Allow multiple dsts in the hash2curve API

elliptic-curve/src/hash2curve/group_digest.rs

@@ -48,10 +48,10 @@ where
     /// [`ExpandMsgXof`]: crate::hash2curve::ExpandMsgXof
     fn hash_from_bytes<'a, X: ExpandMsg<'a>>(
         msgs: &[&[u8]],
-        dst: &'a [u8],
+        dsts: &'a [&'a [u8]],
     ) -> Result<ProjectivePoint<Self>> {
         let mut u = [Self::FieldElement::default(), Self::FieldElement::default()];
-        hash_to_field::<X, _>(msgs, dst, &mut u)?;
+        hash_to_field::<X, _>(msgs, dsts, &mut u)?;
         let q0 = u[0].map_to_curve();
         let q1 = u[1].map_to_curve();
         // Ideally we could add and then clear cofactor once
@@ -88,10 +88,10 @@ where
     /// [`ExpandMsgXof`]: crate::hash2curve::ExpandMsgXof
     fn encode_from_bytes<'a, X: ExpandMsg<'a>>(
         msgs: &[&[u8]],
-        dst: &'a [u8],
+        dsts: &'a [&'a [u8]],
     ) -> Result<ProjectivePoint<Self>> {
         let mut u = [Self::FieldElement::default()];
-        hash_to_field::<X, _>(msgs, dst, &mut u)?;
+        hash_to_field::<X, _>(msgs, dsts, &mut u)?;
         let q0 = u[0].map_to_curve();
         Ok(q0.clear_cofactor().into())
     }
@@ -109,12 +109,15 @@ where
     ///
     /// [`ExpandMsgXmd`]: crate::hash2curve::ExpandMsgXmd
     /// [`ExpandMsgXof`]: crate::hash2curve::ExpandMsgXof
-    fn hash_to_scalar<'a, X: ExpandMsg<'a>>(msgs: &[&[u8]], dst: &'a [u8]) -> Result<Self::Scalar>
+    fn hash_to_scalar<'a, X: ExpandMsg<'a>>(
+        msgs: &[&[u8]],
+        dsts: &'a [&'a [u8]],
+    ) -> Result<Self::Scalar>
     where
         Self::Scalar: FromOkm,
     {
         let mut u = [Self::Scalar::default()];
-        hash_to_field::<X, _>(msgs, dst, &mut u)?;
+        hash_to_field::<X, _>(msgs, dsts, &mut u)?;
         Ok(u[0])
     }
 }

elliptic-curve/src/hash2curve/hash2field.rs

@@ -32,7 +32,7 @@ pub trait FromOkm {
 /// [`ExpandMsgXmd`]: crate::hash2field::ExpandMsgXmd
 /// [`ExpandMsgXof`]: crate::hash2field::ExpandMsgXof
 #[doc(hidden)]
-pub fn hash_to_field<'a, E, T>(data: &[&[u8]], domain: &'a [u8], out: &mut [T]) -> Result<()>
+pub fn hash_to_field<'a, E, T>(data: &[&[u8]], domain: &'a [&'a [u8]], out: &mut [T]) -> Result<()>
 where
     E: ExpandMsg<'a>,
     T: FromOkm + Default,

elliptic-curve/src/hash2curve/hash2field/expand_msg.rs

@@ -25,8 +25,11 @@ pub trait ExpandMsg<'a> {
     ///
     /// Returns an expander that can be used to call `read` until enough
     /// bytes have been consumed
-    fn expand_message(msgs: &[&[u8]], dst: &'a [u8], len_in_bytes: usize)
-        -> Result<Self::Expander>;
+    fn expand_message(
+        msgs: &[&[u8]],
+        dsts: &'a [&'a [u8]],
+        len_in_bytes: usize,
+    ) -> Result<Self::Expander>;
 }
 
