Introduce new trait for integers in field module (#853)

src/field.rs

@@ -166,33 +166,44 @@ pub trait FieldElement:
     }
 }
 
+/// An integer type that accompanies a finite field. Integers and field elements may be converted
+/// back and forth via the natural map between residue classes modulo 'p' and integers between 0
+/// and p - 1.
+pub trait Integer:
+    Debug
+    + Eq
+    + Ord
+    + BitAnd<Output = Self>
+    + Div<Output = Self>
+    + Shl<usize, Output = Self>
+    + Shr<usize, Output = Self>
+    + Add<Output = Self>
+    + Sub<Output = Self>
+    + TryFrom<usize, Error = Self::TryFromUsizeError>
+    + TryInto<u64, Error = Self::TryIntoU64Error>
+{
+    /// The error returned if converting `usize` to this integer type fails.
+    type TryFromUsizeError: std::error::Error;
+
+    /// The error returned if converting this integer type to a `u64` fails.
+    type TryIntoU64Error: std::error::Error;
+
+    /// Returns zero.
+    fn zero() -> Self;
+
+    /// Returns one.
+    fn one() -> Self;
+}
+
 /// Extension trait for field elements that can be converted back and forth to an integer type.
 ///
 /// The `Integer` associated type is an integer (primitive or otherwise) that supports various
 /// arithmetic operations. The order of the field is guaranteed to fit inside the range of the
 /// integer type. This trait also defines methods on field elements, `pow` and `modulus`, that make
 /// use of the associated integer type.
 pub trait FieldElementWithInteger: FieldElement + From<Self::Integer> {
-    /// The error returned if converting `usize` to an `Integer` fails.
-    type IntegerTryFromError: std::error::Error;
-
-    /// The error returned if converting an `Integer` to a `u64` fails.
-    type TryIntoU64Error: std::error::Error;
-
     /// The integer representation of a field element.
-    type Integer: Copy
-        + Debug
-        + Eq
-        + Ord
-        + BitAnd<Output = Self::Integer>
-        + Div<Output = Self::Integer>
-        + Shl<usize, Output = Self::Integer>
-        + Shr<usize, Output = Self::Integer>
-        + Add<Output = Self::Integer>
-        + Sub<Output = Self::Integer>
-        + From<Self>
-        + TryFrom<usize, Error = Self::IntegerTryFromError>
-        + TryInto<u64, Error = Self::TryIntoU64Error>;
+    type Integer: Integer + From<Self> + Copy;
 
     /// Modular exponentation, i.e., `self^exp (mod p)`.
     fn pow(&self, exp: Self::Integer) -> Self;
@@ -216,7 +227,7 @@ pub trait FieldElementWithInteger: FieldElement + From<Self::Integer> {
         // Check if the input value can be represented in the requested number of bits by shifting
         // it. The above check on `bits` ensures this shift won't panic due to the shift width
         // being too large.
-        if input >> bits != Self::zero_integer() {
+        if input >> bits != Self::Integer::zero() {
             return Err(FieldError::InputSizeMismatch);
         }
 
@@ -274,7 +285,7 @@ where
     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
         let bit_offset = self.inner.next()?;
-        Some(F::from((self.input >> bit_offset) & F::one_integer()))
+        Some(F::from((self.input >> bit_offset) & F::Integer::one()))
     }
 }
 
@@ -299,28 +310,14 @@ pub(crate) trait FieldElementWithIntegerExt: FieldElementWithInteger {
         if bits >= 8 * Self::ENCODED_SIZE {
             return false;
         }
-        if Self::modulus() >> bits != Self::zero_integer() {
+        if Self::modulus() >> bits != Self::Integer::zero() {
             return true;
         }
         false
     }
-
-    /// Returns the integer representation of the additive identity.
-    fn zero_integer() -> Self::Integer;
-
-    /// Returns the integer representation of the multiplicative identity.
-    fn one_integer() -> Self::Integer;
 }
 
-impl<F: FieldElementWithInteger> FieldElementWithIntegerExt for F {
-    fn zero_integer() -> Self::Integer {
-        0usize.try_into().unwrap()
-    }
-
-    fn one_integer() -> Self::Integer {
-        1usize.try_into().unwrap()
-    }
-}
+impl<F: FieldElementWithInteger> FieldElementWithIntegerExt for F {}
 
 /// Methods common to all `FieldElement` implementations that are private to the crate.
 pub(crate) trait FieldElementExt: FieldElement {
@@ -711,8 +708,6 @@ macro_rules! make_field {
 
         impl FieldElementWithInteger for $elem {
             type Integer = $int;
-            type IntegerTryFromError = <Self::Integer as TryFrom<usize>>::Error;
-            type TryIntoU64Error = <Self::Integer as TryInto<u64>>::Error;
 
             fn pow(&self, exp: Self::Integer) -> Self {
                 // FieldParameters::pow() relies on mul(), and will always return a value less
@@ -745,6 +740,45 @@ macro_rules! make_field {
     };
 }
 
