diff --git a/ed25519-dalek/README.md b/ed25519-dalek/README.md
index da29362b5..f22a8d1c6 100644
--- a/ed25519-dalek/README.md
+++ b/ed25519-dalek/README.md
@@ -22,7 +22,7 @@ This crate is `#[no_std]` compatible with `default-features = false`.
 | `zeroize`              | ✓        | Implements `Zeroize` and `ZeroizeOnDrop` for `SigningKey` |
 | `rand_core`            |          | Enables `SigningKey::generate` |
 | `batch`                |          | Enables `verify_batch` for verifying many signatures quickly. Also enables `alloc` and `rand_core`. |
-| `digest`               |          | Enables `Context`, `SigningKey::{with_context, sign_prehashed}` and `VerifyingKey::{with_context, verify_prehashed, verify_prehashed_strict}` for Ed25519ph prehashed signatures |
+| `digest`               |          | Enables `Context`, `SigningKey::{with_context, sign_prehashed}` and `VerifyingKey::{with_context, verify_prehashed, verify_prehashed_strict}` for Ed25519ph prehashed signatures. Also implements `KeySizeUser` for `SigningKey` and `VerifyingKey`. |
 | `pkcs8`                |          | Enables [PKCS#8](https://en.wikipedia.org/wiki/PKCS_8) serialization/deserialization for `SigningKey` and `VerifyingKey` |
 | `pem`                  |          | Enables PEM serialization support for PKCS#8 private keys and SPKI public keys. Also enables `alloc`. |
 | `legacy_compatibility` |          | **Unsafe:** Disables certain signature checks. See [below](#malleability-and-the-legacy_compatibility-feature) |
diff --git a/ed25519-dalek/src/signing.rs b/ed25519-dalek/src/signing.rs
index a451b1141..3f7c98e75 100644
--- a/ed25519-dalek/src/signing.rs
+++ b/ed25519-dalek/src/signing.rs
@@ -20,6 +20,9 @@ use rand_core::CryptoRng;
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
+#[cfg(feature = "digest")]
+use curve25519_dalek::digest::{crypto_common::KeySizeUser, typenum::U32};
+
 use sha2::Sha512;
 use subtle::{Choice, ConstantTimeEq};
 
@@ -549,6 +552,11 @@ impl SigningKey {
     }
 }
 
+#[cfg(feature = "digest")]
+impl KeySizeUser for SigningKey {
+    type KeySize = U32;
+}
+
 impl AsRef<VerifyingKey> for SigningKey {
     fn as_ref(&self) -> &VerifyingKey {
         &self.verifying_key
diff --git a/ed25519-dalek/src/verifying.rs b/ed25519-dalek/src/verifying.rs
index 2dc2eba0e..b3b4ef71a 100644
--- a/ed25519-dalek/src/verifying.rs
+++ b/ed25519-dalek/src/verifying.rs
@@ -9,6 +9,9 @@
 
 //! ed25519 public keys.
 
+#[cfg(feature = "digest")]
+use curve25519_dalek::digest::{crypto_common::KeySizeUser, typenum::U32};
+
 use core::fmt::Debug;
 use core::hash::{Hash, Hasher};
 
@@ -116,6 +119,11 @@ impl From<EdwardsPoint> for VerifyingKey {
     }
 }
 
+#[cfg(feature = "digest")]
+impl KeySizeUser for VerifyingKey {
+    type KeySize = U32;
+}
+
 impl VerifyingKey {
     /// Convert this public key to a byte array.
     #[inline]