diff --git a/ed25519-dalek/src/hazmat.rs b/ed25519-dalek/src/hazmat.rs
index 7849613..4be9f0d 100644
--- a/ed25519-dalek/src/hazmat.rs
+++ b/ed25519-dalek/src/hazmat.rs
@@ -106,7 +106,7 @@ impl TryFrom<&[u8]> for ExpandedSecretKey {
 /// calculate the pseudorandomness needed for signing. According to the Ed25519 spec, `CtxDigest =
 /// Sha512`.
 ///
-/// # ⚠️  Unsafe
+/// # ⚠️  Cryptographically Unsafe
 ///
 /// Do NOT use this function unless you absolutely must. Using the wrong values in
 /// `ExpandedSecretKey` can leak your signing key. See
@@ -127,7 +127,7 @@ where
 /// `CtxDigest` is the digest function used to calculate the pseudorandomness needed for signing.
 /// According to the Ed25519 spec, `MsgDigest = CtxDigest = Sha512`.
 ///
-/// # ⚠️  Unsafe
+/// # ⚠️  Cryptographically Unsafe
 //
 /// Do NOT use this function unless you absolutely must. Using the wrong values in
 /// `ExpandedSecretKey` can leak your signing key. See
@@ -169,6 +169,33 @@ where
     esk.raw_sign_prehashed::<CtxDigest, MsgDigest>(prehashed_message, verifying_key, context)
 }
 
+/// Compute an ordinary Ed25519 signature, with the message contents provided incrementally by
+/// updating a digest instance.
+///
+/// The `msg_update` closure provides the message content, updating a hasher argument. It will be
+/// called twice. This closure MUST leave its hasher in the same state (i.e., must hash the same
+/// values) after both calls. Otherwise it will produce an invalid signature.
+///
+/// `CtxDigest` is the digest used to calculate the pseudorandomness needed for signing. According
+/// to the Ed25519 spec, `CtxDigest = Sha512`.
+///
+/// # ⚠️  Cryptographically Unsafe
+///
+/// Do NOT use this function unless you absolutely must. Using the wrong values in
+/// `ExpandedSecretKey` can leak your signing key. See
+/// [here](https://github.com/MystenLabs/ed25519-unsafe-libs) for more details on this attack.
+pub fn raw_sign_byupdate<CtxDigest, F>(
+    esk: &ExpandedSecretKey,
+    msg_update: F,
+    verifying_key: &VerifyingKey,
+) -> Result<Signature, SignatureError>
+where
+    CtxDigest: Digest<OutputSize = U64>,
+    F: Fn(&mut CtxDigest) -> Result<(), SignatureError>,
+{
+    esk.raw_sign_byupdate::<CtxDigest, F>(msg_update, verifying_key)
+}
+
 /// The ordinary non-batched Ed25519 verification check, rejecting non-canonical R
 /// values.`CtxDigest` is the digest used to calculate the pseudorandomness needed for signing.
 /// According to the Ed25519 spec, `CtxDigest = Sha512`.
@@ -263,4 +290,47 @@ mod test {
             .unwrap();
         raw_verify_prehashed::<CtxDigest, MsgDigest>(&vk, h, Some(ctx_str), &sig).unwrap();
     }
+
+    #[test]
+    fn sign_byupdate() {
+        // Generate the keypair
+        let mut rng = OsRng;
+        let esk = ExpandedSecretKey::random(&mut rng);
+        let vk = VerifyingKey::from(&esk);
+
+        let msg = b"realistic";
+        // signatures are deterministic so we can compare with a good one
+        let good_sig = raw_sign::<CtxDigest>(&esk, msg, &vk);
+
+        let sig = raw_sign_byupdate::<CtxDigest, _>(
+            &esk,
+            |h| {
+                h.update(msg);
+                Ok(())
+            },
+            &vk,
+        );
+        assert!(sig.unwrap() == good_sig, "sign byupdate matches");
+
+        let sig = raw_sign_byupdate::<CtxDigest, _>(
+            &esk,
+            |h| {
+                h.update(msg);
+                Err(SignatureError::new())
+            },
+            &vk,
+        );
+        assert!(sig.is_err(), "sign byupdate failure propagates");
+
+        let sig = raw_sign_byupdate::<CtxDigest, _>(
+            &esk,
+            |h| {
+                h.update(&msg[..1]);
+                h.update(&msg[1..]);
+                Ok(())
+            },
+            &vk,
+        );
+        assert!(sig.unwrap() == good_sig, "sign byupdate two part");
+    }
 }
diff --git a/ed25519-dalek/src/signing.rs b/ed25519-dalek/src/signing.rs
index 144569e..5416274 100644
--- a/ed25519-dalek/src/signing.rs
+++ b/ed25519-dalek/src/signing.rs
@@ -833,6 +833,7 @@ impl ExpandedSecretKey {
     /// This definition is loose in its parameters so that end-users of the `hazmat` module can
     /// change how the `ExpandedSecretKey` is calculated and which hash function to use.
     #[allow(non_snake_case)]
+    #[allow(clippy::unwrap_used)]
     #[inline(always)]
     pub(crate) fn raw_sign<CtxDigest>(
         &self,
@@ -841,11 +842,36 @@ impl ExpandedSecretKey {
     ) -> Signature
     where
         CtxDigest: Digest<OutputSize = U64>,
+    {
+        // OK unwrap, update can't fail.
+        self.raw_sign_byupdate(
+            |h: &mut CtxDigest| {
+                h.update(message);
+                Ok(())
+            },
+            verifying_key,
+        )
+        .unwrap()
+    }
+
+    /// Sign a message provided in parts. The `msg_update` closure will be called twice to hash the
+    /// message parts. This closure MUST leave its hasher in the same state (i.e., must hash the
+    /// same values) after both calls. Otherwise it will produce an invalid signature.
+    #[allow(non_snake_case)]
+    #[inline(always)]
+    pub(crate) fn raw_sign_byupdate<CtxDigest, F>(
+        &self,
+        msg_update: F,
+        verifying_key: &VerifyingKey,
+    ) -> Result<Signature, SignatureError>
+    where
+        CtxDigest: Digest<OutputSize = U64>,
+        F: Fn(&mut CtxDigest) -> Result<(), SignatureError>,
     {
         let mut h = CtxDigest::new();
 
         h.update(self.hash_prefix);
-        h.update(message);
+        msg_update(&mut h)?;
 
         let r = Scalar::from_hash(h);
         let R: CompressedEdwardsY = EdwardsPoint::mul_base(&r).compress();
@@ -853,12 +879,12 @@ impl ExpandedSecretKey {
         h = CtxDigest::new();
         h.update(R.as_bytes());
         h.update(verifying_key.as_bytes());
-        h.update(message);
+        msg_update(&mut h)?;
 
         let k = Scalar::from_hash(h);
         let s: Scalar = (k * self.scalar) + r;
 
-        InternalSignature { R, s }.into()
+        Ok(InternalSignature { R, s }.into())
     }
 
     /// The prehashed signing function for Ed25519 (i.e., Ed25519ph). `CtxDigest` is the digest