Switch from const generic to associated const for verify key len.

This saves consumers of the Aggregator trait from needing to specify an
L of their own.

src/vdaf.rs

@@ -170,10 +170,13 @@ where
 }
 
 /// The Aggregator's role in the execution of a VDAF.
-pub trait Aggregator<const L: usize>: Vdaf
+pub trait Aggregator: Vdaf
 where
     for<'a> &'a Self::AggregateShare: Into<Vec<u8>>,
 {
+    /// The length of a verification key, in bytes.
+    const VERIFY_KEY_LEN: usize;
+
     /// State of the Aggregator during the Prepare process.
     type PrepareState: Clone + Debug;
 
@@ -194,7 +197,7 @@ where
     /// message.
     fn prepare_init(
         &self,
-        verify_key: &[u8; L],
+        verify_key: &[u8; Self::VERIFY_KEY_LEN],
         agg_id: usize,
         agg_param: &Self::AggregationParam,
         nonce: &[u8],
@@ -219,7 +222,7 @@ where
         &self,
         state: Self::PrepareState,
         input: Self::PrepareMessage,
-    ) -> Result<PrepareTransition<Self, L>, VdafError>;
+    ) -> Result<PrepareTransition<Self>, VdafError>;
 
     /// Aggregates a sequence of output shares into an aggregate share.
     fn aggregate<M: IntoIterator<Item = Self::OutputShare>>(
@@ -245,7 +248,7 @@ where
 
 /// A state transition of an Aggregator during the Prepare process.
 #[derive(Debug)]
-pub enum PrepareTransition<V: Aggregator<L>, const L: usize>
+pub enum PrepareTransition<V: Aggregator>
 where
     for<'a> &'a V::AggregateShare: Into<Vec<u8>>,
 {

src/vdaf/poplar1.rs

@@ -438,11 +438,13 @@ fn get_level(agg_param: &BTreeSet<IdpfInput>) -> Result<usize, VdafError> {
     }
 }
 
-impl<I, P, const L: usize> Aggregator<L> for Poplar1<I, P, L>
+impl<I, P, const L: usize> Aggregator for Poplar1<I, P, L>
 where
     I: Idpf<2, 2>,
     P: Prg<L>,
 {
+    const VERIFY_KEY_LEN: usize = L;
+
     type PrepareState = Poplar1PrepareState<I::Field>;
     type PrepareShare = Poplar1PrepareMessage<I::Field>;
     type PrepareMessage = Poplar1PrepareMessage<I::Field>;
@@ -571,7 +573,7 @@ where
         &self,
         mut state: Poplar1PrepareState<I::Field>,
         msg: Poplar1PrepareMessage<I::Field>,
-    ) -> Result<PrepareTransition<Self, L>, VdafError> {
+    ) -> Result<PrepareTransition<Self>, VdafError> {
         match &state.sketch {
             SketchState::RoundOne => {
                 if msg.0.len() != 3 {

src/vdaf/prio2.rs

@@ -185,14 +185,16 @@ impl ParameterizedDecode<Prio2PrepareState> for Prio2PrepareShare {
     }
 }
 
-impl Aggregator<32> for Prio2 {
+impl Aggregator for Prio2 {
+    const VERIFY_KEY_LEN: usize = 32;
+
     type PrepareState = Prio2PrepareState;
     type PrepareShare = Prio2PrepareShare;
     type PrepareMessage = ();
 
     fn prepare_init(
         &self,
-        agg_key: &[u8; 32],
+        agg_key: &[u8; Self::VERIFY_KEY_LEN],
         agg_id: usize,
         _agg_param: &(),
         nonce: &[u8],
@@ -239,7 +241,7 @@ impl Aggregator<32> for Prio2 {
         &self,
         state: Prio2PrepareState,
         _input: (),
-    ) -> Result<PrepareTransition<Self, 32>, VdafError> {
+    ) -> Result<PrepareTransition<Self>, VdafError> {
         let data = match state.0 {
             Share::Leader(data) => data,
             Share::Helper(seed) => {

src/vdaf/prio3.rs

@@ -764,11 +764,13 @@ where
     }
 }
 
-impl<T, P, const L: usize> Aggregator<L> for Prio3<T, P, L>
+impl<T, P, const L: usize> Aggregator for Prio3<T, P, L>
 where
     T: Type,
     P: Prg<L>,
 {
+    const VERIFY_KEY_LEN: usize = L;
+
     type PrepareState = Prio3PrepareState<T::Field, L>;
     type PrepareShare = Prio3PrepareShare<T::Field, L>;
     type PrepareMessage = Prio3PrepareMessage<L>;
@@ -943,7 +945,7 @@ where
         &self,
         step: Prio3PrepareState<T::Field, L>,
         msg: Prio3PrepareMessage<L>,
-    ) -> Result<PrepareTransition<Self, L>, VdafError> {
+    ) -> Result<PrepareTransition<Self>, VdafError> {
         if self.typ.joint_rand_len() > 0 {
             // Check that the joint randomness was correct.
             if step.joint_rand_seed.as_ref().unwrap() != msg.joint_rand_seed.as_ref().unwrap() {