General improvements (#56)

* Apply Rust traits to all public types and other improvements

* Move methods into appropriate section

* Check for zero scalars

* Change element and scalar de/serialization from `GenericArray` to slice

* Customize `serde` serialization

src/group/elliptic_curve.rs

@@ -89,7 +89,7 @@ where
         result
     }
 
-    fn deserialize_elem(element_bits: &GenericArray<u8, Self::ElemLen>) -> Result<Self::Elem> {
+    fn deserialize_elem(element_bits: &[u8]) -> Result<Self::Elem> {
         PublicKey::<Self>::from_sec1_bytes(element_bits)
             .map(|public_key| public_key.to_projective())
             .map_err(|_| Error::Deserialization)
@@ -103,6 +103,10 @@ where
         Option::from(scalar.invert()).unwrap()
     }
 
+    fn is_zero_scalar(scalar: Self::Scalar) -> subtle::Choice {
+        scalar.is_zero()
+    }
+
     #[cfg(test)]
     fn zero_scalar() -> Self::Scalar {
         Scalar::<Self>::zero()
@@ -112,7 +116,7 @@ where
         scalar.into()
     }
 
-    fn deserialize_scalar(scalar_bits: &GenericArray<u8, Self::ScalarLen>) -> Result<Self::Scalar> {
+    fn deserialize_scalar(scalar_bits: &[u8]) -> Result<Self::Scalar> {
         SecretKey::<Self>::from_be_bytes(scalar_bits)
             .map(|secret_key| *secret_key.to_nonzero_scalar())
             .map_err(|_| Error::Deserialization)

src/group/mod.rs

@@ -20,7 +20,7 @@ use generic_array::{ArrayLength, GenericArray};
 use rand_core::{CryptoRng, RngCore};
 #[cfg(feature = "ristretto255")]
 pub use ristretto::Ristretto255;
-use subtle::ConstantTimeEq;
+use subtle::{Choice, ConstantTimeEq};
 use zeroize::Zeroize;
 
 use crate::voprf::Mode;
@@ -93,7 +93,7 @@ pub trait Group {
     /// # Errors
     /// [`Error::Deserialization`](crate::Error::Deserialization) if the element
     /// is not a valid point on the group or the identity element.
-    fn deserialize_elem(element_bits: &GenericArray<u8, Self::ElemLen>) -> Result<Self::Elem>;
+    fn deserialize_elem(element_bits: &[u8]) -> Result<Self::Elem>;
 
     /// picks a scalar at random
     fn random_scalar<R: RngCore + CryptoRng>(rng: &mut R) -> Self::Scalar;
@@ -101,6 +101,9 @@ pub trait Group {
     /// The multiplicative inverse of this scalar
     fn invert_scalar(scalar: Self::Scalar) -> Self::Scalar;
 
+    /// Returns `true` if the scalar is zero.
+    fn is_zero_scalar(scalar: Self::Scalar) -> Choice;
+
     /// Returns the scalar representing zero
     #[cfg(test)]
     fn zero_scalar() -> Self::Scalar;
@@ -114,7 +117,7 @@ pub trait Group {
     /// # Errors
     /// [`Error::Deserialization`](crate::Error::Deserialization) if the scalar
     /// is not a valid point on the group or zero.
-    fn deserialize_scalar(scalar_bits: &GenericArray<u8, Self::ScalarLen>) -> Result<Self::Scalar>;
+    fn deserialize_scalar(scalar_bits: &[u8]) -> Result<Self::Scalar>;
 }
 
 #[cfg(test)]

src/group/ristretto.rs

@@ -16,12 +16,16 @@ use generic_array::sequence::Concat;
 use generic_array::typenum::{IsLess, IsLessOrEqual, U256, U32, U64};
 use generic_array::GenericArray;
 use rand_core::{CryptoRng, RngCore};
+use subtle::ConstantTimeEq;
 
 use super::{Group, STR_HASH_TO_GROUP, STR_HASH_TO_SCALAR};
 use crate::voprf::{self, Mode};
 use crate::{CipherSuite, Error, InternalError, Result};
 
 /// [`Group`] implementation for Ristretto255.
+#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
+// `cfg` here is only needed because of a bug in Rust's crate feature documentation. See: https://github.com/rust-lang/rust/issues/83428
+#[cfg(feature = "ristretto255")]
 pub struct Ristretto255;
 
 #[cfg(feature = "ristretto255-ciphersuite")]
@@ -99,7 +103,7 @@ impl Group for Ristretto255 {
         elem.compress().to_bytes().into()
     }
 
-    fn deserialize_elem(element_bits: &GenericArray<u8, Self::ElemLen>) -> Result<Self::Elem> {
+    fn deserialize_elem(element_bits: &[u8]) -> Result<Self::Elem> {
         CompressedRistretto::from_slice(element_bits)
             .decompress()
             .filter(|point| point != &RistrettoPoint::identity())
@@ -124,6 +128,10 @@ impl Group for Ristretto255 {
         scalar.invert()
     }
 
+    fn is_zero_scalar(scalar: Self::Scalar) -> subtle::Choice {
+        scalar.ct_eq(&Scalar::zero())
+    }
+
     #[cfg(test)]
     fn zero_scalar() -> Self::Scalar {
         Scalar::zero()
@@ -133,8 +141,11 @@ impl Group for Ristretto255 {
         scalar.to_bytes().into()
     }
 
-    fn deserialize_scalar(scalar_bits: &GenericArray<u8, Self::ScalarLen>) -> Result<Self::Scalar> {
-        Scalar::from_canonical_bytes((*scalar_bits).into())
+    fn deserialize_scalar(scalar_bits: &[u8]) -> Result<Self::Scalar> {
+        scalar_bits
+            .try_into()
+            .ok()
+            .and_then(Scalar::from_canonical_bytes)
             .filter(|scalar| scalar != &Scalar::zero())
             .ok_or(Error::Deserialization)
     }