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Replace Vdaf::ID with Vdaf::algorithm_id(&self) -> u32
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Instead of defining the algorithm ID as an associated type, define a
method for returning it from an instance of the VDAF. This allows us to
provide more flexibility for variants of (standardized) VDAFs.
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@cjpatton
cjpatton committed Nov 29, 2023
1 parent 8b8e157 commit 5ba8b3b
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Showing 5 changed files with 96 additions and 84 deletions.
10 changes: 5 additions & 5 deletions src/vdaf.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -159,9 +159,6 @@ impl<F: FieldElement, const SEED_SIZE: usize> Encode for Share<F, SEED_SIZE> {
/// and [`Collector`], which define the roles of the various parties involved in the execution of
/// the VDAF.
pub trait Vdaf: Clone + Debug {
/// Algorithm identifier for this VDAF.
const ID: u32;

/// The type of Client measurement to be aggregated.
type Measurement: Clone + Debug;

Expand Down Expand Up @@ -189,17 +186,20 @@ pub trait Vdaf: Clone + Debug {
+ for<'a> ParameterizedDecode<(&'a Self, &'a Self::AggregationParam)>
+ Encode;

/// Return the VDAF's algorithm ID.
fn algorithm_id(&self) -> u32;

/// The number of Aggregators. The Client generates as many input shares as there are
/// Aggregators.
fn num_aggregators(&self) -> usize;

/// Generate the domain separation tag for this VDAF. The output is used for domain separation
/// by the XOF.
fn domain_separation_tag(usage: u16) -> [u8; 8] {
fn domain_separation_tag(&self, usage: u16) -> [u8; 8] {
let mut dst = [0_u8; 8];
dst[0] = VERSION;
dst[1] = 0; // algorithm class
dst[2..6].copy_from_slice(&(Self::ID).to_be_bytes());
dst[2..6].copy_from_slice(&(self.algorithm_id()).to_be_bytes());
dst[6..8].copy_from_slice(&usage.to_be_bytes());
dst
}
Expand Down
6 changes: 4 additions & 2 deletions src/vdaf/dummy.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -98,8 +98,6 @@ impl Default for Vdaf {
}

impl vdaf::Vdaf for Vdaf {
const ID: u32 = 0xFFFF0000;

type Measurement = u8;
type AggregateResult = u8;
type AggregationParam = AggregationParam;
Expand All @@ -108,6 +106,10 @@ impl vdaf::Vdaf for Vdaf {
type OutputShare = OutputShare;
type AggregateShare = AggregateShare;

fn algorithm_id(&self) -> u32 {
0xFFFF0000
}

fn num_aggregators(&self) -> usize {
2
}
Expand Down
132 changes: 68 additions & 64 deletions src/vdaf/poplar1.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,7 @@ pub struct Poplar1<P, const SEED_SIZE: usize> {
impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Poplar1<P, SEED_SIZE> {
/// Construct a `Prng` with the given seed and info-string suffix.
fn init_prng<I, B, F>(
&self,
seed: &[u8; SEED_SIZE],
usage: u16,
binder_chunks: I,
Expand All @@ -75,7 +76,7 @@ impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Poplar1<P, SEED_SIZE> {
P: Xof<SEED_SIZE>,
F: FieldElement,
{
let mut xof = P::init(seed, &Self::domain_separation_tag(usage));
let mut xof = P::init(seed, &self.domain_separation_tag(usage));
for binder_chunk in binder_chunks.into_iter() {
xof.update(binder_chunk.as_ref());
}
Expand Down Expand Up @@ -839,7 +840,6 @@ impl Decode for Poplar1AggregationParam {
}

impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Vdaf for Poplar1<P, SEED_SIZE> {
const ID: u32 = 0x00001000;
type Measurement = IdpfInput;
type AggregateResult = Vec<u64>;
type AggregationParam = Poplar1AggregationParam;
Expand All @@ -848,6 +848,10 @@ impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Vdaf for Poplar1<P, SEED_SIZE> {
type OutputShare = Poplar1FieldVec;
type AggregateShare = Poplar1FieldVec;

fn algorithm_id(&self) -> u32 {
0x00001000
}

fn num_aggregators(&self) -> usize {
2
}
Expand All @@ -870,7 +874,7 @@ impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Poplar1<P, SEED_SIZE> {

