git checkout -f next ./crypto

Proceeds to remove the eVRF DKG after, only keeping what's relevant to this
branch alone.

crypto/ciphersuite/src/dalek.rs

@@ -28,6 +28,12 @@ macro_rules! dalek_curve {
         $Point::generator()
       }
 
+      fn reduce_512(mut scalar: [u8; 64]) -> Self::F {
+        let res = Scalar::from_bytes_mod_order_wide(&scalar);
+        scalar.zeroize();
+        res
+      }
+
       fn hash_to_F(dst: &[u8], data: &[u8]) -> Self::F {
         Scalar::from_hash(Sha512::new_with_prefix(&[dst, data].concat()))
       }

crypto/ciphersuite/src/ed448.rs

@@ -66,6 +66,12 @@ impl Ciphersuite for Ed448 {
     Point::generator()
   }
 
+  fn reduce_512(mut scalar: [u8; 64]) -> Self::F {
+    let res = Self::hash_to_F(b"Ciphersuite-reduce_512", &scalar);
+    scalar.zeroize();
+    res
+  }
+
   fn hash_to_F(dst: &[u8], data: &[u8]) -> Self::F {
     Scalar::wide_reduce(Self::H::digest([dst, data].concat()).as_ref().try_into().unwrap())
   }

crypto/ciphersuite/src/kp256.rs

@@ -6,7 +6,7 @@ use group::ff::PrimeField;
 
 use elliptic_curve::{
   generic_array::GenericArray,
-  bigint::{NonZero, CheckedAdd, Encoding, U384},
+  bigint::{NonZero, CheckedAdd, Encoding, U384, U512},
   hash2curve::{Expander, ExpandMsg, ExpandMsgXmd},
 };
 
@@ -31,6 +31,22 @@ macro_rules! kp_curve {
         $lib::ProjectivePoint::GENERATOR
       }
 
+      fn reduce_512(scalar: [u8; 64]) -> Self::F {
+        let mut modulus = [0; 64];
+        modulus[32 ..].copy_from_slice(&(Self::F::ZERO - Self::F::ONE).to_bytes());
+        let modulus = U512::from_be_slice(&modulus).checked_add(&U512::ONE).unwrap();
+
+        let mut wide =
+          U512::from_be_bytes(scalar).rem(&NonZero::new(modulus).unwrap()).to_be_bytes();
+
+        let mut array = *GenericArray::from_slice(&wide[32 ..]);
+        let res = $lib::Scalar::from_repr(array).unwrap();
+
+        wide.zeroize();
+        array.zeroize();
+        res
+      }
+
       fn hash_to_F(dst: &[u8], msg: &[u8]) -> Self::F {
         // While one of these two libraries does support directly hashing to the Scalar field, the
         // other doesn't. While that's probably an oversight, this is a universally working method

crypto/ciphersuite/src/lib.rs

@@ -62,6 +62,12 @@ pub trait Ciphersuite:
   // While group does provide this in its API, privacy coins may want to use a custom basepoint
   fn generator() -> Self::G;
 
+  /// Reduce 512 bits into a uniform scalar.
+  ///
+  /// If 512 bits is insufficient to perform a reduction into a uniform scalar, the ciphersuite
+  /// will perform a hash to sample the necessary bits.
+  fn reduce_512(scalar: [u8; 64]) -> Self::F;
+
   /// Hash the provided domain-separation tag and message to a scalar. Ciphersuites MAY naively
   /// prefix the tag to the message, enabling transpotion between the two. Accordingly, this
   /// function should NOT be used in any scheme where one tag is a valid substring of another
@@ -99,6 +105,9 @@ pub trait Ciphersuite:
   }
 
   /// Read a canonical point from something implementing std::io::Read.
+  ///
+  /// The provided implementation is safe so long as `GroupEncoding::to_bytes` always returns a
+  /// canonical serialization.
   #[cfg(any(feature = "alloc", feature = "std"))]
   #[allow(non_snake_case)]
   fn read_G<R: Read>(reader: &mut R) -> io::Result<Self::G> {