg
with the following associated parameter:¶
SEED_SIZE: Unsigned
is the size (in bytes) of a seed.¶
A concrete Prg
implements the following class method:¶
Prg(seed: Bytes, info: Bytes)
constructs an instance of Prg
from the given
seed and info string. The seed MUST be of length SEED_SIZE
and MUST be
generated securely (i.e., it is either the output of gen_rand
or a previous
invocation of the PRG). The info string is used for domain separation.¶
prg.next(length: Unsigned)
returns the next length
bytes of output of PRG.
If the seed was securely generated, the output can be treated as pseudorandom.¶
Each Prg
has two derived class methods. The first is used to derive a fresh
seed from an existing one. The second is used to compute a sequence of
pseudorandom field elements. For each method, the seed MUST be of length
SEED_SIZE
and MUST be generated securely (i.e., it is either the output of
gen_rand
or a previous invocation of the PRG).¶
OPEN ISSUE Phillipp points out that a fixedkey mode of AES may be more performant (https://eprint.iacr.org/2019/074.pdf). See issue#32.¶
Our first construction, PrgAes128
, converts a blockcipher, namely AES128,
into a PRG. Seed expansion involves two steps. In the first step, CMAC
[RFC4493] is applied to the seed and info string to get a fresh key. In the
second step, the fresh key is used in CTRmode to produce a key stream for
generating the output. A fixed initialization vector (IV) is used.¶
NOTE This construction has not undergone significant security analysis.¶
This section describes Prio3, a VDAF for Prio [CGB17]. Prio is suitable for a wide variety of aggregation functions, including (but not limited to) sum, mean, standard deviation, estimation of quantiles (e.g., median), and linear regression. In fact, the scheme described in this section is compatible with any aggregation function that has the following structure:¶
At a high level, Prio3 distributes this computation as follows. Each Client first shards its measurement by first encoding it, then splitting the vector into secret shares and sending a share to each Aggregator. Next, in the preparation phase, the Aggregators carry out a multiparty computation to determine if their shares correspond to a valid input (as determined by the arithmetic circuit). This computation involves a "proof" of validity generated by the Client. Next, each Aggregator sums up its input shares locally. Finally, the Collector sums up the aggregate shares and computes the aggregate result.¶
This VDAF does not have an aggregation parameter. Instead, the output share is derived from the input share by applying a fixed map. See Section 8 for an example of a VDAF that makes meaningful use of the aggregation parameter.¶
As the name implies, Prio3 is a descendant of the original Prio construction. A second iteration was deployed in the [ENPA] system, and like the VDAF described here, the ENPA system was built from techniques introduced in [BBCGGI19] that significantly improve communication cost. That system was specialized for a particular aggregation function; the goal of Prio3 is to provide the same level of generality as the original construction.¶
The core component of Prio3 is a "Fully Linear Proof (FLP)" system. Introduced by [BBCGGI19], the FLP encapsulates the functionality required for encoding and validating inputs. Prio3 can be thought of as a transformation of a particular class of FLPs into a VDAF.¶
The remainder of this section is structured as follows. The syntax for FLPs is described in Section 7.1. The generic transformation of an FLP into Prio3 is specified in Section 7.2. Next, a concrete FLP suitable for any validity circuit is specified in Section 7.3. Finally, instantiations of Prio3 for various types of measurements are specified in Section 7.4. Test vectors can be found in Appendix "Test Vectors".¶
Conceptually, an FLP is a twoparty protocol executed by a prover and a verifier. In actual use, however, the prover's computation is carried out by the Client, and the verifier's computation is distributed among the Aggregators. The Client generates a "proof" of its input's validity and distributes shares of the proof to the Aggregators. Each Aggregator then performs some a computation on its input share and proof share locally and sends the result to the other Aggregators. Combining the exchanged messages allows each Aggregator to decide if it holds a share of a valid input. (See Section 7.2 for details.)¶
As usual, we will describe the interface implemented by a concrete FLP in terms
of an abstract base class Flp
that specifies the set of methods and parameters
a concrete FLP must provide.¶
The parameters provided by a concrete FLP are listed in Table 5.¶
Parameter  Description 

PROVE_RAND_LEN

Length of the prover randomness, the number of random field elements consumed by the prover when generating a proof 
QUERY_RAND_LEN

Length of the query randomness, the number of random field elements consumed by the verifier 
JOINT_RAND_LEN

Length of the joint randomness, the number of random field elements consumed by both the prover and verifier 
INPUT_LEN

Length of the encoded measurement (Section 7.1.1) 
OUTPUT_LEN

Length of the aggregatable output (Section 7.1.1) 
PROOF_LEN

Length of the proof 
VERIFIER_LEN

Length of the verifier message generated by querying the input and proof 
Measurement