+impl Integer for u32 {
+    type TryFromUsizeError = <Self as TryFrom<usize>>::Error;
+    type TryIntoU64Error = <Self as TryInto<u64>>::Error;
+
+    fn zero() -> Self {
+        0
+    }
+
+    fn one() -> Self {
+        1
+    }
+}
+
+impl Integer for u64 {
+    type TryFromUsizeError = <Self as TryFrom<usize>>::Error;
+    type TryIntoU64Error = <Self as TryInto<u64>>::Error;
+
+    fn zero() -> Self {
+        0
+    }
+
+    fn one() -> Self {
+        1
+    }
+}
+
+impl Integer for u128 {
+    type TryFromUsizeError = <Self as TryFrom<usize>>::Error;
+    type TryIntoU64Error = <Self as TryInto<u64>>::Error;
+
+    fn zero() -> Self {
+        0
+    }
+
+    fn one() -> Self {
+        1
+    }
+}
+
 make_field!(
     /// Same as Field32, but encoded in little endian for compatibility with Prio v2.
     FieldPrio2,
@@ -850,16 +884,14 @@ pub(crate) fn decode_fieldvec<F: FieldElement>(
 
 #[cfg(test)]
 pub(crate) mod test_utils {
-    use super::{FieldElement, FieldElementWithInteger};
+    use super::{FieldElement, FieldElementWithInteger, Integer};
     use crate::{codec::CodecError, field::FieldError, prng::Prng};
     use assert_matches::assert_matches;
     use std::{
         collections::hash_map::DefaultHasher,
-        convert::{TryFrom, TryInto},
-        fmt::Debug,
+        convert::TryFrom,
         hash::{Hash, Hasher},
         io::Cursor,
-        ops::{Add, BitAnd, Div, Shl, Shr, Sub},
     };
 
     /// A test-only copy of `FieldElementWithInteger`.
@@ -870,54 +902,42 @@ pub(crate) mod test_utils {
     /// requires the `Integer` associated type satisfy `Clone`, not `Copy`, so that it may be used
     /// with arbitrary precision integer implementations.
     pub(crate) trait TestFieldElementWithInteger:
-        FieldElement + From<Self::Integer>
+        FieldElement + From<Self::TestInteger>
     {
         type IntegerTryFromError: std::error::Error;
         type TryIntoU64Error: std::error::Error;
-        type Integer: Clone
-            + Debug
-            + Eq
-            + Ord
-            + BitAnd<Output = Self::Integer>
-            + Div<Output = Self::Integer>
-            + Shl<usize, Output = Self::Integer>
-            + Shr<usize, Output = Self::Integer>
-            + Add<Output = Self::Integer>
-            + Sub<Output = Self::Integer>
-            + From<Self>
-            + TryFrom<usize, Error = Self::IntegerTryFromError>
-            + TryInto<u64, Error = Self::TryIntoU64Error>;
-
-        fn pow(&self, exp: Self::Integer) -> Self;
-
-        fn modulus() -> Self::Integer;
+        type TestInteger: Integer + From<Self> + Clone;
+
+        fn pow(&self, exp: Self::TestInteger) -> Self;
+
+        fn modulus() -> Self::TestInteger;
     }
 
     impl<F> TestFieldElementWithInteger for F
     where
         F: FieldElementWithInteger,
     {
-        type IntegerTryFromError = <F as FieldElementWithInteger>::IntegerTryFromError;
-        type TryIntoU64Error = <F as FieldElementWithInteger>::TryIntoU64Error;
-        type Integer = <F as FieldElementWithInteger>::Integer;
+        type IntegerTryFromError = <F::Integer as Integer>::TryFromUsizeError;
+        type TryIntoU64Error = <F::Integer as Integer>::TryIntoU64Error;
+        type TestInteger = F::Integer;
 
-        fn pow(&self, exp: Self::Integer) -> Self {
+        fn pow(&self, exp: Self::TestInteger) -> Self {
             <F as FieldElementWithInteger>::pow(self, exp)
         }
 
-        fn modulus() -> Self::Integer {
+        fn modulus() -> Self::TestInteger {
             <F as FieldElementWithInteger>::modulus()
         }
     }
 
     pub(crate) fn field_element_test_common<F: TestFieldElementWithInteger>() {
         let mut prng: Prng<F, _> = Prng::new().unwrap();
         let int_modulus = F::modulus();
-        let int_one = F::Integer::try_from(1).unwrap();
+        let int_one = F::TestInteger::try_from(1).unwrap();
         let zero = F::zero();
         let one = F::one();
-        let two = F::from(F::Integer::try_from(2).unwrap());
-        let four = F::from(F::Integer::try_from(4).unwrap());
+        let two = F::from(F::TestInteger::try_from(2).unwrap());
+        let four = F::from(F::TestInteger::try_from(4).unwrap());
 
         // add
         assert_eq!(F::from(int_modulus.clone() - int_one.clone()) + one, zero);
@@ -971,10 +991,22 @@ pub(crate) mod test_utils {
         assert_eq!(a, c);
 
