1 files changed, 31 insertions, 0 deletions
diff --git a/openpgp/src/crypto/backend/botan/kdf.rs b/openpgp/src/crypto/backend/botan/kdf.rs
new file mode 100644
index 00000000..942165e2
--- /dev/null
+++ b/openpgp/src/crypto/backend/botan/kdf.rs
@@ -0,0 +1,31 @@
+use crate::{
+    Result,
+    crypto::{
+        SessionKey,
+        backend::interface::Kdf,
+    },
+};
+
+impl Kdf for super::Backend {
+    fn hkdf_sha256(ikm: &SessionKey, salt: Option<&[u8]>, info: &[u8],
+                   okm: &mut SessionKey)
+                   -> Result<()>
+    {
+        assert!(okm.len() <= 255 * 32);
+
+        const NO_SALT: [u8; 32] = [0; 32];
+        let salt = salt.unwrap_or(&NO_SALT);
+
+        // XXX: It'd be nice to write that directly to `okm`, but botan-rs
+        // does not have such an interface.
+        let okm_heap: SessionKey =
+            botan::kdf("HKDF(SHA-256)", okm.len(), &*ikm, salt, info)?
+            .into();
+
+        // XXX: Now copy the secret.
+        let l = okm.len().min(okm_heap.len());
+        okm[..l].copy_from_slice(&okm_heap[..l]);
+
+        Ok(())
+    }
+}