// Generate the authenticator for each inner level of the IDPF tree.
let mut prng =
Self::init_prng::<_, _, Field64>(&poplar_random[2], DST_SHARD_RANDOMNESS, [&[]]);
self.init_prng::<_, _, Field64>(&poplar_random[2], DST_SHARD_RANDOMNESS, [&[]]);
let auth_inner: Vec<Field64> = (0..self.bits - 1).map(|_| prng.get()).collect();

// Generate the authenticator for the last level of the IDPF tree (i.e., the leaves).
Expand Down Expand Up @@ -900,12 +904,12 @@ impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Poplar1<P, SEED_SIZE> {
let corr_seed_0 = &poplar_random[0];
let corr_seed_1 = &poplar_random[1];
let mut prng = prng.into_new_field::<Field64>();
let mut corr_prng_0 = Self::init_prng::<_, _, Field64>(
let mut corr_prng_0 = self.init_prng::<_, _, Field64>(
corr_seed_0,
DST_CORR_INNER,
[[0].as_slice(), nonce.as_slice()],
);
let mut corr_prng_1 = Self::init_prng::<_, _, Field64>(
let mut corr_prng_1 = self.init_prng::<_, _, Field64>(
corr_seed_1,
DST_CORR_INNER,
[[1].as_slice(), nonce.as_slice()],
Expand All @@ -921,12 +925,12 @@ impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Poplar1<P, SEED_SIZE> {

// Generate the correlated randomness for the leaf nodes.
let mut prng = prng.into_new_field::<Field255>();
let mut corr_prng_0 = Self::init_prng::<_, _, Field255>(
let mut corr_prng_0 = self.init_prng::<_, _, Field255>(
corr_seed_0,
DST_CORR_LEAF,
[[0].as_slice(), nonce.as_slice()],
);
let mut corr_prng_1 = Self::init_prng::<_, _, Field255>(
let mut corr_prng_1 = self.init_prng::<_, _, Field255>(
corr_seed_1,
DST_CORR_LEAF,
[[1].as_slice(), nonce.as_slice()],
Expand All @@ -952,6 +956,59 @@ impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Poplar1<P, SEED_SIZE> {
],
))
}

/// Evaluate the IDPF at the given prefixes and compute the Aggregator's share of the sketch.
fn eval_and_sketch<F>(
&self,
verify_key: &[u8; SEED_SIZE],
agg_id: usize,
nonce: &[u8; 16],
agg_param: &Poplar1AggregationParam,
public_share: &Poplar1PublicShare,
idpf_key: &Seed<16>,
corr_prng: &mut Prng<F, P::SeedStream>,
) -> Result<(Vec<F>, Vec<F>), VdafError>
where
P: Xof<SEED_SIZE>,
F: FieldElement,
Poplar1IdpfValue<F>:
From<IdpfOutputShare<Poplar1IdpfValue<Field64>, Poplar1IdpfValue<Field255>>>,
{
let mut verify_prng = self.init_prng(
verify_key,
DST_VERIFY_RANDOMNESS,
[nonce.as_slice(), agg_param.level.to_be_bytes().as_slice()],
);

let mut out_share = Vec::with_capacity(agg_param.prefixes.len());
let mut sketch_share = vec![
corr_prng.get(), // a_share
corr_prng.get(), // b_share
corr_prng.get(), // c_share
];

let mut idpf_eval_cache = RingBufferCache::new(agg_param.prefixes.len());
let idpf = Idpf::<Poplar1IdpfValue<Field64>, Poplar1IdpfValue<Field255>>::new((), ());
for prefix in agg_param.prefixes.iter() {
let share = Poplar1IdpfValue::<F>::from(idpf.eval(
agg_id,
public_share,
idpf_key,
prefix,
nonce,
&mut idpf_eval_cache,
)?);

let r = verify_prng.get();
let checked_data_share = share.0[0] * r;
sketch_share[0] += checked_data_share;
sketch_share[1] += checked_data_share * r;
sketch_share[2] += share.0[1] * r;
out_share.push(share.0[0]);
}