 /// Expander that, call `read` until enough bytes have been consumed.
@@ -47,54 +50,66 @@ where
     /// > 255
     Hashed(GenericArray<u8, L>),
     /// <= 255
-    Array(&'a [u8]),
+    Array(&'a [&'a [u8]]),
 }
 
 impl<'a, L> Domain<'a, L>
 where
     L: ArrayLength<u8> + IsLess<U256>,
 {
-    pub fn xof<X>(dst: &'a [u8]) -> Result<Self>
+    pub fn xof<X>(dsts: &'a [&'a [u8]]) -> Result<Self>
     where
         X: Default + ExtendableOutput + Update,
     {
-        if dst.is_empty() {
+        if dsts.is_empty() {
             Err(Error)
-        } else if dst.len() > MAX_DST_LEN {
+        } else if dsts.iter().map(|dst| dst.len()).sum::<usize>() > MAX_DST_LEN {
             let mut data = GenericArray::<u8, L>::default();
-            X::default()
-                .chain(OVERSIZE_DST_SALT)
-                .chain(dst)
-                .finalize_xof()
-                .read(&mut data);
+            let mut hash = X::default();
+            hash.update(OVERSIZE_DST_SALT);
+
+            for dst in dsts {
+                hash.update(dst);
+            }
+
+            hash.finalize_xof().read(&mut data);
+
             Ok(Self::Hashed(data))
         } else {
-            Ok(Self::Array(dst))
+            Ok(Self::Array(dsts))
         }
     }
 
-    pub fn xmd<X>(dst: &'a [u8]) -> Result<Self>
+    pub fn xmd<X>(dsts: &'a [&'a [u8]]) -> Result<Self>
     where
         X: Digest<OutputSize = L>,
     {
-        if dst.is_empty() {
+        if dsts.is_empty() {
             Err(Error)
-        } else if dst.len() > MAX_DST_LEN {
+        } else if dsts.iter().map(|dst| dst.len()).sum::<usize>() > MAX_DST_LEN {
             Ok(Self::Hashed({
                 let mut hash = X::new();
                 hash.update(OVERSIZE_DST_SALT);
-                hash.update(dst);
+
+                for dst in dsts {
+                    hash.update(dst);
+                }
+
                 hash.finalize()
             }))
         } else {
-            Ok(Self::Array(dst))
+            Ok(Self::Array(dsts))
         }
     }
 
-    pub fn data(&self) -> &[u8] {
+    pub fn update_hash<HashT: Update>(&self, hash: &mut HashT) {
         match self {
-            Self::Hashed(d) => &d[..],
-            Self::Array(d) => d,
+            Self::Hashed(d) => hash.update(d),
+            Self::Array(d) => {
+                for d in d.iter() {
+                    hash.update(d)
+                }
+            }
         }
     }
 
@@ -103,13 +118,28 @@ where
             // Can't overflow because it's enforced on a type level.
             Self::Hashed(_) => L::to_u8(),
             // Can't overflow because it's checked on creation.
-            Self::Array(d) => u8::try_from(d.len()).expect("length overflow"),
+            Self::Array(d) => {
+                u8::try_from(d.iter().map(|d| d.len()).sum::<usize>()).expect("length overflow")
+            }
         }
     }
 
     #[cfg(test)]
     pub fn assert(&self, bytes: &[u8]) {
-        assert_eq!(self.data(), &bytes[..bytes.len() - 1]);
+        let data = match self {
+            Domain::Hashed(d) => d.to_vec(),
+            Domain::Array(d) => d.iter().copied().flatten().copied().collect(),
+        };
+        assert_eq!(data, bytes);
+    }
+
+    #[cfg(test)]
+    pub fn assert_dst(&self, bytes: &[u8]) {
+        let data = match self {
+            Domain::Hashed(d) => d.to_vec(),
+            Domain::Array(d) => d.iter().copied().flatten().copied().collect(),
+        };
+        assert_eq!(data, &bytes[..bytes.len() - 1]);
         assert_eq!(self.len(), bytes[bytes.len() - 1]);
     }
 }

elliptic-curve/src/hash2curve/hash2field/expand_msg/xmd.rs

@@ -10,7 +10,7 @@ use digest::{
         typenum::{IsLess, IsLessOrEqual, Unsigned, U256},
         GenericArray,
     },
-    Digest,
+    FixedOutput, HashMarker,
 };
 
