Adds HKDF to the Crypto trait (#610)

libraries/opensk/src/api/crypto/hkdf256.rs

@@ -0,0 +1,26 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use super::HASH_SIZE;
+
+/// HKDF using SHA256.
+pub trait Hkdf256 {
+    /// Computes the HKDF with empty salt and 256 bit (one block) output.
+    ///
+    /// # Arguments
+    ///
+    /// * `ikm` - Input keying material
+    /// * `info` - Optional context and application specific information
+    fn hkdf_empty_salt_256(ikm: &[u8], info: &[u8]) -> [u8; HASH_SIZE];
+}