Type of the measurement 
Field

As defined in (Section 6.1) 
An FLP specifies the following algorithms for generating and verifying proofs of validity (encoding is described below in Section 7.1.1):¶
Flp.prove(input: Vec[Field], prove_rand: Vec[Field], joint_rand: Vec[Field])
> Vec[Field]
is the deterministic proofgeneration algorithm run by the
prover. Its inputs are the encoded input, the "prover randomness"
prove_rand
, and the "joint randomness" joint_rand
. The proof randomness is
used only by the prover, but the joint randomness is shared by both the prover
and verifier.¶
Flp.query(input: Vec[Field], proof: Vec[Field], query_rand: Vec[Field],
joint_rand: Vec[Field], num_shares: Unsigned) > Vec[Field]
is the
querygeneration algorithm run by the verifier. This is used to "query" the
input and proof. The result of the query (i.e., the output of this function)
is called the "verifier message". In addition to the input and proof, this
algorithm takes as input the query randomness query_rand
and the joint
randomness joint_rand
. The former is used only by the verifier. The
semantics of num_shares
is discussed below.¶
Flp.decide(verifier: Vec[Field]) > Bool
is the deterministic decision
algorithm run by the verifier. It takes as input the verifier message and
outputs a boolean indicating if the input from which it was generated is
valid.¶
Our application requires that the FLP is "fully linear" in the sense defined in [BBCGGI19]. As a practical matter, what this property implies is that, when run on a share of the input and proof, the querygeneration algorithm outputs a share of the verifier message. Furthermore, the "strong zeroknowledge" property of the FLP system ensures that the verifier message reveals nothing about the input's validity. Therefore, to decide if an input is valid, the Aggregators will run the querygeneration algorithm locally, exchange verifier shares, combine them to recover the verifier message, and run the decision algorithm.¶
The querygeneration algorithm includes a parameter num_shares
that specifies
the number of shares of the input and proof that were generated. If these data
are not secret shared, then num_shares == 1
. This parameter is useful for
certain FLP constructions. For example, the FLP in Section 7.3 is defined in
terms of an arithmetic circuit; when the circuit contains constants, it is
sometimes necessary to normalize those constants to ensure that the circuit's
output, when run on a valid input, is the same regardless of the number of
shares.¶
An FLP is executed by the prover and verifier as follows:¶
The proof system is constructed so that, if input
is a valid input, then
run_flp(Flp, input, 1)
always returns True
. On the other hand, if input
is
invalid, then as long as joint_rand
and query_rand
are generated uniform
randomly, the output is False
with overwhelming probability.¶
We remark that [BBCGGI19] defines a much larger class of fully linear proof systems than we consider here. In particular, what is called an "FLP" here is called a 1.5round, publiccoin, interactive oracle proof system in their paper.¶
The type of measurement being aggregated is defined by the FLP. Hence, the FLP also specifies a method of encoding raw measurements as a vector of field elements:¶
Flp.encode(measurement: Measurement) > Vec[Field]
encodes a raw measurement
as a vector of field elements. The return value MUST be of length INPUT_LEN
.¶
For some FLPs, the encoded input also includes redundant field elements that are
useful for checking the proof, but which are not needed after the proof has been
checked. An example is the "integer sum" data type from [CGB17] in which an
integer in range [0, 2^k)
is encoded as a vector of k
field elements (this
type is also defined in Section 7.4). After consuming this vector,
all that is needed is the integer it represents. Thus the FLP defines an
algorithm for truncating the input to the length of the aggregated output:¶
Flp.truncate(input: Vec[Field]) > Vec[Field]
maps an encoded input to an
aggregatable output. The length of the input MUST be INPUT_LEN
and the length
of the output MUST be OUTPUT_LEN
.¶
We remark that, taken together, these two functionalities correspond roughly to the notion of "Affineaggregatable encodings (AFEs)" from [CGB17].¶
This section specifies Prio3
, an implementation of the Vdaf
interface
(Section 5). It has two generic parameters: an Flp
(Section 7.1) and a Prg
(Section 6.2). The associated constants and types required by the Vdaf
interface
are defined in Table 6. The methods required for sharding, preparation,
aggregation, and unsharding are described in the remaining subsections.¶
Parameter  Value 

VERIFY_KEY_SIZE

Prg.SEED_SIZE

ROUNDS

1

SHARES

in [2, 255)

Measurement

Flp.Measurement

AggParam

None

Prep

Tuple[Vec[Flp.Field], Optional[Bytes], Bytes]

OutShare

Vec[Flp.Field]

AggResult

Vec[Unsigned]

