feat: round-6 abstractions — HealthCheckedPool, HkdfKeyDeriver, BackpressureEnforcer

crates/mytheclipse-crypto/Cargo.toml

@@ -22,6 +22,8 @@ encryption = ["dep:aead", "dep:aes-gcm", "dep:rand_core", "dep:rand"]
 tokens = ["encryption", "dep:serde", "dep:serde_json", "dep:base64", "dep:jsonwebtoken"]
 paseto = ["encryption", "dep:serde", "dep:serde_json", "dep:base64", "dep:pasetors"]
 rate-limit = ["dep:hashbrown", "dep:tokio"]
+# HKDF-SHA256 key derivation (RFC 5869).
+derivation = ["dep:hkdf", "dep:sha2"]
 
 [dependencies]
 tracing = "0.1"
@@ -37,6 +39,8 @@ serde = { version = "1", optional = true, features = ["derive"] }
 serde_json = { version = "1", optional = true }
 rand = { version = "0.8", default-features = false, features = ["std", "std_rng"], optional = true }
 rand_core = { version = "0.6", optional = true }
+hkdf = { version = "0.12", default-features = false, optional = true }
+sha2 = { version = "0.10", optional = true }
 pasetors = { version = "0.6", optional = true, default-features = false, features = ["v4"] }
 hashbrown = { version = "0.15", optional = true }
 tokio = { version = "1.53", features = ["sync", "time"], optional = true }

crates/mytheclipse-crypto/src/hkdf.rs

@@ -0,0 +1,70 @@
+//! HKDF-SHA256 key derivation (feature `derivation`).
+//!
+//! [`HkdfKeyDeriver`] wraps the HKDF construction (RFC 5869) to derive
+//! domain-specific sub-keys from a single master secret. Each purpose
+//! string acts as the `info` parameter for domain separation.
+
+use sha2::Sha256;
+use hkdf::Hkdf;
+
+/// Derives sub-keys from a master secret using HKDF-SHA256.
+pub struct HkdfKeyDeriver {
+    hk: Hkdf<Sha256>,
+}
+
+impl HkdfKeyDeriver {
+    /// Creates a deriver from the given master secret (IKM).
+    pub fn new(master: &[u8]) -> Self {
+        let hk = Hkdf::<Sha256>::new(None, master);
+        Self { hk }
+    }
+
+    /// Derives a sub-key for the given `purpose` (used as the `info` parameter).
+    ///
+    /// Returns `Ok(key)` on success, or an error if `output_len` exceeds the
+    /// maximum for SHA-256 HKDF.
+    pub fn derive_key(&self, purpose: &str, output_len: usize) -> Vec<u8> {
+        let mut okm = vec![0u8; output_len];
+        self.hk
+            .expand(purpose.as_bytes(), &mut okm)
+            .expect("HKDF expand failed — output_len too large");
+        okm
+    }
+
+    /// Convenience: derive a 32-byte AES-256 key for `purpose`.
+    pub fn derive_aes256_key(&self, purpose: &str) -> [u8; 32] {
+        let v = self.derive_key(purpose, 32);
+        let mut key = [0u8; 32];
+        key.copy_from_slice(&v);
+        key
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn derive_key_is_deterministic() {
+        let deriver = HkdfKeyDeriver::new(b"master-secret");
+        let k1 = deriver.derive_key("encryption", 32);
+        let k2 = deriver.derive_key("encryption", 32);
+        assert_eq!(k1, k2);
+        assert_eq!(k1.len(), 32);
+    }
+
+    #[test]
+    fn derive_key_different_purposes_yield_different_keys() {
+        let deriver = HkdfKeyDeriver::new(b"master-secret");
+        let enc = deriver.derive_key("encryption", 32);
+        let auth = deriver.derive_key("auth", 32);
+        assert_ne!(enc, auth);
+    }
+
+    #[test]
+    fn derive_aes256_key_length() {
+        let deriver = HkdfKeyDeriver::new(b"master-secret");
+        let key = deriver.derive_aes256_key("signing");
+        assert_eq!(key.len(), 32);
+    }
+}

crates/mytheclipse-crypto/src/lib.rs

@@ -55,6 +55,8 @@ pub mod token;
 
 #[cfg(feature = "paseto")]
 pub mod paseto;
+#[cfg(feature = "derivation")]
+pub mod hkdf;
 
 #[cfg(feature = "password")]
 pub use password::PasswordHasher;
@@ -71,6 +73,9 @@ pub use paseto::{PasetoSigner, PasetoClaims};
 pub use key_ring::KeyRing;
 pub use key_registry::TypedKeyRegistry;
 
+#[cfg(feature = "derivation")]
+pub use hkdf::HkdfKeyDeriver;
+
 /// Errors returned across mytheclipse-crypto primitives.
 #[non_exhaustive]
 #[derive(Debug)]