diff --git a/async-signature/src/hazmat.rs b/async-signature/src/hazmat.rs
new file mode 100644
index 000000000..4a130d578
--- /dev/null
+++ b/async-signature/src/hazmat.rs
@@ -0,0 +1,53 @@
+//! Hazardous Materials: low-level APIs which can be insecure if misused.
+//!
+//! The traits in this module are not generally recommended, and should only be
+//! used in special cases where they are specifically needed.
+//!
+//! Using them incorrectly can introduce security vulnerabilities. Please
+//! carefully read the documentation before attempting to use them.
+
+use signature::Error;
+
+#[cfg(feature = "rand_core")]
+use signature::rand_core::CryptoRngCore;
+
+/// Asynchronously sign the provided message prehash, returning a digital signature.
+#[allow(async_fn_in_trait)]
+pub trait AsyncPrehashSigner<S> {
+    /// Attempt to sign the given message digest, returning a digital signature
+    /// on success, or an error if something went wrong.
+    ///
+    /// The `prehash` parameter should be the output of a secure cryptographic
+    /// hash function.
+    ///
+    /// This API takes a `prehash` byte slice as there can potentially be many
+    /// compatible lengths for the message digest for a given concrete signature
+    /// algorithm.
+    ///
+    /// Allowed lengths are algorithm-dependent and up to a particular
+    /// implementation to decide.
+    async fn sign_prehash_async(&self, prehash: &[u8]) -> Result<S, Error>;
+}
+
+/// Asynchronously sign the provided message prehash using the provided external randomness source, returning a digital signature.
+#[cfg(feature = "rand_core")]
+#[allow(async_fn_in_trait)]
+pub trait AsyncRandomizedPrehashSigner<S> {
+    /// Attempt to sign the given message digest, returning a digital signature
+    /// on success, or an error if something went wrong.
+    ///
+    /// The `prehash` parameter should be the output of a secure cryptographic
+    /// hash function.
+    ///
+    /// This API takes a `prehash` byte slice as there can potentially be many
+    /// compatible lengths for the message digest for a given concrete signature
+    /// algorithm.
+    ///
+    /// Allowed lengths are algorithm-dependent and up to a particular
+    /// implementation to decide.
+    async fn sign_prehash_with_rng_async(
+        &self,
+        rng: &mut impl CryptoRngCore,
+        prehash: &[u8],
+    ) -> Result<S, Error>;
+}
diff --git a/async-signature/src/lib.rs b/async-signature/src/lib.rs
index 20ebd06d6..56d40e841 100644
--- a/async-signature/src/lib.rs
+++ b/async-signature/src/lib.rs
@@ -13,6 +13,8 @@
     missing_debug_implementations
 )]
 
+pub mod hazmat;
+
 pub use signature::{self, Error};
 
 #[cfg(feature = "digest")]