This section describes the process of recovering output shares from the input shares. The highlevel idea is that each Aggregator first queries its input and proof share locally, then exchanges its verifier share with the other Aggregators. The verifier shares are then combined into the verifier message, which is used to decide whether to accept.¶
In addition, the Aggregators must ensure that they have all used the same joint randomness for the querygeneration algorithm. The joint randomness is generated by a PRG seed. Each Aggregator derives an XOR secret share of this seed from its input share and the "blind" generated by the client. Thus, before running the querygeneration algorithm, it must first gather the XOR secret shares derived by the other Aggregators.¶
In order to avoid extra round of communication, the Client sends each Aggregator a "hint" equal to the XOR of the other Aggregators' shares of the joint randomness seed. This leaves open the possibility that the Client cheated by, say, forcing the Aggregators to use joint randomness that biases the proof check procedure some way in its favor. To mitigate this, the Aggregators also check that they have all computed the same joint randomness seed before accepting their output shares. To do so, they exchange their XOR shares of the PRG seed along with their verifier shares.¶
NOTE This optimization somewhat diverges from Section 6.2.3 of [BBCGGI19]. Security analysis is needed.¶
The algorithms required for preparation are defined as follows. These algorithms make use of encoding and decoding methods defined in Section 7.2.5.¶
Aggregating a set of output shares is simply a matter of adding up the vectors elementwise.¶
To unshard a set of aggregate shares, the Collector first adds up the vectors elementwise. It then converts each element of the vector into an integer.¶
This section describes an FLP based on the construction from in [BBCGGI19], Section 4.2. We begin in Section 7.3.1 with an overview of their proof system and the extensions to their proof system made here. The construction is specified in Section 7.3.3.¶
OPEN ISSUE We're not yet sure if specifying this generalpurpose FLP is desirable. It might be preferable to specify specialized FLPs for each data type that we want to standardize, for two reasons. First, clear and concise specifications are likely easier to write for specialized FLPs rather than the general one. Second, we may end up tailoring each FLP to the measurement type in a way that improves performance, but breaks compatibility with the generalpurpose FLP.¶
In any case, we can't make this decision until we know which data types to standardize, so for now, we'll stick with the generalpurpose construction. The reference implementation can be found at https://github.com/cfrg/draftirtfcfrgvdaf/tree/main/poc.¶
OPEN ISSUE Chris Wood points out that the this section reads more like a paper than a standard. Eventually we'll want to work this into something that is readily consumable by the CFRG.¶
In the proof system of [BBCGGI19], validity is defined via an arithmetic circuit evaluated over the input: If the circuit output is zero, then the input is deemed valid; otherwise, if the circuit output is nonzero, then the input is deemed invalid. Thus the goal of the proof system is merely to allow the verifier to evaluate the validity circuit over the input. For our application (Section 7), this computation is distributed among multiple Aggregators, each of which has only a share of the input.¶
Suppose for a moment that the validity circuit C
is affine, meaning its only
operations are addition and multiplicationbyconstant. In particular, suppose
the circuit does not contain a multiplication gate whose operands are both
nonconstant. Then to decide if an input x
is valid, each Aggregator could
evaluate C
on its share of x
locally, broadcast the output share to its
peers, then combine the output shares locally to recover C(x)
. This is true
because for any SHARES
way secret sharing of x
it holds that¶
C(x_shares[0] + ... + x_shares[SHARES1]) = C(x_shares[0]) + ... + C(x_shares[SHARES1])¶
(Note that, for this equality to hold, it may be necessary to scale any
constants in the circuit by SHARES
.) However this is not the case if C
is
notaffine (i.e., it contains at least one multiplication gate whose operands
are nonconstant). In the proof system of [BBCGGI19], the proof is designed to
allow the (distributed) verifier to compute the nonaffine operations using only
linear operations on (its share of) the input and proof.¶
To make this work, the proof system is restricted to validity circuits that
exhibit a special structure. Specifically, an arithmetic circuit with "Ggates"
(see [BBCGGI19], Definition 5.2) is composed of affine gates and any number of
instances of a distinguished gate G
, which may be nonaffine. We will refer to
this class of circuits as 'gadget circuits' and to G
as the "gadget".¶
As an illustrative example, consider a validity circuit C
that recognizes the
set L = set([0], [1])
. That is, C
takes as input a length1 vector x
and
returns 0 if x[0]
is in [0,2)
and outputs something else otherwise. This
circuit can be expressed as the following degree2 polynomial:¶
C(x) = (x[0]  1) * x[0] = x[0]^2  x[0]¶
This polynomial recognizes L
because x[0]^2 = x[0]
is only true if x[0] ==
0
or x[0] == 1
. Notice that the polynomial involves a nonaffine operation,
x[0]^2
. In order to apply [BBCGGI19], Theorem 4.3, the circuit needs to be
rewritten in terms of a gadget that subsumes this nonaffine operation. For
example, the gadget might be multiplication:¶
Mul(left, right) = left * right¶
The validity circuit can then be rewritten in terms of Mul
like so:¶
C(x[0]) = Mul(x[0], x[0])  x[0]¶
The proof system of [BBCGGI19] allows the verifier to evaluate each instance
of the gadget (i.e., Mul(x[0], x[0])
in our example) using a linear function
of the input and proof. The proof is constructed roughly as follows. Let C
be
the validity circuit and suppose the gadget is arityL
(i.e., it has L
input
wires.). Let wire[j1,k1]
denote the value of the j
th wire of the k
th
call to the gadget during the evaluation of C(x)
. Suppose there are M
such
calls and fix distinct field elements alpha[0], ..., alpha[M1]
. (We will
require these points to have a special property, as we'll discuss in
Section 7.3.1.1; but for the moment it is only important
that they are distinct.)¶
The prover constructs from wire
and alpha
a polynomial that, when evaluated
at alpha[k1]
, produces the output of the k
th call to the gadget. Let us
call this the "gadget polynomial". Polynomial evaluation is linear, which means
that, in the distributed setting, the Client can disseminate additive shares of
the gadget polynomial that the Aggregators then use to compute additive shares
of each gadget output, allowing each Aggregator to compute its share of C(x)
locally.¶
There is one more wrinkle, however: It is still possible for a malicious prover
to produce a gadget polynomial that would result in C(x)
being computed
incorrectly, potentially resulting in an invalid input being accepted. To
prevent this, the verifier performs a probabilistic test to check that the
gadget polynomial is wellformed. This test, and the procedure for constructing
the gadget polynomial, are described in detail in Section 7.3.3.¶
The FLP described in the next section extends the proof system [BBCGGI19], Section 4.2 in three ways.¶
First, the validity circuit in our construction includes an additional, random
input (this is the "joint randomness" derived from the input shares in Prio3;
see Section 7.2). This allows for circuit optimizations that trade a
small soundness error for a shorter proof. For example, consider a circuit that
recognizes the set of lengthN
vectors for which each element is either one or
zero. A deterministic circuit could be constructed for this language, but it
would involve a large number of multiplications that would result in a large
proof. (See the discussion in [BBCGGI19], Section 5.2 for details). A much
shorter proof can be constructed for the following randomized circuit:¶
C(inp, r) = r * Range2(inp[0]) + ... + r^N * Range2(inp[N1])¶
(Note that this is a special case of [BBCGGI19], Theorem 5.2.) Here inp
is
the lengthN
input and r
is a random field element. The gadget circuit
Range2
is the "rangecheck" polynomial described above, i.e., Range2(x) = x^2 
x
. The idea is that, if inp
is valid (i.e., each inp[j]
is in [0,2)
),
then the circuit will evaluate to 0 regardless of the value of r
; but if
inp[j]
is not in [0,2)
for some j
, the output will be nonzero with high
probability.¶
The second extension implemented by our FLP allows the validity circuit to
contain multiple gadget types. (This generalization was suggested in
[BBCGGI19], Remark 4.5.) For example, the following circuit is allowed, where
Mul
and Range2
are the gadgets defined above (the input has length N+1
):¶
C(inp, r) = r * Range2(inp[0]) + ... + r^N * Range2(inp[N1]) + \ 2^0 * inp[0] + ... + 2^(N1) * inp[N1]  \ Mul(inp[N], inp[N])¶
Finally, [BBCGGI19], Theorem 4.3 makes no restrictions on the choice of the
fixed points alpha[0], ..., alpha[M1]
, other than to require that the points
are distinct. In this document, the fixed points are chosen so that the gadget
polynomial can be constructed efficiently using the CooleyTukey FFT ("Fast
Fourier Transform") algorithm. Note that this requires the field to be
"FFTfriendly" as defined in Section 6.1.2.¶
The FLP described in Section 7.3.3 is defined in terms of a
validity circuit Valid
that implements the interface described here.¶
A concrete Valid
defines the following parameters:¶
Parameter  Description 

