Move szk module from flp to vdaf::mastic

Some of the implementation details of SZK are, for the moment, specific
to Mastic. For example, the domain separation tag for each XOF
invocation is prefixed by `b"mastic"`. It therefore makes sense to move
this module to `mastic` until such time we need to refactor in order to
use SZK in multiple places.

src/flp.rs

@@ -57,8 +57,6 @@ use std::convert::TryFrom;
 use std::fmt::Debug;
 
 pub mod gadgets;
-#[cfg(all(feature = "crypto-dependencies", feature = "experimental"))]
-pub mod szk;
 pub mod types;
 
 /// Errors propagated by methods in this module.

src/vdaf.rs

@@ -8,10 +8,10 @@
 #[cfg(feature = "experimental")]
 use crate::dp::DifferentialPrivacyStrategy;
 #[cfg(all(feature = "crypto-dependencies", feature = "experimental"))]
-use crate::flp::szk::SzkError;
-#[cfg(all(feature = "crypto-dependencies", feature = "experimental"))]
 use crate::idpf::IdpfError;
 #[cfg(all(feature = "crypto-dependencies", feature = "experimental"))]
+use crate::vdaf::mastic::szk::SzkError;
+#[cfg(all(feature = "crypto-dependencies", feature = "experimental"))]
 use crate::vidpf::VidpfError;
 use crate::{
     codec::{CodecError, Decode, Encode, ParameterizedDecode},

src/vdaf/mastic.rs

@@ -8,10 +8,7 @@ use crate::{
     bt::BinaryTree,
     codec::{CodecError, Decode, Encode, ParameterizedDecode},
     field::{decode_fieldvec, FieldElement, FieldElementWithInteger},
-    flp::{
-        szk::{Szk, SzkInputShare, SzkJointShare, SzkQueryShare, SzkQueryState},
-        Type,
-    },
+    flp::Type,
     vdaf::{
         poplar1::{Poplar1, Poplar1AggregationParam},
         xof::{Seed, Xof},
@@ -24,6 +21,8 @@ use crate::{
     },
 };
 
+use szk::{Szk, SzkInputShare, SzkJointShare, SzkQueryShare, SzkQueryState};
+
 use rand::prelude::*;
 use std::io::{Cursor, Read};
 use std::ops::BitAnd;
@@ -33,6 +32,8 @@ use subtle::{Choice, ConstantTimeEq};
 
 use super::xof::XofTurboShake128;
 
+pub(crate) mod szk;
+
 pub(crate) const SEED_SIZE: usize = 32;
 pub(crate) const NONCE_SIZE: usize = 16;
 
@@ -319,9 +320,9 @@ impl<T: Type> Client<16> for Mastic<T> {
 
 /// Mastic preparation state.
 ///
-/// State held by an aggregator waiting for a message during Mastic preparation. Includes intermediate
-/// state for [`Szk`] verification, the output shares currently being validated, and
-/// parameters of Mastic used for encoding.
+/// State held by an aggregator waiting for a message during Mastic preparation. Includes
+/// intermediate state for the evaluation check, the range check (if applicable) verification, and
+/// the output shares currently being validated.
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct MasticPrepareState<F: FieldElement> {
     /// The counter and truncated weight for each candidate prefix.