 /// Placeholder type for implementing `expand_message_xmd` based on a hash function
@@ -22,14 +22,14 @@ use digest::{
 /// - `len_in_bytes > 255 * HashT::OutputSize`
 pub struct ExpandMsgXmd<HashT>(PhantomData<HashT>)
 where
-    HashT: Digest + BlockSizeUser,
+    HashT: BlockSizeUser + Default + FixedOutput + HashMarker,
     HashT::OutputSize: IsLess<U256>,
     HashT::OutputSize: IsLessOrEqual<HashT::BlockSize>;
 
 /// ExpandMsgXmd implements expand_message_xmd for the ExpandMsg trait
 impl<'a, HashT> ExpandMsg<'a> for ExpandMsgXmd<HashT>
 where
-    HashT: Digest + BlockSizeUser,
+    HashT: BlockSizeUser + Default + FixedOutput + HashMarker,
     // If `len_in_bytes` is bigger then 256, length of the `DST` will depend on
     // the output size of the hash, which is still not allowed to be bigger then 256:
     // https://www.ietf.org/archive/id/draft-irtf-cfrg-hash-to-curve-13.html#section-5.4.1-6
@@ -42,7 +42,7 @@ where
 
     fn expand_message(
         msgs: &[&[u8]],
-        dst: &'a [u8],
+        dsts: &'a [&'a [u8]],
         len_in_bytes: usize,
     ) -> Result<Self::Expander> {
         if len_in_bytes == 0 {
@@ -54,26 +54,26 @@ where
         let b_in_bytes = HashT::OutputSize::to_usize();
         let ell = u8::try_from((len_in_bytes + b_in_bytes - 1) / b_in_bytes).map_err(|_| Error)?;
 
-        let domain = Domain::xmd::<HashT>(dst)?;
-        let mut b_0 = HashT::new();
-        b_0.update(GenericArray::<u8, HashT::BlockSize>::default());
+        let domain = Domain::xmd::<HashT>(dsts)?;
+        let mut b_0 = HashT::default();
+        b_0.update(&GenericArray::<u8, HashT::BlockSize>::default());
 
         for msg in msgs {
             b_0.update(msg);
         }
 
-        b_0.update(len_in_bytes_u16.to_be_bytes());
-        b_0.update([0]);
-        b_0.update(domain.data());
-        b_0.update([domain.len()]);
-        let b_0 = b_0.finalize();
+        b_0.update(&len_in_bytes_u16.to_be_bytes());
+        b_0.update(&[0]);
+        domain.update_hash(&mut b_0);
+        b_0.update(&[domain.len()]);
+        let b_0 = b_0.finalize_fixed();
 
-        let mut b_vals = HashT::new();
+        let mut b_vals = HashT::default();
         b_vals.update(&b_0[..]);
-        b_vals.update([1u8]);
-        b_vals.update(domain.data());
-        b_vals.update([domain.len()]);
-        let b_vals = b_vals.finalize();
+        b_vals.update(&[1u8]);
+        domain.update_hash(&mut b_vals);
+        b_vals.update(&[domain.len()]);
+        let b_vals = b_vals.finalize_fixed();
 
         Ok(ExpanderXmd {
             b_0,
@@ -89,7 +89,7 @@ where
 /// [`Expander`] type for [`ExpandMsgXmd`].
 pub struct ExpanderXmd<'a, HashT>
 where
-    HashT: Digest + BlockSizeUser,
+    HashT: BlockSizeUser + Default + FixedOutput + HashMarker,
     HashT::OutputSize: IsLess<U256>,
     HashT::OutputSize: IsLessOrEqual<HashT::BlockSize>,
 {
@@ -103,7 +103,7 @@ where
 
 impl<'a, HashT> ExpanderXmd<'a, HashT>
 where
-    HashT: Digest + BlockSizeUser,
+    HashT: BlockSizeUser + Default + FixedOutput + HashMarker,
     HashT::OutputSize: IsLess<U256>,
     HashT::OutputSize: IsLessOrEqual<HashT::BlockSize>,
 {
@@ -118,12 +118,12 @@ where
                 .zip(&self.b_vals[..])
                 .enumerate()
                 .for_each(|(j, (b0val, bi1val))| tmp[j] = b0val ^ bi1val);
-            let mut b_vals = HashT::new();
-            b_vals.update(tmp);
-            b_vals.update([self.index]);
-            b_vals.update(self.domain.data());
-            b_vals.update([self.domain.len()]);
-            self.b_vals = b_vals.finalize();
+            let mut b_vals = HashT::default();
+            b_vals.update(&tmp);
+            b_vals.update(&[self.index]);
+            self.domain.update_hash(&mut b_vals);
+            b_vals.update(&[self.domain.len()]);
+            self.b_vals = b_vals.finalize_fixed();
             true
         } else {
             false
@@ -133,7 +133,7 @@ where
 