GADGETS

A list of gadgets 
GADGET_CALLS

Number of times each gadget is called 
INPUT_LEN

Length of the input 
OUTPUT_LEN

Length of the aggregatable output 
JOINT_RAND_LEN

Length of the random input 
Measurement

The type of measurement 
Field

An FFTfriendly finite field as defined in Section 6.1.2 
Each gadget G
in GADGETS
defines a constant DEGREE
that specifies the
circuit's "arithmetic degree". This is defined to be the degree of the
polynomial that computes it. For example, the Mul
circuit in
Section 7.3.1 is defined by the polynomial Mul(x) = x * x
, which
has degree 2
. Hence, the arithmetic degree of this gadget is 2
.¶
Each gadget also defines a parameter ARITY
that specifies the circuit's arity
(i.e., the number of input wires).¶
A concrete Valid
provides the following methods for encoding a measurement as
an input vector and truncating an input vector to the length of an aggregatable
output:¶
Valid.encode(measurement: Measurement) > Vec[Field]
returns a vector of
length INPUT_LEN
representing a measurement.¶
Valid.truncate(input: Vec[Field]) > Vec[Field]
returns a vector of length
OUTPUT_LEN
representing an aggregatable output.¶
Finally, the following class methods are derived for each concrete Valid
:¶
This section specifies FlpGeneric
, an implementation of the Flp
interface
(Section 7.1). It has as a generic parameter a validity circuit Valid
implementing
the interface defined in Section 7.3.2.¶
NOTE A reference implementation can be found in https://github.com/cfrg/draftirtfcfrgvdaf/blob/main/poc/flp_generic.sage.¶
The FLP parameters for FlpGeneric
are defined in Table 8. The
required methods for generating the proof, generating the verifier, and deciding
validity are specified in the remaining subsections.¶
In the remainder, we let [n]
denote the set {1, ..., n}
for positive integer
n
. We also define the following constants:¶
Parameter  Value 