Ok((out_share, sketch_share))
}
}

impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Client<16> for Poplar1<P, SEED_SIZE> {
Expand Down Expand Up @@ -1000,7 +1057,7 @@ impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Aggregator<SEED_SIZE, 16>
};

if usize::from(agg_param.level) < self.bits - 1 {
let mut corr_prng = Self::init_prng::<_, _, Field64>(
let mut corr_prng = self.init_prng::<_, _, Field64>(
input_share.corr_seed.as_ref(),
DST_CORR_INNER,
[[agg_id as u8].as_slice(), nonce.as_slice()],
Expand All @@ -1011,7 +1068,7 @@ impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Aggregator<SEED_SIZE, 16>
corr_prng.get();
}

let (output_share, sketch_share) = eval_and_sketch::<P, Field64, SEED_SIZE>(
let (output_share, sketch_share) = self.eval_and_sketch::<Field64>(
verify_key,
agg_id,
nonce,
Expand All @@ -1033,13 +1090,13 @@ impl<P: Xof<SEED_SIZE>, const SEED_SIZE: usize> Aggregator<SEED_SIZE, 16>
Poplar1FieldVec::Inner(sketch_share),
))
} else {
let corr_prng = Self::init_prng::<_, _, Field255>(
let corr_prng = self.init_prng::<_, _, Field255>(
input_share.corr_seed.as_ref(),
DST_CORR_LEAF,
[[agg_id as u8].as_slice(), nonce.as_slice()],
);

let (output_share, sketch_share) = eval_and_sketch::<P, Field255, SEED_SIZE>(
let (output_share, sketch_share) = self.eval_and_sketch::<Field255>(
verify_key,
agg_id,
nonce,
Expand Down Expand Up @@ -1257,59 +1314,6 @@ fn compute_next_corr_shares<F: FieldElement + From<u64>, S: RngCore>(
(corr_0, corr_1)
}

/// Evaluate the IDPF at the given prefixes and compute the Aggregator's share of the sketch.
fn eval_and_sketch<P, F, const SEED_SIZE: usize>(
verify_key: &[u8; SEED_SIZE],
agg_id: usize,
nonce: &[u8; 16],
agg_param: &Poplar1AggregationParam,
public_share: &Poplar1PublicShare,
idpf_key: &Seed<16>,
corr_prng: &mut Prng<F, P::SeedStream>,
) -> Result<(Vec<F>, Vec<F>), VdafError>
where
P: Xof<SEED_SIZE>,
F: FieldElement,
Poplar1IdpfValue<F>:
From<IdpfOutputShare<Poplar1IdpfValue<Field64>, Poplar1IdpfValue<Field255>>>,
{
// TODO(cjpatton) spec: Consider not encoding the prefixes here.
let mut verify_prng = Poplar1::<P, SEED_SIZE>::init_prng(
verify_key,
DST_VERIFY_RANDOMNESS,
[nonce.as_slice(), agg_param.level.to_be_bytes().as_slice()],
);

let mut out_share = Vec::with_capacity(agg_param.prefixes.len());
let mut sketch_share = vec![
corr_prng.get(), // a_share
corr_prng.get(), // b_share
corr_prng.get(), // c_share
];

let mut idpf_eval_cache = RingBufferCache::new(agg_param.prefixes.len());
let idpf = Idpf::<Poplar1IdpfValue<Field64>, Poplar1IdpfValue<Field255>>::new((), ());
for prefix in agg_param.prefixes.iter() {
let share = Poplar1IdpfValue::<F>::from(idpf.eval(
agg_id,
public_share,
idpf_key,
prefix,
nonce,
&mut idpf_eval_cache,
)?);

let r = verify_prng.get();
let checked_data_share = share.0[0] * r;
sketch_share[0] += checked_data_share;
sketch_share[1] += checked_data_share * r;
sketch_share[2] += share.0[1] * r;
out_share.push(share.0[0]);
}

Ok((out_share, sketch_share))
}

/// Compute the Aggregator's share of the sketch verifier. The shares should sum to zero.
#[allow(non_snake_case)]
fn finish_sketch<F: FieldElement>(
Expand Down
5 changes: 4 additions & 1 deletion src/vdaf/prio2.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -117,7 +117,6 @@ impl Prio2 {
}

impl Vdaf for Prio2 {
const ID: u32 = 0xFFFF0000;
type Measurement = Vec<u32>;
type AggregateResult = Vec<u32>;
type AggregationParam = ();
Expand All @@ -126,6 +125,10 @@ impl Vdaf for Prio2 {
type OutputShare = OutputShare<FieldPrio2>;
type AggregateShare = AggregateShare<FieldPrio2>;

fn algorithm_id(&self) -> u32 {
0xFFFF0000
}

fn num_aggregators(&self) -> usize {
// Prio2 can easily be extended to support more than two Aggregators.
2
Expand Down
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