         // integer conversion
-        assert_eq!(F::Integer::from(zero), F::Integer::try_from(0).unwrap());
-        assert_eq!(F::Integer::from(one), F::Integer::try_from(1).unwrap());
-        assert_eq!(F::Integer::from(two), F::Integer::try_from(2).unwrap());
-        assert_eq!(F::Integer::from(four), F::Integer::try_from(4).unwrap());
+        assert_eq!(
+            F::TestInteger::from(zero),
+            F::TestInteger::try_from(0).unwrap()
+        );
+        assert_eq!(
+            F::TestInteger::from(one),
+            F::TestInteger::try_from(1).unwrap()
+        );
+        assert_eq!(
+            F::TestInteger::from(two),
+            F::TestInteger::try_from(2).unwrap()
+        );
+        assert_eq!(
+            F::TestInteger::from(four),
+            F::TestInteger::try_from(4).unwrap()
+        );
 
         // serialization
         let test_inputs = vec![
@@ -1032,7 +1064,7 @@ pub(crate) mod test_utils {
         // various products that should be equal have the same hash. Three is chosen as a generator
         // here because it happens to generate fairly large subgroups of (Z/pZ)* for all four
         // primes.
-        let three = F::from(F::Integer::try_from(3).unwrap());
+        let three = F::from(F::TestInteger::try_from(3).unwrap());
         let mut powers_of_three = Vec::with_capacity(500);
         let mut power = one;
         for _ in 0..500 {

src/field/field255.rs

@@ -341,7 +341,7 @@ mod tests {
         codec::Encode,
         field::{
             test_utils::{field_element_test_common, TestFieldElementWithInteger},
-            FieldElement, FieldError,
+            FieldElement, FieldError, Integer,
         },
     };
     use assert_matches::assert_matches;
@@ -375,16 +375,30 @@ mod tests {
         }
     }
 
+    impl Integer for BigUint {
+        type TryFromUsizeError = <Self as TryFrom<usize>>::Error;
+
+        type TryIntoU64Error = <Self as TryInto<u64>>::Error;
+
+        fn zero() -> Self {
+            Self::new(Vec::new())
+        }
+
+        fn one() -> Self {
+            Self::new(Vec::from([1]))
+        }
+    }
+
     impl TestFieldElementWithInteger for Field255 {
-        type Integer = BigUint;
-        type IntegerTryFromError = <Self::Integer as TryFrom<usize>>::Error;
-        type TryIntoU64Error = <Self::Integer as TryInto<u64>>::Error;
+        type TestInteger = BigUint;
+        type IntegerTryFromError = <Self::TestInteger as TryFrom<usize>>::Error;
+        type TryIntoU64Error = <Self::TestInteger as TryInto<u64>>::Error;
 
-        fn pow(&self, _exp: Self::Integer) -> Self {
+        fn pow(&self, _exp: Self::TestInteger) -> Self {
             unimplemented!("Field255::pow() is not implemented because it's not needed yet")
         }
 
-        fn modulus() -> Self::Integer {
+        fn modulus() -> Self::TestInteger {
             MODULUS.clone()
         }
     }

src/flp/types/fixedpoint_l2.rs

@@ -172,15 +172,17 @@
 pub mod compatible_float;
 
 use crate::dp::{distributions::ZCdpDiscreteGaussian, DifferentialPrivacyStrategy, DpError};
-use crate::field::{Field128, FieldElement, FieldElementWithInteger, FieldElementWithIntegerExt};
+use crate::field::{
+    Field128, FieldElement, FieldElementWithInteger, FieldElementWithIntegerExt, Integer,
+};
 use crate::flp::gadgets::{Mul, ParallelSumGadget, PolyEval};
 use crate::flp::types::fixedpoint_l2::compatible_float::CompatibleFloat;
 use crate::flp::types::parallel_sum_range_checks;
 use crate::flp::{FlpError, Gadget, Type, TypeWithNoise};
 use crate::vdaf::xof::SeedStreamTurboShake128;
 use fixed::traits::Fixed;
 use num_bigint::{BigInt, BigUint, TryFromBigIntError};
-use num_integer::Integer;
+use num_integer::Integer as _;
 use num_rational::Ratio;
 use rand::{distributions::Distribution, Rng};
 use rand_core::SeedableRng;
@@ -250,7 +252,7 @@ where
     /// fixed point vector with `entries` entries.
     pub fn new(entries: usize) -> Result<Self, FlpError> {
         // (0) initialize constants
-        let fi_one = Field128::one_integer();
+        let fi_one = <Field128 as FieldElementWithInteger>::Integer::one();
 
         // (I) Check that the fixed type is compatible.
         //
@@ -537,7 +539,7 @@ where
 
             // Chunks which are too short need to be extended with a share of the
             // encoded zero value, that is: 1/num_shares * (2^(n-1))
-            let fi_one = Field128::one_integer();
+            let fi_one = <Field128 as FieldElementWithInteger>::Integer::one();
             let zero_enc = Field128::from(fi_one << (self.bits_per_entry - 1));
             let zero_enc_share = zero_enc * num_shares_inverse;