PROVE_RAND_LEN

Valid.prove_rand_len() (see Section 7.3.2) 
QUERY_RAND_LEN

Valid.query_rand_len() (see Section 7.3.2) 
JOINT_RAND_LEN

Valid.JOINT_RAND_LEN

INPUT_LEN

Valid.INPUT_LEN

OUTPUT_LEN

Valid.OUTPUT_LEN

PROOF_LEN

Valid.proof_len() (see Section 7.3.2) 
VERIFIER_LEN

Valid.verifier_len() (see Section 7.3.2) 
Measurement

Valid.Measurement

Field

Valid.Field

On input inp
, prove_rand
, and joint_rand
, the proof is computed as
follows:¶
i
in [H]
create an empty table wire_i
.¶
prove_rand
into subvectors seed_1, ...,
seed_H
where len(seed_i) == L_i
for all i
in [H]
. Let us call these
the "wire seeds" of each gadget.¶
Valid
on input of inp
and joint_rand
, recording the inputs of
each gadget in the corresponding table. Specifically, for every i
in [H]
,
set wire_i[j1,k1]
to the value on the j
th wire into the k
th call to
gadget G_i
.¶
Compute the "wire polynomials". That is, for every i
in [H]
and j
in
[L_i]
, construct poly_wire_i[j1]
, the j
th wire polynomial for the
i
th gadget, as follows:¶
w = [seed_i[j1], wire_i[j1,0], ..., wire_i[j1,M_i1]]
.¶
padded_w = w + Field.zeros(P_i  len(w))
.¶
NOTE We pad w
to the nearest power of 2 so that we can use FFT for
interpolating the wire polynomials. Perhaps there is some clever math for
picking wire_inp
in a way that avoids having to pad.¶
poly_wire_i[j1]
be the lowest degree polynomial for which
poly_wire_i[j1](alpha_i^k) == padded_w[k]
for all k
in [P_i]
.¶
Compute the "gadget polynomials". That is, for every i
in [H]
:¶
poly_gadget_i = G_i(poly_wire_i[0], ..., poly_wire_i[L_i1])
. That
is, evaluate the circuit G_i
on the wire polynomials for the i
th
gadget. (Arithmetic is in the ring of polynomials over Field
.)¶
The proof is the vector proof = seed_1 + coeff_1 + ... + seed_H + coeff_H
,
where coeff_i
is the vector of coefficients of poly_gadget_i
for each i
in
[H]
.¶
On input of inp
, proof
, query_rand
, and joint_rand
, the verifier message
is generated as follows:¶
i
in [H]
create an empty table wire_i
.¶
proof
into the subvectors seed_1
, coeff_1
, ..., seed_H
,
coeff_H
defined in Section 7.3.3.1.¶
Valid
on input of inp
and joint_rand
, recording the inputs of
each gadget in the corresponding table. This step is similar to the prover's
step (3.) except the verifier does not evaluate the gadgets. Instead, it
computes the output of the k
th call to G_i
by evaluating
poly_gadget_i(alpha_i^k)
. Let v
denote the output of the circuit
evaluation.¶
Compute the tests for wellformedness of the gadget polynomials. That is, for
every i
in [H]
:¶
The verifier message is the vector verifier = [v] + x_1 + [y_1] + ... + x_H +
[y_H]
.¶
This section specifies instantiations of Prio3 for various measurement types.
Each uses FlpGeneric
as the FLP (Section 7.3) and is determined by a
validity circuit (Section 7.3.2) and a PRG (Section 6.2). Test vectors for
each can be found in Appendix "Test Vectors".¶
NOTE Reference implementations of each of these VDAFs can be found in https://github.com/cfrg/draftirtfcfrgvdaf/blob/main/poc/vdaf_prio3.sage.¶
Our first instance of Prio3 is for a simple counter: Each measurement is either one or zero and the aggregate result is the sum of the measurements.¶
This instance uses PrgAes128
(Section 6.2.1) as its PRG. Its validity
circuit, denoted Count
, uses Field64
(Table 3) as its finite field. Its
gadget, denoted Mul
, is the degree2, arity2 gadget defined as¶
def Mul(x, y): return x * y¶
The validity circuit is defined as¶
def Count(inp: Vec[Field64]): return Mul(inp[0], inp[0])  inp[0]¶
The measurement is encoded as a singleton vector in the natural way. The parameters for this circuit are summarized below.¶
Parameter  Value 

GADGETS

[Mul]

GADGET_CALLS

[1]

INPUT_LEN

1

OUTPUT_LEN

1

JOINT_RAND_LEN

0

Measurement

Unsigned , in range [0,2)

Field

Field64 (Table 3) 
The next instance of Prio3 supports summing of integers in a predetermined
range. Each measurement is an integer in range [0, 2^bits)
, where bits
is an
associated parameter.¶
This instance of Prio3 uses PrgAes128
(Section 6.2.1) as its PRG.
Its validity circuit, denoted Sum
, uses Field128
(Table 4) as its
finite field. The measurement is encoded as a lengthbits
vector of field
elements, where the l
th element of the vector represents the l
th bit of the
summand:¶
def encode(Sum, measurement: Integer): if 0 > measurement or measurement >= 2^Sum.INPUT_LEN: raise ERR_INPUT encoded = [] for l in range(Sum.INPUT_LEN): encoded.append(Sum.Field((measurement >> l) & 1)) return encoded def truncate(Sum, inp): decoded = Sum.Field(0) for (l, b) in enumerate(inp): w = Sum.Field(1 << l) decoded += w * b return [decoded]¶
The validity circuit checks that the input comprised of ones and zeros. Its
gadget, denoted Range2
, is the degree2, arity1 gadget defined as¶
def Range2(x): return x^2  x¶
The validity circuit is defined as¶
def Sum(inp: Vec[Field128], joint_rand: Vec[Field128]): out = Field128(0) r = joint_rand[0] for x in inp: out += r * Range2(x) r *= joint_rand[0] return out¶
Parameter  Value 

GADGETS

[Range2]

GADGET_CALLS

[bits]

INPUT_LEN

bits

OUTPUT_LEN

1

JOINT_RAND_LEN

1

Measurement

Unsigned , in range [0, 2^bits)

Field

Field128 (Table 4) 
This instance of Prio3 allows for estimating the distribution of the measurements by computing a simple histogram. Each measurement is an arbitrary integer and the aggregate result counts the number of measurements that fall in a set of fixed buckets.¶
This instance of Prio3 uses PrgAes128
(Section 6.2.1) as its PRG. Its
validity circuit, denoted Histogram
, uses Field128
(Table 4) as its
finite field. The measurement is encoded as a onehot vector representing the
bucket into which the measurement falls (let bucket
denote a sequence of
monotonically increasing integers):¶
def encode(Histogram, measurement: Integer): boundaries = buckets + [Infinity] encoded = [Field128(0) for _ in range(len(boundaries))] for i in range(len(boundaries)): if measurement <= boundaries[i]: encoded[i] = Field128(1) return encoded def truncate(Histogram, inp: Vec[Field128]): return inp¶
The validity circuit uses Range2
(see Section 7.4.2) as its single gadget. It
checks for onehotness in two steps, as follows:¶
def Histogram(inp: Vec[Field128], joint_rand: Vec[Field128], num_shares: Unsigned): # Check that each bucket is one or zero. range_check = Field128(0) r = joint_rand[0] for x in inp: range_check += r * Range2(x) r *= joint_rand[0] # Check that the buckets sum to 1. sum_check = Field128(1) * Field128(num_shares).inv() for b in inp: sum_check += b out = joint_rand[1] * range_check + \ joint_rand[1]^2 * sum_check return out¶
Note that this circuit depends on the number of shares into which the input is sharded. This is provided to the FLP by Prio3.¶
Parameter  Value 

GADGETS

[Range2]

GADGET_CALLS

[buckets + 1]