 impl<'a, HashT> Expander for ExpanderXmd<'a, HashT>
 where
-    HashT: Digest + BlockSizeUser,
+    HashT: BlockSizeUser + Default + FixedOutput + HashMarker,
     HashT::OutputSize: IsLess<U256>,
     HashT::OutputSize: IsLessOrEqual<HashT::BlockSize>,
 {
@@ -165,7 +165,7 @@ mod test {
         len_in_bytes: u16,
         bytes: &[u8],
     ) where
-        HashT: Digest + BlockSizeUser,
+        HashT: BlockSizeUser + Default + FixedOutput + HashMarker,
         HashT::OutputSize: IsLess<U256>,
     {
         let block = HashT::BlockSize::to_usize();
@@ -183,8 +183,8 @@ mod test {
         let pad = l + mem::size_of::<u8>();
         assert_eq!([0], &bytes[l..pad]);
 
-        let dst = pad + domain.data().len();
-        assert_eq!(domain.data(), &bytes[pad..dst]);
+        let dst = pad + usize::from(domain.len());
+        domain.assert(&bytes[pad..dst]);
 
         let dst_len = dst + mem::size_of::<u8>();
         assert_eq!([domain.len()], &bytes[dst..dst_len]);
@@ -205,13 +205,14 @@ mod test {
             domain: &Domain<'_, HashT::OutputSize>,
         ) -> Result<()>
         where
-            HashT: Digest + BlockSizeUser,
+            HashT: BlockSizeUser + Default + FixedOutput + HashMarker,
             HashT::OutputSize: IsLess<U256> + IsLessOrEqual<HashT::BlockSize>,
         {
             assert_message::<HashT>(self.msg, domain, L::to_u16(), self.msg_prime);
 
+            let dst = [dst];
             let mut expander =
-                ExpandMsgXmd::<HashT>::expand_message(&[self.msg], dst, L::to_usize())?;
+                ExpandMsgXmd::<HashT>::expand_message(&[self.msg], &dst, L::to_usize())?;
 
             let mut uniform_bytes = GenericArray::<u8, L>::default();
             expander.fill_bytes(&mut uniform_bytes);
@@ -227,8 +228,8 @@ mod test {
         const DST_PRIME: &[u8] =
             &hex!("515555582d5630312d435330322d776974682d657870616e6465722d5348413235362d31323826");
 
-        let dst_prime = Domain::xmd::<Sha256>(DST)?;
-        dst_prime.assert(DST_PRIME);
+        let dst_prime = Domain::xmd::<Sha256>(&[DST])?;
+        dst_prime.assert_dst(DST_PRIME);
 
         const TEST_VECTORS_32: &[TestVector] = &[
             TestVector {
@@ -299,8 +300,8 @@ mod test {
         const DST_PRIME: &[u8] =
             &hex!("412717974da474d0f8c420f320ff81e8432adb7c927d9bd082b4fb4d16c0a23620");
 
-        let dst_prime = Domain::xmd::<Sha256>(DST)?;
-        dst_prime.assert(DST_PRIME);
+        let dst_prime = Domain::xmd::<Sha256>(&[DST])?;
+        dst_prime.assert_dst(DST_PRIME);
 
         const TEST_VECTORS_32: &[TestVector] = &[
             TestVector {
@@ -377,8 +378,8 @@ mod test {
         const DST_PRIME: &[u8] =
             &hex!("515555582d5630312d435330322d776974682d657870616e6465722d5348413531322d32353626");
 
-        let dst_prime = Domain::xmd::<Sha512>(DST)?;
-        dst_prime.assert(DST_PRIME);
+        let dst_prime = Domain::xmd::<Sha512>(&[DST])?;
+        dst_prime.assert_dst(DST_PRIME);
 
         const TEST_VECTORS_32: &[TestVector] = &[
             TestVector {