INPUT_LEN

buckets + 1

OUTPUT_LEN

buckets + 1

JOINT_RAND_LEN

2

Measurement

Integer

Field

Field128 (Table 4) 
TODO Update this section in light of removing the public parameter and replacing the verification parameter.¶
NOTE The spec for Poplar1 is still a workinprogress. A partial implementation can be found at https://github.com/divviup/libpriors/blob/main/src/vdaf/poplar1.rs. The verification logic is nearly complete, however as of this draft the code is missing the IDPF. An implementation of the IDPF can be found at https://github.com/google/distributed_point_functions/.¶
This section specifies Poplar1, a VDAF for the following task. Each Client holds
a BITS
bit string and the Aggregators hold a set of l
bit strings, where l
<= BITS
. We will refer to the latter as the set of "candidate prefixes". The
Aggregators' goal is to count how many inputs are prefixed by each candidate
prefix.¶
This functionality is the core component of Poplar [BBCGGI21]. At a high level, the protocol works as follows.¶
n
bit string and
sends an input share to each Aggregator.¶
0
and
1
.¶
H
denote the set of prefixes that occurred at least t
times. If the
prefixes all have length BITS
, then H
is the set of t
heavyhitters.
Otherwise compute the next set of candidate prefixes as follows. For each p
in H
, add add p  0
and p  1
to the set. Repeat step 3 with the new
set of candidate prefixes.¶
Poplar1 is constructed from an "Incremental Distributed Point Function (IDPF)", a primitive described by [BBCGGI21] that generalizes the notion of a Distributed Point Function (DPF) [GI14]. Briefly, a DPF is used to distribute the computation of a "point function", a function that evaluates to zero on every input except at a programmable "point". The computation is distributed in such a way that no one party knows either the point or what it evaluates to.¶
An IDPF generalizes this "point" to a path on a full binary tree from the root to one of the leaves. It is evaluated on an "index" representing a unique node of the tree. If the node is on the path, then function evaluates to to a nonzero value; otherwise it evaluates to zero. This structure allows an IDPF to provide the functionality required for the above protocol, while at the same time ensuring the same degree of privacy as a DPF.¶
Our VDAF composes an IDPF with the "secure sketching" protocol of [BBCGGI21]. This protocol ensures that evaluating a set of input shares on a unique set of candidate prefixes results in shares of a "onehot" vector, i.e., a vector that is zero everywhere except for one element, which is equal to one.¶
An IDPF is defined over a domain of size 2^BITS
, where BITS
is constant
defined by the IDPF. The Client specifies an index alpha
and a pair of values
beta
, one for each "level" 1 <= l <= BITS
. The key generation generates two
IDPF keys, one for each Aggregator. When evaluated at index 0 <= x < 2^l
, each
IDPF share returns an additive share of beta[l]
if x
is the l
bit prefix
of alpha
and shares of zero otherwise.¶
CP What does it mean for x
to be the l
bit prefix of alpha
? We need to
be a bit more precise here.¶
CP Why isn't the domain size actually 2^(BITS+1)
, i.e., the number of nodes
in a binary tree of height BITS
(excluding the root)?¶
Each beta[l]
is a pair of elements of a finite field. Each level MAY have
different field parameters. Thus a concrete IDPF specifies associated types
Field[1]
, Field[2]
, ..., and Field[BITS]
defining, respectively, the field
parameters at level 1, level 2, ..., and level BITS
.¶
An IDPF is comprised of the following algorithms (let type Value[l]
denote
(Field[l], Field[l])
for each level l
):¶
idpf_gen(alpha: Unsigned, beta: (Value[1], ..., Value[BITS])) > key:
(IDPFKey, IDPFKey)
is the randomized keygeneration algorithm run by the
client. Its inputs are the index alpha
and the values beta
. The value of
alpha
MUST be in range [0, 2^BITS)
.¶
IDPFKey.eval(l: Unsigned, x: Unsigned) > value: Value[l])
is deterministic,
stateless keyevaluation algorithm run by each Aggregator. It returns the
value corresponding to index x
. The value of l
MUST be in [1, BITS]
and
the value of x
MUST be in range [2^(l1), 2^l)
.¶
A concrete IDPF specifies a single associated constant:¶
BITS: Unsigned
is the length of each Client input.¶
A concrete IDPF also specifies the following associated types:¶
Field[l]
for each level 1 <= l <= BITS
. Each defines the same methods and
associated constants as Field
in Section 7.¶
Note that IDPF construction of [BBCGGI21] uses one field for the inner nodes of the tree and a different, larger field for the leaf nodes. See [BBCGGI21], Section 4.3.¶
Finally, an implementation note. The interface for IDPFs specified here is stateless, in the sense that there is no state carried between IDPF evaluations. This is to align the IDPF syntax with the VDAF abstraction boundary, which does not include shared state across across VDAF evaluations. In practice, of course, it will often be beneficial to expose a stateful API for IDPFs and carry the state across evaluations.¶
The VDAF involves two rounds of communication (ROUNDS == 2
) and is defined for
two Aggregators (SHARES == 2
).¶
The verification parameter is a symmetric key shared by both Aggregators. This VDAF has no public parameter.¶
The client's input is an IDPF index, denoted alpha
. The values are pairs of
field elements (1, k)
where each k
is chosen at random. This random value is
used as part of the secure sketching protocol of [BBCGGI21]. After evaluating
their IDPF key shares on the set of candidate prefixes, the sketching protocol
is used by the Aggregators to verify that they hold shares of a onehot vector.
In addition, for each level of the tree, the prover generates random elements
a
, b
, and c
and computes¶
A = 2*a + k B = a*a + b  k*a + c¶
and sends additive shares of a
, b
, c
, A
and B
to the Aggregators.
Putting everything together, the inputdistribution algorithm is defined as
follows. Function encode_input_share
is defined in Section 8.2.5.¶
The aggregation parameter encodes a sequence of candidate prefixes. When an
Aggregator receives an input share from the Client, it begins by evaluating its
IDPF share on each candidate prefix, recovering a pair of vectors of field
elements data_share
and auth_share
, The Aggregators use auth_share
and the
correlation shares provided by the Client to verify that their data_share
vectors are additive shares of a onehot vector.¶
CP Consider adding aggregation parameter as input to k_verify_rand
derivation.¶
TODO Specify the following functionalities:¶
encode_input_share
is used to encode an input share, consisting of an IDPF
key share and correlation shares.¶
decode_input_share
is used to decode an input share.¶
decode_indexes(encoded: Bytes) > (l: Unsigned, indexes: Vec[Unsigned])
decodes a sequence of indexes, i.e., candidate indexes for IDPF evaluation.
The value of l
MUST be in range [1, BITS]
and indexes[i]
MUST be in range
[2^(l1), 2^l)
for all i
. An error is raised if encoded
cannot be
decoded.¶
NOTE: This is a brief outline of the security considerations. This section will be filled out more as the draft matures and security analyses are completed.¶
VDAFs have two essential security goals:¶
Note that, to achieve robustness, it is important to ensure that the
verification key distributed to the Aggregators (verify_key
, see Section 8.2.1) is
never revealed to the Clients.¶
It is also possible to consider a stronger form of robustness, where the
attacker also controls a subset of Aggregators (see [BBCGGI19], Section 6.3).
To satisfy this stronger notion of robustness, it is necessary to prevent the
attacker from sharing the verification key with the Clients. It is therefore
RECOMMENDED that the Aggregators generate verify_key
only after a set of
Client inputs has been collected for verification, and regenerate them for each
such set of inputs.¶
In order to achieve robustness, the Aggregators MUST ensure that the nonces used to process the measurements in a batch are all unique.¶
A VDAF is the core cryptographic primitive of a protocol that achieves the above privacy and robustness goals. It is not sufficient on its own, however. The application will need to assure a few security properties, for example:¶
Establishing secure channels:¶
In such an environment, a VDAF provides the highlevel privacy property described above: The Collector learns only the aggregate measurement, and nothing about individual measurements aside from what can be inferred from the aggregate result. The Aggregators learn neither individual measurements nor the aggregate result. The Collector is assured that the aggregate statistic accurately reflects the inputs as long as the Aggregators correctly executed their role in the VDAF.¶
On their own, VDAFs do not mitigate Sybil attacks [Dou02]. In this attack, the adversary observes a subset of input shares transmitted by a Client it is interested in. It allows the input shares to be processed, but corrupts and picks bogus inputs for the remaining Clients. Applications can guard against these risks by adding additional controls on measurement submission, such as client authentication and rate limits.¶
VDAFs do not inherently provide differential privacy [Dwo06]. The VDAF approach to private measurement can be viewed as complementary to differential privacy, relying on noncollusion instead of statistical noise to protect the privacy of the inputs. It is possible that a future VDAF could incorporate differential privacy features, e.g., by injecting noise before the sharding stage and removing it after unsharding.¶
This document makes no request of IANA.¶
Thanks to David Cook, Henry CorriganGibbs, Armando FazHernandez, Simon Friedberger, Tim Geoghegan, Mariana Raykova, Jacob Rothstein, and Christopher Wood for useful feedback on and contributions to the spec.¶
NOTE Machinereadable test vectors can be found at https://github.com/cfrg/draftirtfcfrgvdaf/tree/main/poc/test_vec.¶
Test vectors cover the generation of input shares and the conversion of input
shares into output shares. Vectors specify the verification key, measurements,
aggregation parameter, and any parameters needed to construct the VDAF. (For
example, for Prio3AesSum
, the user specifies the number of bits for
representing each summand.)¶
Byte strings are encoded in hexadecimal To make the tests deterministic,
gen_rand()
was replaced with a function that returns the requested number of
0x01
octets.¶
verify_key: "01010101010101010101010101010101" upload_0: measurement: 1 nonce: "01010101010101010101010101010101" input_share_0: > ae5483343eb35a52fcb36a62271a7ddb47f09d0ea2c6613807f84ac2e16814c82bca bdc9db5080fdf4f4f778734644fc input_share_1: > 0101010101010101010101010101010101010101010101010101010101010101 round_0: prep_share_0: > 22ce013d3aaa7e7574ed01fe1d074cd845dfbbbc5901cabd487d4e2e228274cc prep_share_1: > dd31fec1c555818c51ab7ccac14ca5b00aae1c33d835c76dfa9406011a92a8e9 prep_message: > out_share_0:  12561809521056635474 out_share_1:  5884934548357948848 agg_share_0: > ae5483343eb35a52 agg_share_1: > 51ab7ccac14ca5b0 agg_result: [1]¶
bits: 8 verify_key: "01010101010101010101010101010101" upload_0: measurement: 100 nonce: "01010101010101010101010101010101" input_share_0: > ae5483353eb35a3371beec8f796e9afd086cb72d05a83a3dbefbe273acb0410787b1 afba2065df5389011fd8963091e3004fa07fc91018af378da47c89abf1bd85047e40 874e2cdc5f3bc48f363b89f746770a402a777bed31b5a10c7319b3908d72de0c6512 15ba78d3cf681e07c564c0b4a9a4508df645bad8fef61e3ddf37fcb36a63271a7dbe 47f09d0ea2c661396ad006d8915d149ad88f9b1cdb86e1d13d683c359b7ac899a245 4316051e4e235dfd566f3459c336826555ed7f1baabf241e9a697d458912f3bd3778 225e832b78cd4f17e57c9b9678cf6043894aff0d0f2e06828982ac3493ae5ded0c98 86ea13d52bc0f209dc2f4e676c42b95b548a413f67b03ff18e9e6b699338400e9dff a800563abb495364acffc17126bf0bf8ff3c5caba82333e91352e03c637d44dc4db1 59a1b19d8db4d5a3fce356f6f2fca4adc9bcf65bec8d4d962b2b40f7ea413aead099 79d4958707bf4098bb28829b79e381aaad8f69b7f2e6c159bbcb342ef7df2d9c56a9 06b171ab61b025b7c19aad8de495a8a97af2baab6d1240d30df417d1cc0fe7a90ada ad8115924c0987fe1d16abe0c8a3c297d58a3112b818df72a10a41b34aa6b4ae370b 1340a6085c8dcd597eead5d2584fdb160f0a086a56ea6a7736666ae34d3012fdb2c2 4af3d4b2a6ae735edfe837eaab1309eaa2d8273e7dbfe0fd4166d545ce8354e1237b 48456715d12e38d02cd64c96b9daa01a2281d8a930817088c648b7c115e1550ada14 b6072ada49be3c7e3f184db2461160d29937caa97db6020a5598063f1dc05653d1d3 80b34e923bd7170eeeb811bfc3ce12c1df55cf552e986a823743fac4723a48bda6c1 8ffec653c1f182890197e9fc74631dcbd0283c4258933c03aee9404f010101010101 01010101010101010101fb0c701f7c07b9407a4a7b77d1ea017e input_share_1: > 01010101010101010101010101010101010101010101010101010101010101010101 01010101010101010101010101012d7667bffd0f81b078896503385f6f13 round_0: prep_share_0: > 9f7aca77f790b930b46e8cd786ff1a239aa00e7aaaa734cc2bbcb121eb7c5bc00a ef22fd95a24cd1a0054bde0dba06062d7667bffd0f81b078896503385f6f13 prep_share_1: > 60853588086f46b34b9173287900e5de2becb1bdb8a8009d2cdc258674f08e8157 e3a202a38282c20e220e733ab61e4cfb0c701f7c07b9407a4a7b77d1ea017e prep_message: > d67a17a0810838f002c31e74e9b56e6d out_share_0:  242787699414660215404830418280405596120 out_share_1:  97494667506278247542035355087495170189 agg_share_0: > b6a735c5636efee29c0c1455e0c0f7d8 agg_share_1: > 4958ca3a9c91010163f3ebaa1f3f088d agg_result: [100]¶
buckets: [1, 10, 100] verify_key: "01010101010101010101010101010101" upload_0: measurement: 50 nonce: "01010101010101010101010101010101" input_share_0: > ae5483353eb35a3371beec8f796e9afd086cb72d05a83a3dbefbe273acb0410787b1 afba2065df5389011fd8963091e3004fa07fc91018af378da47c89abf1bdfcb36a63 271a7dbe47f09d0ea2c6613956dfe44e1302160dd2ade0205aa0409225caf0f966df 97691568169000ef0af27c0985636e34889bc3fef4df192d7ead56e0dd51187bdc66 62505cbd2962843cf2a1929642367f32058c531a6c611d76441e4ba82d136ba4aab1 6f2a612df63678d42d527e59d8a0b4cb2f07ed8aaf04199819a25fad1b8cad62fb2e c5a9bd78b2e013a50250c8bd44a15ad7d5edac35a58bed81a4088c72430afbd6fe34 635a737cb7c4d29ffc9947b6b0fb8f3fdede9d8bd495b4d47e8400bded8aa53e4a5a 2d063c6091c29613e044082b0555ce74c45b823aa8c5804aacdd3dc92a6ac0058755 7770972dcdc37eefb42eef43a1b401010101010101010101010101010101d5bf864d e68bac19204e29697bf9504d input_share_1: > 01010101010101010101010101010101010101010101010101010101010101010101 01010101010101010101010101018e2e553b5e45c62e0ced57ec947c8627 round_0: prep_share_0: > 93b9dd4a3b46d8941fe7524a5cf1cd47ff8ee9c0c2e9b8230b1b940b665263b7b1 c4b370652a333ee774ec9cd379b6e78e2e553b5e45c62e0ced57ec947c8627 prep_share_1: > 6c4622b5c4b9274fe018adb5a30e32baa310ddd3e5ed87892dca520d1bed7d0299 8e38190652ba60a225e19211d77e22d5bf864de68bac19204e29697bf9504d prep_message: > 5b91d376b8ce6a372ca37e85ef85d66a out_share_0:  231724485416847873323492487111470127869  11198307274976669387765744195748249863  180368380143069850478496598824148046307  413446761563421317675646300023681469 out_share_1:  108557881504090589623373286256430638340  329084059645961793559100029172152516346  159913986777868612468369174543752719903  339868920159375041629190127067877084740 agg_share_0: > ae5483353eb35a3371beec8f796e9afd086cb72d05a83a3dbefbe273acb0410787b1af ba2065df5389011fd8963091e3004fa07fc91018af378da47c89abf1bd agg_share_1: > 51ab7ccac14ca5b08e41137086916504f79348d2fa57c5a641041d8c534fbefa784e50 45df9a209076fee02769cf6e1fffb05f8036efe734c8725b8376540e44 agg_result: [0, 0, 1, 0]¶